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<div class="section" id="basic-module-usage">
<h1>Basic module usage<a class="headerlink" href="#basic-module-usage" title="Permalink to this headline">¶</a></h1>
<p id="index-0">The basic Psycopg usage is common to all the database adapters implementing
the <a class="reference external" href="http://www.python.org/dev/peps/pep-0249/">DB API 2.0</a> protocol. Here is an interactive session showing some of the
basic commands:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">psycopg2</span>
<span class="go"># Connect to an existing database</span>
<span class="gp">>>> </span><span class="n">conn</span> <span class="o">=</span> <span class="n">psycopg2</span><span class="o">.</span><span class="n">connect</span><span class="p">(</span><span class="s">"dbname=test user=postgres"</span><span class="p">)</span>
<span class="go"># Open a cursor to perform database operations</span>
<span class="gp">>>> </span><span class="n">cur</span> <span class="o">=</span> <span class="n">conn</span><span class="o">.</span><span class="n">cursor</span><span class="p">()</span>
<span class="go"># Execute a command: this creates a new table</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"CREATE TABLE test (id serial PRIMARY KEY, num integer, data varchar);"</span><span class="p">)</span>
<span class="go"># Pass data to fill a query placeholders and let Psycopg perform</span>
<span class="go"># the correct conversion (no more SQL injections!)</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"INSERT INTO test (num, data) VALUES (</span><span class="si">%s</span><span class="s">, </span><span class="si">%s</span><span class="s">)"</span><span class="p">,</span>
<span class="gp">... </span> <span class="p">(</span><span class="mi">100</span><span class="p">,</span> <span class="s">"abc'def"</span><span class="p">))</span>
<span class="go"># Query the database and obtain data as Python objects</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"SELECT * FROM test;"</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">fetchone</span><span class="p">()</span>
<span class="go">(1, 100, "abc'def")</span>
<span class="go"># Make the changes to the database persistent</span>
<span class="gp">>>> </span><span class="n">conn</span><span class="o">.</span><span class="n">commit</span><span class="p">()</span>
<span class="go"># Close communication with the database</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">conn</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
<p>The main entry points of Psycopg are:</p>
<ul class="simple">
<li>The function <a class="reference internal" href="module.html#psycopg2.connect" title="psycopg2.connect"><tt class="xref py py-obj docutils literal"><span class="pre">connect()</span></tt></a> creates a new database session and
returns a new <a class="reference internal" href="connection.html#connection" title="connection"><tt class="xref py py-obj docutils literal"><span class="pre">connection</span></tt></a> instance.</li>
<li>The class <a class="reference internal" href="connection.html#connection" title="connection"><tt class="xref py py-obj docutils literal"><span class="pre">connection</span></tt></a> encapsulates a database session. It allows to:<ul>
<li>create new <a class="reference internal" href="cursor.html#cursor" title="cursor"><tt class="xref py py-obj docutils literal"><span class="pre">cursor</span></tt></a>s using the <a class="reference internal" href="connection.html#connection.cursor" title="connection.cursor"><tt class="xref py py-obj docutils literal"><span class="pre">cursor()</span></tt></a> method to
execute database commands and queries,</li>
<li>terminate the session using the methods <a class="reference internal" href="connection.html#connection.commit" title="connection.commit"><tt class="xref py py-obj docutils literal"><span class="pre">commit()</span></tt></a> or
<a class="reference internal" href="connection.html#connection.rollback" title="connection.rollback"><tt class="xref py py-obj docutils literal"><span class="pre">rollback()</span></tt></a>.</li>
</ul>
</li>
<li>The class <a class="reference internal" href="cursor.html#cursor" title="cursor"><tt class="xref py py-obj docutils literal"><span class="pre">cursor</span></tt></a> allows interaction with the database:<ul>
<li>send commands to the database using methods such as <a class="reference internal" href="cursor.html#cursor.execute" title="cursor.execute"><tt class="xref py py-obj docutils literal"><span class="pre">execute()</span></tt></a>
and <a class="reference internal" href="cursor.html#cursor.executemany" title="cursor.executemany"><tt class="xref py py-obj docutils literal"><span class="pre">executemany()</span></tt></a>,</li>
<li>retrieve data from the database <a class="reference internal" href="cursor.html#cursor-iterable"><em>by iteration</em></a> or
using methods such as <a class="reference internal" href="cursor.html#cursor.fetchone" title="cursor.fetchone"><tt class="xref py py-obj docutils literal"><span class="pre">fetchone()</span></tt></a>, <a class="reference internal" href="cursor.html#cursor.fetchmany" title="cursor.fetchmany"><tt class="xref py py-obj docutils literal"><span class="pre">fetchmany()</span></tt></a>,
<a class="reference internal" href="cursor.html#cursor.fetchall" title="cursor.fetchall"><tt class="xref py py-obj docutils literal"><span class="pre">fetchall()</span></tt></a>.</li>
</ul>
</li>
</ul>
<div class="section" id="passing-parameters-to-sql-queries">
<span id="query-parameters"></span><span id="index-1"></span><h2>Passing parameters to SQL queries<a class="headerlink" href="#passing-parameters-to-sql-queries" title="Permalink to this headline">¶</a></h2>
<p>Psycopg casts Python variables to SQL literals by type. Many standard Python types
are already <a class="reference internal" href="#python-types-adaptation">adapted to the correct SQL representation</a>.</p>
<p>Example: the Python function call:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span>
<span class="gp">... </span> <span class="sd">"""INSERT INTO some_table (an_int, a_date, a_string)</span>
<span class="gp">... </span><span class="sd"> VALUES (%s, %s, %s);"""</span><span class="p">,</span>
<span class="gp">... </span> <span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="n">datetime</span><span class="o">.</span><span class="n">date</span><span class="p">(</span><span class="mi">2005</span><span class="p">,</span> <span class="mi">11</span><span class="p">,</span> <span class="mi">18</span><span class="p">),</span> <span class="s">"O'Reilly"</span><span class="p">))</span>
</pre></div>
</div>
<p>is converted into the SQL command:</p>
<div class="highlight-python"><div class="highlight"><pre>INSERT INTO some_table (an_int, a_date, a_string)
VALUES (10, '2005-11-18', 'O''Reilly');
</pre></div>
</div>
<p>Named arguments are supported too using <tt class="samp docutils literal"><span class="pre">%(</span><em><span class="pre">name</span></em><span class="pre">)s</span></tt> placeholders.
Using named arguments the values can be passed to the query in any order and
many placeholders can use the same values:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span>
<span class="gp">... </span> <span class="sd">"""INSERT INTO some_table (an_int, a_date, another_date, a_string)</span>
<span class="gp">... </span><span class="sd"> VALUES (%(int)s, %(date)s, %(date)s, %(str)s);"""</span><span class="p">,</span>
<span class="gp">... </span> <span class="p">{</span><span class="s">'int'</span><span class="p">:</span> <span class="mi">10</span><span class="p">,</span> <span class="s">'str'</span><span class="p">:</span> <span class="s">"O'Reilly"</span><span class="p">,</span> <span class="s">'date'</span><span class="p">:</span> <span class="n">datetime</span><span class="o">.</span><span class="n">date</span><span class="p">(</span><span class="mi">2005</span><span class="p">,</span> <span class="mi">11</span><span class="p">,</span> <span class="mi">18</span><span class="p">)})</span>
</pre></div>
</div>
<p>While the mechanism resembles regular Python strings manipulation, there are a
few subtle differences you should care about when passing parameters to a
query:</p>
<ul>
<li><p class="first">The Python string operator <tt class="docutils literal"><span class="pre">%</span></tt> is not used: the <a class="reference internal" href="cursor.html#cursor.execute" title="cursor.execute"><tt class="xref py py-obj docutils literal"><span class="pre">execute()</span></tt></a>
method accepts a tuple or dictionary of values as second parameter.
<a class="reference internal" href="#sql-injection"><strong>Never</strong> use <tt class="docutils literal"><span class="pre">%</span></tt> or <tt class="docutils literal"><span class="pre">+</span></tt> to merge values
into queries</a>.</p>
</li>
<li><p class="first">The variables placeholder must <em>always be a</em> <tt class="docutils literal"><span class="pre">%s</span></tt>, even if a different
placeholder (such as a <tt class="docutils literal"><span class="pre">%d</span></tt> for integers or <tt class="docutils literal"><span class="pre">%f</span></tt> for floats) may look
more appropriate:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"INSERT INTO numbers VALUES (</span><span class="si">%d</span><span class="s">)"</span><span class="p">,</span> <span class="p">(</span><span class="mi">42</span><span class="p">,))</span> <span class="c"># WRONG</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"INSERT INTO numbers VALUES (</span><span class="si">%s</span><span class="s">)"</span><span class="p">,</span> <span class="p">(</span><span class="mi">42</span><span class="p">,))</span> <span class="c"># correct</span>
</pre></div>
</div>
</li>
<li><p class="first">For positional variables binding, <em>the second argument must always be a
sequence</em>, even if it contains a single variable. And remember that Python
requires a comma to create a single element tuple:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"INSERT INTO foo VALUES (</span><span class="si">%s</span><span class="s">)"</span><span class="p">,</span> <span class="s">"bar"</span><span class="p">)</span> <span class="c"># WRONG</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"INSERT INTO foo VALUES (</span><span class="si">%s</span><span class="s">)"</span><span class="p">,</span> <span class="p">(</span><span class="s">"bar"</span><span class="p">))</span> <span class="c"># WRONG</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"INSERT INTO foo VALUES (</span><span class="si">%s</span><span class="s">)"</span><span class="p">,</span> <span class="p">(</span><span class="s">"bar"</span><span class="p">,))</span> <span class="c"># correct</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"INSERT INTO foo VALUES (</span><span class="si">%s</span><span class="s">)"</span><span class="p">,</span> <span class="p">[</span><span class="s">"bar"</span><span class="p">])</span> <span class="c"># correct</span>
</pre></div>
</div>
</li>
<li><p class="first">Only variable values should be bound via this method: it shouldn’t be used
to set table or field names. For these elements, ordinary string formatting
should be used before running <a class="reference internal" href="cursor.html#cursor.execute" title="cursor.execute"><tt class="xref py py-obj docutils literal"><span class="pre">execute()</span></tt></a>.</p>
</li>
</ul>
<div class="section" id="the-problem-with-the-query-parameters">
<span id="sql-injection"></span><span id="index-2"></span><h3>The problem with the query parameters<a class="headerlink" href="#the-problem-with-the-query-parameters" title="Permalink to this headline">¶</a></h3>
<p>The SQL representation for many data types is often not the same of the Python
string representation. The classic example is with single quotes in
strings: SQL uses them as string constants bounds and requires them to be
escaped, whereas in Python single quotes can be left unescaped in strings
bounded by double quotes. For this reason a naïve approach to the composition
of query strings, e.g. using string concatenation, is a recipe for terrible
problems:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">SQL</span> <span class="o">=</span> <span class="s">"INSERT INTO authors (name) VALUES ('</span><span class="si">%s</span><span class="s">');"</span> <span class="c"># NEVER DO THIS</span>
<span class="gp">>>> </span><span class="n">data</span> <span class="o">=</span> <span class="p">(</span><span class="s">"O'Reilly"</span><span class="p">,</span> <span class="p">)</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="n">SQL</span> <span class="o">%</span> <span class="n">data</span><span class="p">)</span> <span class="c"># THIS WILL FAIL MISERABLY</span>
<span class="go">ProgrammingError: syntax error at or near "Reilly"</span>
<span class="go">LINE 1: INSERT INTO authors (name) VALUES ('O'Reilly')</span>
<span class="go"> ^</span>
</pre></div>
</div>
<p>If the variable containing the data to be sent to the database comes from an
untrusted source (e.g. a form published on a web site) an attacker could
easily craft a malformed string, either gaining access to unauthorized data or
performing destructive operations on the database. This form of attack is
called <a class="reference external" href="http://en.wikipedia.org/wiki/SQL_injection">SQL injection</a> and is known to be one of the most widespread forms of
attack to servers. Before continuing, please print <a class="reference external" href="http://xkcd.com/327/">this page</a> as a memo and
hang it onto your desk.</p>
<p>Psycopg can <a class="reference internal" href="#python-types-adaptation">automatically convert Python objects to and from SQL
literals</a>: using this feature your code will be more robust and
reliable. We must stress this point:</p>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">Never, <strong>never</strong>, <strong>NEVER</strong> use Python string concatenation (<tt class="docutils literal"><span class="pre">+</span></tt>) or
string parameters interpolation (<tt class="docutils literal"><span class="pre">%</span></tt>) to pass variables to a SQL query
string. Not even at gunpoint.</p>
</div>
<p>The correct way to pass variables in a SQL command is using the second
argument of the <a class="reference internal" href="cursor.html#cursor.execute" title="cursor.execute"><tt class="xref py py-obj docutils literal"><span class="pre">execute()</span></tt></a> method:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">SQL</span> <span class="o">=</span> <span class="s">"INSERT INTO authors (name) VALUES (</span><span class="si">%s</span><span class="s">);"</span> <span class="c"># Note: no quotes</span>
<span class="gp">>>> </span><span class="n">data</span> <span class="o">=</span> <span class="p">(</span><span class="s">"O'Reilly"</span><span class="p">,</span> <span class="p">)</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="n">SQL</span><span class="p">,</span> <span class="n">data</span><span class="p">)</span> <span class="c"># Note: no % operator</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="adaptation-of-python-values-to-sql-types">
<span id="python-types-adaptation"></span><span id="index-3"></span><h2>Adaptation of Python values to SQL types<a class="headerlink" href="#adaptation-of-python-values-to-sql-types" title="Permalink to this headline">¶</a></h2>
<p>Many standards Python types are adapted into SQL and returned as Python
objects when a query is executed.</p>
<p>If you need to convert other Python types to and from PostgreSQL data types,
see <a class="reference internal" href="advanced.html#adapting-new-types"><em>Adapting new Python types to SQL syntax</em></a> and <a class="reference internal" href="advanced.html#type-casting-from-sql-to-python"><em>Type casting of SQL types into Python objects</em></a>. You
can also find a few other specialized adapters in the <a class="reference internal" href="extras.html#module-psycopg2.extras" title="psycopg2.extras"><tt class="xref py py-obj docutils literal"><span class="pre">psycopg2.extras</span></tt></a>
module.</p>
<p>In the following examples the method <a class="reference internal" href="cursor.html#cursor.mogrify" title="cursor.mogrify"><tt class="xref py py-obj docutils literal"><span class="pre">mogrify()</span></tt></a> is used to show
the SQL string that would be sent to the database.</p>
<span class="target" id="adapt-consts"></span><ul id="index-4">
<li><p class="first">Python <a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/constants.html#None" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">None</span></tt></a> and boolean values <a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/constants.html#True" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">True</span></tt></a> and <a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/constants.html#False" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">False</span></tt></a> are converted into the
proper SQL literals:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">mogrify</span><span class="p">(</span><span class="s">"SELECT </span><span class="si">%s</span><span class="s">, </span><span class="si">%s</span><span class="s">, </span><span class="si">%s</span><span class="s">;"</span><span class="p">,</span> <span class="p">(</span><span class="bp">None</span><span class="p">,</span> <span class="bp">True</span><span class="p">,</span> <span class="bp">False</span><span class="p">))</span>
<span class="gp">>>> </span><span class="s">'SELECT NULL, true, false;'</span>
</pre></div>
</div>
</li>
</ul>
<span class="target" id="adapt-numbers"></span><ul id="index-5">
<li><p class="first">Numeric objects: <a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/functions.html#int" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">int</span></tt></a>, <a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/functions.html#long" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">long</span></tt></a>, <a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/functions.html#float" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">float</span></tt></a>, <a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/decimal.html#decimal.Decimal" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">Decimal</span></tt></a> are converted in
the PostgreSQL numerical representation:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">mogrify</span><span class="p">(</span><span class="s">"SELECT </span><span class="si">%s</span><span class="s">, </span><span class="si">%s</span><span class="s">, </span><span class="si">%s</span><span class="s">, </span><span class="si">%s</span><span class="s">;"</span><span class="p">,</span> <span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="il">10L</span><span class="p">,</span> <span class="mf">10.0</span><span class="p">,</span> <span class="n">Decimal</span><span class="p">(</span><span class="s">"10.00"</span><span class="p">)))</span>
<span class="gp">>>> </span><span class="s">'SELECT 10, 10, 10.0, 10.00;'</span>
</pre></div>
</div>
</li>
</ul>
<span class="target" id="adapt-string"></span><ul class="simple" id="index-6">
<li>String types: <a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/functions.html#str" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">str</span></tt></a>, <a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/functions.html#unicode" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">unicode</span></tt></a> are converted in SQL string syntax.
<tt class="xref py py-obj docutils literal"><span class="pre">unicode</span></tt> objects (<tt class="xref py py-obj docutils literal"><span class="pre">str</span></tt> in Python 3) are encoded in the connection
<a class="reference internal" href="connection.html#connection.encoding" title="connection.encoding"><tt class="xref py py-obj docutils literal"><span class="pre">encoding</span></tt></a> to be sent to the backend: trying to send a character
not supported by the encoding will result in an error. Received data can be
converted either as <tt class="xref py py-obj docutils literal"><span class="pre">str</span></tt> or <tt class="xref py py-obj docutils literal"><span class="pre">unicode</span></tt>: see <a class="reference internal" href="#unicode-handling"><em>Unicode handling</em></a>.</li>
</ul>
<span class="target" id="adapt-binary"></span><ul id="index-7">
<li><p class="first">Binary types: Python types representing binary objects are converted into
PostgreSQL binary string syntax, suitable for <tt class="sql docutils literal"><span class="pre">bytea</span></tt> fields. Such
types are <a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/functions.html#buffer" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">buffer</span></tt></a> (only available in Python 2), <a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/stdtypes.html#memoryview" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">memoryview</span></tt></a> (available
from Python 2.7), <a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/functions.html#bytearray" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">bytearray</span></tt></a> (available from Python 2.6) and <tt class="xref py py-obj docutils literal"><span class="pre">bytes</span></tt>
(only from Python 3: the name is available from Python 2.6 but it’s only an
alias for the type <tt class="xref py py-obj docutils literal"><span class="pre">str</span></tt>). Any object implementing the <a class="reference external" href="http://www.python.org/dev/peps/pep-3118/">Revised Buffer
Protocol</a> should be usable as binary type where the protocol is supported
(i.e. from Python 2.6). Received data is returned as <tt class="xref py py-obj docutils literal"><span class="pre">buffer</span></tt> (in Python 2)
or <tt class="xref py py-obj docutils literal"><span class="pre">memoryview</span></tt> (in Python 3).</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 2.4: </span>only strings were supported before.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 2.4.1: </span>can parse the ‘hex’ format from 9.0 servers without relying on the
version of the client library.</p>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p>In Python 2, if you have binary data in a <tt class="xref py py-obj docutils literal"><span class="pre">str</span></tt> object, you can pass them
to a <tt class="sql docutils literal"><span class="pre">bytea</span></tt> field using the <a class="reference internal" href="module.html#psycopg2.Binary" title="psycopg2.Binary"><tt class="xref py py-obj docutils literal"><span class="pre">psycopg2.Binary</span></tt></a> wrapper:</p>
<div class="last highlight-python"><div class="highlight"><pre><span class="n">mypic</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="s">'picture.png'</span><span class="p">,</span> <span class="s">'rb'</span><span class="p">)</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="n">curs</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"insert into blobs (file) values (</span><span class="si">%s</span><span class="s">)"</span><span class="p">,</span>
<span class="p">(</span><span class="n">psycopg2</span><span class="o">.</span><span class="n">Binary</span><span class="p">(</span><span class="n">mypic</span><span class="p">),))</span>
</pre></div>
</div>
</div>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">Since version 9.0 PostgreSQL uses by default <a class="reference external" href="http://www.postgresql.org/docs/current/static/datatype-binary.html">a new “hex” format</a> to
emit <tt class="sql docutils literal"><span class="pre">bytea</span></tt> fields. Starting from Psycopg 2.4.1 the format is
correctly supported. If you use a previous version you will need some
extra care when receiving bytea from PostgreSQL: you must have at least
libpq 9.0 installed on the client or alternatively you can set the
<a class="reference external" href="http://www.postgresql.org/docs/current/static/runtime-config-client.html#GUC-BYTEA-OUTPUT">bytea_output</a> configuration parameter to <tt class="docutils literal"><span class="pre">escape</span></tt>, either in the
server configuration file or in the client session (using a query such as
<tt class="docutils literal"><span class="pre">SET</span> <span class="pre">bytea_output</span> <span class="pre">TO</span> <span class="pre">escape;</span></tt>) before receiving binary data.</p>
</div>
</li>
</ul>
<span class="target" id="adapt-date"></span><ul id="index-8">
<li><p class="first">Date and time objects: builtin <a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/datetime.html#datetime.datetime" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">datetime</span></tt></a>, <a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/datetime.html#datetime.date" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">date</span></tt></a>,
<a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/datetime.html#datetime.time" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">time</span></tt></a>, <a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/datetime.html#datetime.timedelta" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">timedelta</span></tt></a> are converted into PostgreSQL’s
<tt class="sql docutils literal"><span class="pre">timestamp</span></tt>, <tt class="sql docutils literal"><span class="pre">date</span></tt>, <tt class="sql docutils literal"><span class="pre">time</span></tt>, <tt class="sql docutils literal"><span class="pre">interval</span></tt> data types.
Time zones are supported too. The Egenix <a class="reference external" href="http://www.egenix.com/products/python/mxBase/mxDateTime/">mx.DateTime</a> objects are adapted
the same way:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">dt</span> <span class="o">=</span> <span class="n">datetime</span><span class="o">.</span><span class="n">datetime</span><span class="o">.</span><span class="n">now</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">dt</span>
<span class="go">datetime.datetime(2010, 2, 8, 1, 40, 27, 425337)</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">mogrify</span><span class="p">(</span><span class="s">"SELECT </span><span class="si">%s</span><span class="s">, </span><span class="si">%s</span><span class="s">, </span><span class="si">%s</span><span class="s">;"</span><span class="p">,</span> <span class="p">(</span><span class="n">dt</span><span class="p">,</span> <span class="n">dt</span><span class="o">.</span><span class="n">date</span><span class="p">(),</span> <span class="n">dt</span><span class="o">.</span><span class="n">time</span><span class="p">()))</span>
<span class="go">"SELECT '2010-02-08T01:40:27.425337', '2010-02-08', '01:40:27.425337';"</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">mogrify</span><span class="p">(</span><span class="s">"SELECT </span><span class="si">%s</span><span class="s">;"</span><span class="p">,</span> <span class="p">(</span><span class="n">dt</span> <span class="o">-</span> <span class="n">datetime</span><span class="o">.</span><span class="n">datetime</span><span class="p">(</span><span class="mi">2010</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">1</span><span class="p">),))</span>
<span class="go">"SELECT '38 days 6027.425337 seconds';"</span>
</pre></div>
</div>
</li>
</ul>
<span class="target" id="adapt-list"></span><ul id="index-9">
<li><p class="first">Python lists are converted into PostgreSQL <tt class="sql docutils literal"><span class="pre">ARRAY</span></tt>s:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">mogrify</span><span class="p">(</span><span class="s">"SELECT </span><span class="si">%s</span><span class="s">;"</span><span class="p">,</span> <span class="p">([</span><span class="mi">10</span><span class="p">,</span> <span class="mi">20</span><span class="p">,</span> <span class="mi">30</span><span class="p">],</span> <span class="p">))</span>
<span class="go">'SELECT ARRAY[10, 20, 30];'</span>
</pre></div>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Reading back from PostgreSQL, arrays are converted to list of Python
objects as expected, but only if the types are known one. Arrays of
unknown types are returned as represented by the database (e.g.
<tt class="docutils literal"><span class="pre">{a,b,c}</span></tt>). You can easily create a typecaster for <a class="reference internal" href="extensions.html#cast-array-unknown"><em>array of
unknown types</em></a>.</p>
</div>
</li>
</ul>
<span class="target" id="adapt-tuple"></span><ul id="index-10">
<li><p class="first">Python tuples are converted in a syntax suitable for the SQL <tt class="sql docutils literal"><span class="pre">IN</span></tt>
operator and to represent a composite type:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">mogrify</span><span class="p">(</span><span class="s">"SELECT </span><span class="si">%s</span><span class="s"> IN </span><span class="si">%s</span><span class="s">;"</span><span class="p">,</span> <span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="mi">20</span><span class="p">,</span> <span class="mi">30</span><span class="p">)))</span>
<span class="go">'SELECT 10 IN (10, 20, 30);'</span>
</pre></div>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">SQL doesn’t allow an empty list in the IN operator, so your code should
guard against empty tuples.</p>
</div>
<p>If you want PostgreSQL composite types to be converted into a Python
tuple/namedtuple you can use the <a class="reference internal" href="extras.html#psycopg2.extras.register_composite" title="psycopg2.extras.register_composite"><tt class="xref py py-obj docutils literal"><span class="pre">register_composite()</span></tt></a>
function.</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 2.0.6: </span>the tuple <tt class="sql docutils literal"><span class="pre">IN</span></tt> adaptation.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 2.0.14: </span>the tuple <tt class="sql docutils literal"><span class="pre">IN</span></tt> adapter is always active. In previous releases it
was necessary to import the <a class="reference internal" href="extensions.html#module-psycopg2.extensions" title="psycopg2.extensions"><tt class="xref py py-obj docutils literal"><span class="pre">extensions</span></tt></a> module to have it
registered.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 2.3: </span><a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/collections.html#collections.namedtuple" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">namedtuple</span></tt></a> instances are adapted like regular tuples and
can thus be used to represent composite types.</p>
</div>
</li>
</ul>
<span class="target" id="adapt-dict"></span><ul id="index-11">
<li><p class="first">Python dictionaries are converted into the <a class="reference external" href="http://www.postgresql.org/docs/current/static/hstore.html"><tt class="sql docutils literal"><span class="pre">hstore</span></tt></a> data type. By default
the adapter is not enabled: see <a class="reference internal" href="extras.html#psycopg2.extras.register_hstore" title="psycopg2.extras.register_hstore"><tt class="xref py py-obj docutils literal"><span class="pre">register_hstore()</span></tt></a> for
further details.</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 2.3: </span>the <tt class="sql docutils literal"><span class="pre">hstore</span></tt> adaptation.</p>
</div>
</li>
</ul>
<div class="section" id="unicode-handling">
<span id="index-12"></span><span id="id9"></span><h3>Unicode handling<a class="headerlink" href="#unicode-handling" title="Permalink to this headline">¶</a></h3>
<p>Psycopg can exchange Unicode data with a PostgreSQL database. Python
<tt class="xref py py-obj docutils literal"><span class="pre">unicode</span></tt> objects are automatically <em>encoded</em> in the client encoding
defined on the database connection (the <a class="reference external" href="http://www.postgresql.org/docs/current/static/multibyte.html">PostgreSQL encoding</a>, available in
<a class="reference internal" href="connection.html#connection.encoding" title="connection.encoding"><tt class="xref py py-obj docutils literal"><span class="pre">connection.encoding</span></tt></a>, is translated into a <a class="reference external" href="http://docs.python.org/library/codecs.html#standard-encodings">Python codec</a> using the
<a class="reference internal" href="extensions.html#psycopg2.extensions.encodings" title="psycopg2.extensions.encodings"><tt class="xref py py-obj docutils literal"><span class="pre">encodings</span></tt></a> mapping):</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">print</span> <span class="n">u</span><span class="p">,</span> <span class="nb">type</span><span class="p">(</span><span class="n">u</span><span class="p">)</span>
<span class="go">àèìòù€ <type 'unicode'></span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"INSERT INTO test (num, data) VALUES (</span><span class="si">%s</span><span class="s">,</span><span class="si">%s</span><span class="s">);"</span><span class="p">,</span> <span class="p">(</span><span class="mi">74</span><span class="p">,</span> <span class="n">u</span><span class="p">))</span>
</pre></div>
</div>
<p>When reading data from the database, in Python 2 the strings returned are
usually 8 bit <tt class="xref py py-obj docutils literal"><span class="pre">str</span></tt> objects encoded in the database client encoding:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">print</span> <span class="n">conn</span><span class="o">.</span><span class="n">encoding</span>
<span class="go">UTF8</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"SELECT data FROM test WHERE num = 74"</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="n">cur</span><span class="o">.</span><span class="n">fetchone</span><span class="p">()[</span><span class="mi">0</span><span class="p">]</span>
<span class="gp">>>> </span><span class="k">print</span> <span class="n">x</span><span class="p">,</span> <span class="nb">type</span><span class="p">(</span><span class="n">x</span><span class="p">),</span> <span class="nb">repr</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="go">àèìòù€ <type 'str'> '\xc3\xa0\xc3\xa8\xc3\xac\xc3\xb2\xc3\xb9\xe2\x82\xac'</span>
<span class="gp">>>> </span><span class="n">conn</span><span class="o">.</span><span class="n">set_client_encoding</span><span class="p">(</span><span class="s">'LATIN9'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"SELECT data FROM test WHERE num = 74"</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="n">cur</span><span class="o">.</span><span class="n">fetchone</span><span class="p">()[</span><span class="mi">0</span><span class="p">]</span>
<span class="gp">>>> </span><span class="k">print</span> <span class="nb">type</span><span class="p">(</span><span class="n">x</span><span class="p">),</span> <span class="nb">repr</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="go"><type 'str'> '\xe0\xe8\xec\xf2\xf9\xa4'</span>
</pre></div>
</div>
<p>In Python 3 instead the strings are automatically <em>decoded</em> in the connection
<a class="reference internal" href="connection.html#connection.encoding" title="connection.encoding"><tt class="xref py py-obj docutils literal"><span class="pre">encoding</span></tt></a>, as the <tt class="xref py py-obj docutils literal"><span class="pre">str</span></tt> object can represent Unicode characters.
In Python 2 you must register a <a class="reference internal" href="advanced.html#type-casting-from-sql-to-python"><em>typecaster</em></a> in order to receive <tt class="xref py py-obj docutils literal"><span class="pre">unicode</span></tt> objects:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">psycopg2</span><span class="o">.</span><span class="n">extensions</span><span class="o">.</span><span class="n">register_type</span><span class="p">(</span><span class="n">psycopg2</span><span class="o">.</span><span class="n">extensions</span><span class="o">.</span><span class="n">UNICODE</span><span class="p">,</span> <span class="n">cur</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"SELECT data FROM test WHERE num = 74"</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="n">cur</span><span class="o">.</span><span class="n">fetchone</span><span class="p">()[</span><span class="mi">0</span><span class="p">]</span>
<span class="gp">>>> </span><span class="k">print</span> <span class="n">x</span><span class="p">,</span> <span class="nb">type</span><span class="p">(</span><span class="n">x</span><span class="p">),</span> <span class="nb">repr</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="go">àèìòù€ <type 'unicode'> u'\xe0\xe8\xec\xf2\xf9\u20ac'</span>
</pre></div>
</div>
<p>In the above example, the <a class="reference internal" href="extensions.html#psycopg2.extensions.UNICODE" title="psycopg2.extensions.UNICODE"><tt class="xref py py-obj docutils literal"><span class="pre">UNICODE</span></tt></a> typecaster is
registered only on the cursor. It is also possible to register typecasters on
the connection or globally: see the function
<a class="reference internal" href="extensions.html#psycopg2.extensions.register_type" title="psycopg2.extensions.register_type"><tt class="xref py py-obj docutils literal"><span class="pre">register_type()</span></tt></a> and
<a class="reference internal" href="advanced.html#type-casting-from-sql-to-python"><em>Type casting of SQL types into Python objects</em></a> for details.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p>In Python 2, if you want to uniformly receive all your database input in
Unicode, you can register the related typecasters globally as soon as
Psycopg is imported:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">psycopg2</span>
<span class="kn">import</span> <span class="nn">psycopg2.extensions</span>
<span class="n">psycopg2</span><span class="o">.</span><span class="n">extensions</span><span class="o">.</span><span class="n">register_type</span><span class="p">(</span><span class="n">psycopg2</span><span class="o">.</span><span class="n">extensions</span><span class="o">.</span><span class="n">UNICODE</span><span class="p">)</span>
<span class="n">psycopg2</span><span class="o">.</span><span class="n">extensions</span><span class="o">.</span><span class="n">register_type</span><span class="p">(</span><span class="n">psycopg2</span><span class="o">.</span><span class="n">extensions</span><span class="o">.</span><span class="n">UNICODEARRAY</span><span class="p">)</span>
</pre></div>
</div>
<p class="last">and then forget about this story.</p>
</div>
</div>
<div class="section" id="time-zones-handling">
<span id="tz-handling"></span><span id="index-13"></span><h3>Time zones handling<a class="headerlink" href="#time-zones-handling" title="Permalink to this headline">¶</a></h3>
<p>The PostgreSQL type <tt class="sql docutils literal"><span class="pre">timestamp</span> <span class="pre">with</span> <span class="pre">time</span> <span class="pre">zone</span></tt> is converted into Python
<a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/datetime.html#datetime.datetime" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">datetime</span></tt></a> objects with a <a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/datetime.html#datetime.datetime.tzinfo" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">tzinfo</span></tt></a> attribute set
to a <a class="reference internal" href="tz.html#psycopg2.tz.FixedOffsetTimezone" title="psycopg2.tz.FixedOffsetTimezone"><tt class="xref py py-obj docutils literal"><span class="pre">FixedOffsetTimezone</span></tt></a> instance.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"SET TIME ZONE 'Europe/Rome';"</span><span class="p">)</span> <span class="c"># UTC + 1 hour</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"SELECT '2010-01-01 10:30:45'::timestamptz;"</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">fetchone</span><span class="p">()[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">tzinfo</span>
<span class="go">psycopg2.tz.FixedOffsetTimezone(offset=60, name=None)</span>
</pre></div>
</div>
<p>Note that only time zones with an integer number of minutes are supported:
this is a limitation of the Python <a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/datetime.html#module-datetime" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">datetime</span></tt></a> module. A few historical time
zones had seconds in the UTC offset: these time zones will have the offset
rounded to the nearest minute, with an error of up to 30 seconds.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"SET TIME ZONE 'Asia/Calcutta';"</span><span class="p">)</span> <span class="c"># offset was +5:53:20</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"SELECT '1930-01-01 10:30:45'::timestamptz;"</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">cur</span><span class="o">.</span><span class="n">fetchone</span><span class="p">()[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">tzinfo</span>
<span class="go">psycopg2.tz.FixedOffsetTimezone(offset=353, name=None)</span>
</pre></div>
</div>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 2.2.2: </span>timezones with seconds are supported (with rounding). Previously such
timezones raised an error. In order to deal with them in previous
versions use <a class="reference internal" href="extras.html#psycopg2.extras.register_tstz_w_secs" title="psycopg2.extras.register_tstz_w_secs"><tt class="xref py py-obj docutils literal"><span class="pre">psycopg2.extras.register_tstz_w_secs()</span></tt></a>.</p>
</div>
</div>
</div>
<div class="section" id="transactions-control">
<span id="index-14"></span><span id="id12"></span><h2>Transactions control<a class="headerlink" href="#transactions-control" title="Permalink to this headline">¶</a></h2>
<p>In Psycopg transactions are handled by the <a class="reference internal" href="connection.html#connection" title="connection"><tt class="xref py py-obj docutils literal"><span class="pre">connection</span></tt></a> class. By
default, the first time a command is sent to the database (using one of the
<a class="reference internal" href="cursor.html#cursor" title="cursor"><tt class="xref py py-obj docutils literal"><span class="pre">cursor</span></tt></a>s created by the connection), a new transaction is created.
The following database commands will be executed in the context of the same
transaction – not only the commands issued by the first cursor, but the ones
issued by all the cursors created by the same connection. Should any command
fail, the transaction will be aborted and no further command will be executed
until a call to the <a class="reference internal" href="connection.html#connection.rollback" title="connection.rollback"><tt class="xref py py-obj docutils literal"><span class="pre">rollback()</span></tt></a> method.</p>
<p>The connection is responsible to terminate its transaction, calling either the
<a class="reference internal" href="connection.html#connection.commit" title="connection.commit"><tt class="xref py py-obj docutils literal"><span class="pre">commit()</span></tt></a> or <a class="reference internal" href="connection.html#connection.rollback" title="connection.rollback"><tt class="xref py py-obj docutils literal"><span class="pre">rollback()</span></tt></a> method. Committed
changes are immediately made persistent into the database. Closing the
connection using the <a class="reference internal" href="connection.html#connection.close" title="connection.close"><tt class="xref py py-obj docutils literal"><span class="pre">close()</span></tt></a> method or destroying the
connection object (using <tt class="xref py py-obj docutils literal"><span class="pre">del</span></tt> or letting it fall out of scope)
will result in an implicit <tt class="xref py py-obj docutils literal"><span class="pre">rollback()</span></tt> call.</p>
<p>It is possible to set the connection in <em>autocommit</em> mode: this way all the
commands executed will be immediately committed and no rollback is possible. A
few commands (e.g. <tt class="sql docutils literal"><span class="pre">CREATE</span> <span class="pre">DATABASE</span></tt>, <tt class="sql docutils literal"><span class="pre">VACUUM</span></tt>...) require to be run
outside any transaction: in order to be able to run these commands from
Psycopg, the session must be in autocommit mode: you can use the
<a class="reference internal" href="connection.html#connection.autocommit" title="connection.autocommit"><tt class="xref py py-obj docutils literal"><span class="pre">autocommit</span></tt></a> property (<a class="reference internal" href="connection.html#connection.set_isolation_level" title="connection.set_isolation_level"><tt class="xref py py-obj docutils literal"><span class="pre">set_isolation_level()</span></tt></a> in
older versions).</p>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">By default even a simple <tt class="sql docutils literal"><span class="pre">SELECT</span></tt> will start a transaction: in
long-running programs, if no further action is taken, the session will
remain “idle in transaction”, a condition non desiderable for several
reasons (locks are held by the session, tables bloat...). For long lived
scripts, either make sure to terminate a transaction as soon as possible or
use an autocommit connection.</p>
</div>
<p>A few other transaction properties can be set session-wide by the
<tt class="xref py py-obj docutils literal"><span class="pre">connection</span></tt>: for instance it is possible to have read-only transactions or
change the isolation level. See the <a class="reference internal" href="connection.html#connection.set_session" title="connection.set_session"><tt class="xref py py-obj docutils literal"><span class="pre">set_session()</span></tt></a> method for all
the details.</p>
</div>
<div class="section" id="server-side-cursors">
<span id="index-15"></span><span id="id13"></span><h2>Server side cursors<a class="headerlink" href="#server-side-cursors" title="Permalink to this headline">¶</a></h2>
<p>When a database query is executed, the Psycopg <a class="reference internal" href="cursor.html#cursor" title="cursor"><tt class="xref py py-obj docutils literal"><span class="pre">cursor</span></tt></a> usually fetches
all the records returned by the backend, transferring them to the client
process. If the query returned an huge amount of data, a proportionally large
amount of memory will be allocated by the client.</p>
<p>If the dataset is too large to be practically handled on the client side, it is
possible to create a <em>server side</em> cursor. Using this kind of cursor it is
possible to transfer to the client only a controlled amount of data, so that a
large dataset can be examined without keeping it entirely in memory.</p>
<p>Server side cursor are created in PostgreSQL using the <a class="reference external" href="http://www.postgresql.org/docs/current/static/sql-declare.html"><tt class="sql docutils literal"><span class="pre">DECLARE</span></tt></a> command and
subsequently handled using <tt class="sql docutils literal"><span class="pre">MOVE</span></tt>, <tt class="sql docutils literal"><span class="pre">FETCH</span></tt> and <tt class="sql docutils literal"><span class="pre">CLOSE</span></tt> commands.</p>
<p>Psycopg wraps the database server side cursor in <em>named cursors</em>. A named
cursor is created using the <a class="reference internal" href="connection.html#connection.cursor" title="connection.cursor"><tt class="xref py py-obj docutils literal"><span class="pre">cursor()</span></tt></a> method specifying the
<em>name</em> parameter. Such cursor will behave mostly like a regular cursor,
allowing the user to move in the dataset using the <a class="reference internal" href="cursor.html#cursor.scroll" title="cursor.scroll"><tt class="xref py py-obj docutils literal"><span class="pre">scroll()</span></tt></a>
method and to read the data using <a class="reference internal" href="cursor.html#cursor.fetchone" title="cursor.fetchone"><tt class="xref py py-obj docutils literal"><span class="pre">fetchone()</span></tt></a> and
<a class="reference internal" href="cursor.html#cursor.fetchmany" title="cursor.fetchmany"><tt class="xref py py-obj docutils literal"><span class="pre">fetchmany()</span></tt></a> methods.</p>
<p>Named cursors are also <a class="reference internal" href="cursor.html#cursor-iterable"><em>iterable</em></a> like regular cursors.
Note however that before Psycopg 2.4 iteration was performed fetching one
record at time from the backend, resulting in a large overhead. The attribute
<a class="reference internal" href="cursor.html#cursor.itersize" title="cursor.itersize"><tt class="xref py py-obj docutils literal"><span class="pre">itersize</span></tt></a> now controls how many records are fetched at time
during the iteration: the default value of 2000 allows to fetch about 100KB
per roundtrip assuming records of 10-20 columns of mixed number and strings;
you may decrease this value if you are dealing with huge records.</p>
<p>Named cursors are usually created <tt class="sql docutils literal"><span class="pre">WITHOUT</span> <span class="pre">HOLD</span></tt>, meaning they live only
as long as the current transaction. Trying to fetch from a named cursor after
a <a class="reference internal" href="connection.html#connection.commit" title="connection.commit"><tt class="xref py py-obj docutils literal"><span class="pre">commit()</span></tt></a> or to create a named cursor when the <a class="reference internal" href="connection.html#connection" title="connection"><tt class="xref py py-obj docutils literal"><span class="pre">connection</span></tt></a>
transaction isolation level is set to <tt class="xref py py-obj docutils literal"><span class="pre">AUTOCOMMIT</span></tt> will result in an exception.
It is possible to create a <tt class="sql docutils literal"><span class="pre">WITH</span> <span class="pre">HOLD</span></tt> cursor by specifying a <tt class="xref py py-obj docutils literal"><span class="pre">True</span></tt>
value for the <tt class="xref py py-obj docutils literal"><span class="pre">withhold</span></tt> parameter to <a class="reference internal" href="connection.html#connection.cursor" title="connection.cursor"><tt class="xref py py-obj docutils literal"><span class="pre">cursor()</span></tt></a> or by setting the
<a class="reference internal" href="cursor.html#cursor.withhold" title="cursor.withhold"><tt class="xref py py-obj docutils literal"><span class="pre">withhold</span></tt></a> attribute to <tt class="xref py py-obj docutils literal"><span class="pre">True</span></tt> before calling <a class="reference internal" href="cursor.html#cursor.execute" title="cursor.execute"><tt class="xref py py-obj docutils literal"><span class="pre">execute()</span></tt></a> on
the cursor. It is extremely important to always <a class="reference internal" href="cursor.html#cursor.close" title="cursor.close"><tt class="xref py py-obj docutils literal"><span class="pre">close()</span></tt></a> such cursors,
otherwise they will continue to hold server-side resources until the connection
will be eventually closed. Also note that while <tt class="sql docutils literal"><span class="pre">WITH</span> <span class="pre">HOLD</span></tt> cursors
lifetime extends well after <a class="reference internal" href="connection.html#connection.commit" title="connection.commit"><tt class="xref py py-obj docutils literal"><span class="pre">commit()</span></tt></a>, calling
<a class="reference internal" href="connection.html#connection.rollback" title="connection.rollback"><tt class="xref py py-obj docutils literal"><span class="pre">rollback()</span></tt></a> will automatically close the cursor.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p>It is also possible to use a named cursor to consume a cursor created
in some other way than using the <tt class="sql docutils literal"><span class="pre">DECLARE</span></tt> executed by
<a class="reference internal" href="cursor.html#cursor.execute" title="cursor.execute"><tt class="xref py py-obj docutils literal"><span class="pre">execute()</span></tt></a>. For example, you may have a PL/pgSQL function
returning a cursor:</p>
<div class="highlight-python"><div class="highlight"><pre>CREATE FUNCTION reffunc(refcursor) RETURNS refcursor AS $$
BEGIN
OPEN $1 FOR SELECT col FROM test;
RETURN $1;
END;
$$ LANGUAGE plpgsql;
</pre></div>
</div>
<p>You can read the cursor content by calling the function with a regular,
non-named, Psycopg cursor:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">cur1</span> <span class="o">=</span> <span class="n">conn</span><span class="o">.</span><span class="n">cursor</span><span class="p">()</span>
<span class="n">cur1</span><span class="o">.</span><span class="n">callproc</span><span class="p">(</span><span class="s">'reffunc'</span><span class="p">,</span> <span class="p">[</span><span class="s">'curname'</span><span class="p">])</span>
</pre></div>
</div>
<p>and then use a named cursor in the same transaction to “steal the cursor”:</p>
<div class="last highlight-python"><div class="highlight"><pre><span class="n">cur2</span> <span class="o">=</span> <span class="n">conn</span><span class="o">.</span><span class="n">cursor</span><span class="p">(</span><span class="s">'curname'</span><span class="p">)</span>
<span class="k">for</span> <span class="n">record</span> <span class="ow">in</span> <span class="n">cur2</span><span class="p">:</span> <span class="c"># or cur2.fetchone, fetchmany...</span>
<span class="c"># do something with record</span>
<span class="k">pass</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="thread-and-process-safety">
<span id="thread-safety"></span><span id="index-16"></span><h2>Thread and process safety<a class="headerlink" href="#thread-and-process-safety" title="Permalink to this headline">¶</a></h2>
<p>The Psycopg module and the <a class="reference internal" href="connection.html#connection" title="connection"><tt class="xref py py-obj docutils literal"><span class="pre">connection</span></tt></a> objects are <em>thread-safe</em>: many
threads can access the same database either using separate sessions and
creating a <tt class="xref py py-obj docutils literal"><span class="pre">connection</span></tt> per thread or using the same
connection and creating separate <a class="reference internal" href="cursor.html#cursor" title="cursor"><tt class="xref py py-obj docutils literal"><span class="pre">cursor</span></tt></a>s. In <a class="reference external" href="http://www.python.org/dev/peps/pep-0249/">DB API 2.0</a> parlance, Psycopg is
<em>level 2 thread safe</em>.</p>
<p>The difference between the above two approaches is that, using different
connections, the commands will be executed in different sessions and will be
served by different server processes. On the other hand, using many cursors on
the same connection, all the commands will be executed in the same session
(and in the same transaction if the connection is not in <a class="reference internal" href="#transactions-control"><em>autocommit</em></a> mode), but they will be serialized.</p>
<p>The above observations are only valid for regular threads: they don’t apply to
forked processes nor to green threads. <tt class="xref py py-obj docutils literal"><span class="pre">libpq</span></tt> connections <a class="reference external" href="http://www.postgresql.org/docs/current/static/libpq-connect.html#LIBPQ-CONNECT">shouldn’t be used by a
forked processes</a>, so when using a module such as <a class="reference external" href="/usr/share/doc/python2.7-doc/html/library/multiprocessing.html#module-multiprocessing" title="(in Python v2.7)"><tt class="xref py py-obj docutils literal"><span class="pre">multiprocessing</span></tt></a> or a
forking web deploy method such as FastCGI make sure to create the connections
<em>after</em> the fork.</p>
<p>Connections shouldn’t be shared either by different green threads: see
<a class="reference internal" href="advanced.html#green-support"><em>Support to coroutine libraries</em></a> for further details.</p>
</div>
<div class="section" id="using-copy-to-and-copy-from">
<span id="copy"></span><span id="index-17"></span><h2>Using COPY TO and COPY FROM<a class="headerlink" href="#using-copy-to-and-copy-from" title="Permalink to this headline">¶</a></h2>
<p>Psycopg <a class="reference internal" href="cursor.html#cursor" title="cursor"><tt class="xref py py-obj docutils literal"><span class="pre">cursor</span></tt></a> objects provide an interface to the efficient
PostgreSQL <a class="reference external" href="http://www.postgresql.org/docs/current/static/sql-copy.html"><tt class="sql docutils literal"><span class="pre">COPY</span></tt></a> command to move data from files to tables and back.
The methods exposed are:</p>
<dl class="docutils">
<dt><a class="reference internal" href="cursor.html#cursor.copy_from" title="cursor.copy_from"><tt class="xref py py-obj docutils literal"><span class="pre">copy_from()</span></tt></a></dt>
<dd>Reads data <em>from</em> a file-like object appending them to a database table
(<tt class="sql docutils literal"><span class="pre">COPY</span> <span class="pre">table</span> <span class="pre">FROM</span> <span class="pre">file</span></tt> syntax). The source file must have both
<tt class="xref py py-obj docutils literal"><span class="pre">read()</span></tt> and <tt class="xref py py-obj docutils literal"><span class="pre">readline()</span></tt> method.</dd>
<dt><a class="reference internal" href="cursor.html#cursor.copy_to" title="cursor.copy_to"><tt class="xref py py-obj docutils literal"><span class="pre">copy_to()</span></tt></a></dt>
<dd>Writes the content of a table <em>to</em> a file-like object (<tt class="sql docutils literal"><span class="pre">COPY</span> <span class="pre">table</span> <span class="pre">TO</span>
<span class="pre">file</span></tt> syntax). The target file must have a <tt class="xref py py-obj docutils literal"><span class="pre">write()</span></tt> method.</dd>
<dt><a class="reference internal" href="cursor.html#cursor.copy_expert" title="cursor.copy_expert"><tt class="xref py py-obj docutils literal"><span class="pre">copy_expert()</span></tt></a></dt>
<dd>Allows to handle more specific cases and to use all the <tt class="sql docutils literal"><span class="pre">COPY</span></tt>
features available in PostgreSQL.</dd>
</dl>
<p>Please refer to the documentation of the single methods for details and
examples.</p>
</div>
<div class="section" id="access-to-postgresql-large-objects">
<span id="large-objects"></span><span id="index-18"></span><h2>Access to PostgreSQL large objects<a class="headerlink" href="#access-to-postgresql-large-objects" title="Permalink to this headline">¶</a></h2>
<p>PostgreSQL offers support for <a class="reference external" href="http://www.postgresql.org/docs/current/static/largeobjects.html">large objects</a>, which provide stream-style
access to user data that is stored in a special large-object structure. They
are useful with data values too large to be manipulated conveniently as a
whole.</p>
<p>Psycopg allows access to the large object using the
<a class="reference internal" href="extensions.html#psycopg2.extensions.lobject" title="psycopg2.extensions.lobject"><tt class="xref py py-obj docutils literal"><span class="pre">lobject</span></tt></a> class. Objects are generated using the
<a class="reference internal" href="connection.html#connection.lobject" title="connection.lobject"><tt class="xref py py-obj docutils literal"><span class="pre">connection.lobject()</span></tt></a> factory method. Data can be retrieved either as bytes
or as Unicode strings.</p>
<p>Psycopg large object support efficient import/export with file system files
using the <a class="reference external" href="http://www.postgresql.org/docs/current/static/lo-interfaces.html#LO-IMPORT"><tt class="xref py py-obj docutils literal"><span class="pre">lo_import()</span></tt></a> and <a class="reference external" href="http://www.postgresql.org/docs/current/static/lo-interfaces.html#LO-EXPORT"><tt class="xref py py-obj docutils literal"><span class="pre">lo_export()</span></tt></a> libpq functions.</p>
</div>
<div class="section" id="two-phase-commit-protocol-support">
<span id="tpc"></span><span id="index-19"></span><h2>Two-Phase Commit protocol support<a class="headerlink" href="#two-phase-commit-protocol-support" title="Permalink to this headline">¶</a></h2>
<div class="versionadded">
<p><span class="versionmodified">New in version 2.3.</span></p>
</div>
<p>Psycopg exposes the two-phase commit features available since PostgreSQL 8.1
implementing the <em>two-phase commit extensions</em> proposed by the DB API 2.0.</p>
<p>The DB API 2.0 model of two-phase commit is inspired by the <a class="reference external" href="http://www.opengroup.org/bookstore/catalog/c193.htm">XA specification</a>,
according to which transaction IDs are formed from three components:</p>
<ul class="simple">
<li>a format ID (non-negative 32 bit integer)</li>
<li>a global transaction ID (string not longer than 64 bytes)</li>
<li>a branch qualifier (string not longer than 64 bytes)</li>
</ul>
<p>For a particular global transaction, the first two components will be the same
for all the resources. Every resource will be assigned a different branch
qualifier.</p>
<p>According to the DB API 2.0 specification, a transaction ID is created using the
<a class="reference internal" href="connection.html#connection.xid" title="connection.xid"><tt class="xref py py-obj docutils literal"><span class="pre">connection.xid()</span></tt></a> method. Once you have a transaction id, a distributed
transaction can be started with <a class="reference internal" href="connection.html#connection.tpc_begin" title="connection.tpc_begin"><tt class="xref py py-obj docutils literal"><span class="pre">connection.tpc_begin()</span></tt></a>, prepared using
<a class="reference internal" href="connection.html#connection.tpc_prepare" title="connection.tpc_prepare"><tt class="xref py py-obj docutils literal"><span class="pre">tpc_prepare()</span></tt></a> and completed using <a class="reference internal" href="connection.html#connection.tpc_commit" title="connection.tpc_commit"><tt class="xref py py-obj docutils literal"><span class="pre">tpc_commit()</span></tt></a> or
<a class="reference internal" href="connection.html#connection.tpc_rollback" title="connection.tpc_rollback"><tt class="xref py py-obj docutils literal"><span class="pre">tpc_rollback()</span></tt></a>. Transaction IDs can also be retrieved from the
database using <a class="reference internal" href="connection.html#connection.tpc_recover" title="connection.tpc_recover"><tt class="xref py py-obj docutils literal"><span class="pre">tpc_recover()</span></tt></a> and completed using the above
<tt class="xref py py-obj docutils literal"><span class="pre">tpc_commit()</span></tt> and <tt class="xref py py-obj docutils literal"><span class="pre">tpc_rollback()</span></tt>.</p>
<p>PostgreSQL doesn’t follow the XA standard though, and the ID for a PostgreSQL
prepared transaction can be any string up to 200 characters long.
Psycopg’s <a class="reference internal" href="extensions.html#psycopg2.extensions.Xid" title="psycopg2.extensions.Xid"><tt class="xref py py-obj docutils literal"><span class="pre">Xid</span></tt></a> objects can represent both XA-style
transactions IDs (such as the ones created by the <tt class="xref py py-obj docutils literal"><span class="pre">xid()</span></tt> method) and
PostgreSQL transaction IDs identified by an unparsed string.</p>
<p>The format in which the Xids are converted into strings passed to the
database is the same employed by the <a class="reference external" href="http://jdbc.postgresql.org/">PostgreSQL JDBC driver</a>: this should
allow interoperation between tools written in Python and in Java. For example
a recovery tool written in Python would be able to recognize the components of
transactions produced by a Java program.</p>
<p>For further details see the documentation for the above methods.</p>
</div>
</div>
</div>
</div>
</div>
<div class="sphinxsidebar">
<div class="sphinxsidebarwrapper">
<h3><a href="index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Basic module usage</a><ul>
<li><a class="reference internal" href="#passing-parameters-to-sql-queries">Passing parameters to SQL queries</a><ul>
<li><a class="reference internal" href="#the-problem-with-the-query-parameters">The problem with the query parameters</a></li>
</ul>
</li>
<li><a class="reference internal" href="#adaptation-of-python-values-to-sql-types">Adaptation of Python values to SQL types</a><ul>
<li><a class="reference internal" href="#unicode-handling">Unicode handling</a></li>
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<li><a class="reference internal" href="#using-copy-to-and-copy-from">Using COPY TO and COPY FROM</a></li>
<li><a class="reference internal" href="#access-to-postgresql-large-objects">Access to PostgreSQL large objects</a></li>
<li><a class="reference internal" href="#two-phase-commit-protocol-support">Two-Phase Commit protocol support</a></li>
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