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  <div class="section" id="blob-input-output">
<span id="blobio"></span><h1>Blob Input/Output<a class="headerlink" href="#blob-input-output" title="Permalink to this headline"></a></h1>
<p>A <a class="reference external" href="http://en.wikipedia.org/wiki/Binary_large_object">blob</a> is a
SQLite <a class="reference external" href="https://sqlite.org/datatype3.html">datatype</a> representing
a sequence of bytes.  It can be zero or more bytes in size.</p>
<p>SQLite blobs have an absolute maximum size of 2GB and a <a class="reference external" href="https://sqlite.org/c3ref/c_limit_attached.html">default
maximum size</a> of
1GB.</p>
<p>An alternate approach to using blobs is to store the data in files and
store the filename in the database.  Doing so loses the <a class="reference external" href="https://sqlite.org/transactional.html">ACID</a> properties of SQLite.</p>
<div class="section" id="zeroblob-class">
<h2>zeroblob class<a class="headerlink" href="#zeroblob-class" title="Permalink to this headline"></a></h2>
<dl class="class">
<dt id="apsw.zeroblob">
<em class="property">class </em><tt class="descname">zeroblob</tt><big>(</big><em>size</em><big>)</big><a class="headerlink" href="#apsw.zeroblob" title="Permalink to this definition"></a></dt>
<dd><p>If you want to insert a blob into a row, you previously needed to
supply the entire blob in one go.  To read just one byte also
required retrieving the blob in its entireity. For example to insert
a 100MB file you would have done:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">largedata</span><span class="o">=</span><span class="nb">open</span><span class="p">(</span><span class="s">&quot;largefile&quot;</span><span class="p">,</span> <span class="s">&quot;rb&quot;</span><span class="p">)</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="n">cur</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">&quot;insert into foo values(?)&quot;</span><span class="p">,</span> <span class="p">(</span><span class="nb">buffer</span><span class="p">(</span><span class="n">largedata</span><span class="p">),))</span>
</pre></div>
</div>
<p>SQLite 3.5 allowed for incremental Blob I/O so you can read and
write blobs in small amounts.  You cannot change the size of a blob
so you need to reserve space which you do through zeroblob which
creates a blob of the specified size but full of zero bytes.  For
example you would reserve space for your 100MB one of these two
ways:</p>
<div class="highlight-python"><pre>cur.execute("insert into foo values(zeroblob(100000000))")
cur.execute("insert into foo values(?),
             (apsw.zeroblob(100000000),))</pre>
</div>
<p>This class is used for the second way.  Once a blob exists in the
database, you then use the <a class="reference internal" href="#apsw.blob" title="apsw.blob"><tt class="xref py py-class docutils literal"><span class="pre">blob</span></tt></a> class to read and write its
contents.</p>
</dd></dl>

<dl class="method">
<dt id="apsw.zeroblob.length">
<tt class="descclassname">zeroblob.</tt><tt class="descname">length</tt><big>(</big><big>)</big> &rarr; int<a class="headerlink" href="#apsw.zeroblob.length" title="Permalink to this definition"></a></dt>
<dd><p>Size of zero blob in bytes.</p>
</dd></dl>

</div>
<div class="section" id="blob-class">
<h2>blob class<a class="headerlink" href="#blob-class" title="Permalink to this headline"></a></h2>
<dl class="class">
<dt id="apsw.blob">
<em class="property">class </em><tt class="descname">blob</tt><a class="headerlink" href="#apsw.blob" title="Permalink to this definition"></a></dt>
<dd><p>This object is created by <a class="reference internal" href="connection.html#apsw.Connection.blobopen" title="apsw.Connection.blobopen"><tt class="xref py py-meth docutils literal"><span class="pre">Connection.blobopen()</span></tt></a> and provides
access to a blob in the database.  It behaves like a Python file.
At the C level it wraps a <a class="reference external" href="https://sqlite.org/c3ref/blob.html">sqlite3_blob</a>.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">You cannot change the size of a blob using this object. You should
create it with the correct size in advance either by using
<a class="reference internal" href="#apsw.zeroblob" title="apsw.zeroblob"><tt class="xref py py-class docutils literal"><span class="pre">zeroblob</span></tt></a> or the <a class="reference external" href="https://sqlite.org/lang_corefunc.html">zeroblob()</a> function.</p>
</div>
<p>See the <a class="reference internal" href="example.html#example-blobio"><em>example</em></a>.</p>
</dd></dl>

<dl class="method">
<dt id="apsw.blob.__enter__">
<tt class="descclassname">blob.</tt><tt class="descname">__enter__</tt><big>(</big><big>)</big> &rarr; context<a class="headerlink" href="#apsw.blob.__enter__" title="Permalink to this definition"></a></dt>
<dd><p>You can use a blob as a <a class="reference external" href="http://docs.python.org/reference/datamodel.html#with-statement-context-managers">context manager</a>
as defined in <span class="target" id="index-0"></span><a class="pep reference external" href="http://www.python.org/dev/peps/pep-0343"><strong>PEP 0343</strong></a>.  When you use <em>with</em> statement,
the blob is always <tt class="xref py py-meth docutils literal"><span class="pre">closed</span></tt> on exit from the block, even if an
exception occurred in the block.</p>
<p>For example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">with</span> <span class="n">connection</span><span class="o">.</span><span class="n">blobopen</span><span class="p">()</span> <span class="k">as</span> <span class="n">blob</span><span class="p">:</span>
    <span class="n">blob</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s">&quot;...&quot;</span><span class="p">)</span>
    <span class="n">res</span><span class="o">=</span><span class="n">blob</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="mi">1024</span><span class="p">)</span>
</pre></div>
</div>
</dd></dl>

<dl class="method">
<dt id="apsw.blob.__exit__">
<tt class="descclassname">blob.</tt><tt class="descname">__exit__</tt><big>(</big><big>)</big> &rarr; False<a class="headerlink" href="#apsw.blob.__exit__" title="Permalink to this definition"></a></dt>
<dd><p>Implements context manager in conjunction with
<a class="reference internal" href="#apsw.blob.__enter__" title="apsw.blob.__enter__"><tt class="xref py py-meth docutils literal"><span class="pre">__enter__()</span></tt></a>.  Any exception that happened in the
<em>with</em> block is raised after closing the blob.</p>
</dd></dl>

<span class="target" id="index-1"></span><dl class="method">
<dt id="apsw.blob.close">
<tt class="descclassname">blob.</tt><tt class="descname">close</tt><big>(</big><span class="optional">[</span><em>force=False</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#apsw.blob.close" title="Permalink to this definition"></a></dt>
<dd><p>Closes the blob.  Note that even if an error occurs the blob is
still closed.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">In some cases errors that technically occurred in the
<a class="reference internal" href="#apsw.blob.read" title="apsw.blob.read"><tt class="xref py py-meth docutils literal"><span class="pre">read()</span></tt></a> and <a class="reference internal" href="#apsw.blob.write" title="apsw.blob.write"><tt class="xref py py-meth docutils literal"><span class="pre">write()</span></tt></a> routines may not be
reported until close is called.  Similarly errors that occurred
in those methods (eg calling <a class="reference internal" href="#apsw.blob.write" title="apsw.blob.write"><tt class="xref py py-meth docutils literal"><span class="pre">write()</span></tt></a> on a read-only
blob) may also be re-reported in <a class="reference internal" href="#apsw.blob.close" title="apsw.blob.close"><tt class="xref py py-meth docutils literal"><span class="pre">close()</span></tt></a>.  (This
behaviour is what the underlying SQLite APIs do - it is not APSW
doing it.)</p>
</div>
<p>It is okay to call <a class="reference internal" href="#apsw.blob.close" title="apsw.blob.close"><tt class="xref py py-meth docutils literal"><span class="pre">close()</span></tt></a> multiple times.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Parameters:</th><td class="field-body"><strong>force</strong> &#8211; Ignores any errors during close.</td>
</tr>
</tbody>
</table>
<p>Calls: <a class="reference external" href="https://sqlite.org/c3ref/blob_close.html">sqlite3_blob_close</a></p>
</dd></dl>

<span class="target" id="index-2"></span><dl class="method">
<dt id="apsw.blob.length">
<tt class="descclassname">blob.</tt><tt class="descname">length</tt><big>(</big><big>)</big> &rarr; int<a class="headerlink" href="#apsw.blob.length" title="Permalink to this definition"></a></dt>
<dd><p>Returns the size of the blob in bytes.</p>
<p>Calls: <a class="reference external" href="https://sqlite.org/c3ref/blob_bytes.html">sqlite3_blob_bytes</a></p>
</dd></dl>

<span class="target" id="index-3"></span><dl class="method">
<dt id="apsw.blob.read">
<tt class="descclassname">blob.</tt><tt class="descname">read</tt><big>(</big><span class="optional">[</span><em>nbytes</em><span class="optional">]</span><big>)</big> &rarr; bytes<a class="headerlink" href="#apsw.blob.read" title="Permalink to this definition"></a></dt>
<dd><p>Reads amount of data requested, or till end of file, whichever is
earlier. Attempting to read beyond the end of the blob returns the
empty string/bytes, in the same manner as end of file on normal file
objects.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Return type:</th><td class="field-body">(Python 2) string  (Python 3) bytes</td>
</tr>
</tbody>
</table>
<p>Calls: <a class="reference external" href="https://sqlite.org/c3ref/blob_read.html">sqlite3_blob_read</a></p>
</dd></dl>

<span class="target" id="index-4"></span><dl class="method">
<dt id="apsw.blob.readinto">
<tt class="descclassname">blob.</tt><tt class="descname">readinto</tt><big>(</big><em>buffer</em><span class="optional">[</span>, <em>offset=0</em>, <em>length=remaining-buffer</em><span class="optional">]</span><big>)</big> &rarr; None<a class="headerlink" href="#apsw.blob.readinto" title="Permalink to this definition"></a></dt>
<dd><p>Reads from the blob into a buffer you have supplied.  This method is
useful if you already have a buffer like object that data is being
assembled in, and avoids allocating results in <a class="reference internal" href="#apsw.blob.read" title="apsw.blob.read"><tt class="xref py py-meth docutils literal"><span class="pre">blob.read()</span></tt></a> and
then copying into buffer.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first last simple">
<li><strong>buffer</strong> &#8211; A writable buffer like object.  In Python 2.6 onwards
there is a bytearray type that is very useful.
<tt class="xref py py-class docutils literal"><span class="pre">array.array</span></tt> also works.</li>
<li><strong>offset</strong> &#8211; The position to start writing into the buffer
defaulting to the beginning.</li>
<li><strong>length</strong> &#8211; How much of the blob to read.  The default is the
remaining space left in the buffer.  Note that if
there is more space available than blob left then you
will get a <tt class="xref py py-exc docutils literal"><span class="pre">ValueError</span></tt> exception.</li>
</ul>
</td>
</tr>
</tbody>
</table>
<p>Calls: <a class="reference external" href="https://sqlite.org/c3ref/blob_read.html">sqlite3_blob_read</a></p>
</dd></dl>

<span class="target" id="index-5"></span><dl class="method">
<dt id="apsw.blob.reopen">
<tt class="descclassname">blob.</tt><tt class="descname">reopen</tt><big>(</big><em>rowid</em><big>)</big><a class="headerlink" href="#apsw.blob.reopen" title="Permalink to this definition"></a></dt>
<dd><p>Change this blob object to point to a different row.  It can be
faster than closing an existing blob an opening a new one.</p>
<p>Calls: <a class="reference external" href="https://sqlite.org/c3ref/blob_reopen.html">sqlite3_blob_reopen</a></p>
</dd></dl>

<dl class="method">
<dt id="apsw.blob.seek">
<tt class="descclassname">blob.</tt><tt class="descname">seek</tt><big>(</big><em>offset</em><span class="optional">[</span>, <em>whence=0</em><span class="optional">]</span><big>)</big> &rarr; None<a class="headerlink" href="#apsw.blob.seek" title="Permalink to this definition"></a></dt>
<dd><p>Changes current position to <em>offset</em> biased by <em>whence</em>.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first simple">
<li><strong>offset</strong> &#8211; New position to seek to.  Can be positive or negative number.</li>
<li><strong>whence</strong> &#8211; Use 0 if <em>offset</em> is relative to the begining of the blob,
1 if <em>offset</em> is relative to the current position,
and 2 if <em>offset</em> is relative to the end of the blob.</li>
</ul>
</td>
</tr>
<tr class="field-even field"><th class="field-name" colspan="2">Raises ValueError:</th></tr>
<tr class="field-even field"><td>&nbsp;</td><td class="field-body"><p class="first last">If the resulting offset is before the begining (less than zero) or beyond the end of the blob.</p>
</td>
</tr>
</tbody>
</table>
</dd></dl>

<dl class="method">
<dt id="apsw.blob.tell">
<tt class="descclassname">blob.</tt><tt class="descname">tell</tt><big>(</big><big>)</big> &rarr; int<a class="headerlink" href="#apsw.blob.tell" title="Permalink to this definition"></a></dt>
<dd><p>Returns the current offset.</p>
</dd></dl>

<span class="target" id="index-6"></span><dl class="method">
<dt id="apsw.blob.write">
<tt class="descclassname">blob.</tt><tt class="descname">write</tt><big>(</big><em>data</em><big>)</big> &rarr; None<a class="headerlink" href="#apsw.blob.write" title="Permalink to this definition"></a></dt>
<dd><p>Writes the data to the blob.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Parameters:</th><td class="field-body"><p class="first"><strong>data</strong> &#8211; (Python 2) buffer or string. (Python 3) buffer or bytes.</p>
</td>
</tr>
<tr class="field-even field"><th class="field-name">Raises:</th><td class="field-body"><ul class="first last simple">
<li><strong>TypeError</strong> &#8211; Wrong data type</li>
<li><strong>ValueError</strong> &#8211; If the data would go beyond the end of the blob.
You cannot increase the size of a blob by writing beyond the end.
You need to use <a class="reference internal" href="#apsw.zeroblob" title="apsw.zeroblob"><tt class="xref py py-class docutils literal"><span class="pre">zeroblob</span></tt></a> to set the desired size first when
inserting the blob.</li>
</ul>
</td>
</tr>
</tbody>
</table>
<p>Calls: <a class="reference external" href="https://sqlite.org/c3ref/blob_write.html">sqlite3_blob_write</a></p>
</dd></dl>

</div>
</div>


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  <ul>
<li><a class="reference internal" href="#">Blob Input/Output</a><ul>
<li><a class="reference internal" href="#zeroblob-class">zeroblob class</a></li>
<li><a class="reference internal" href="#blob-class">blob class</a></li>
</ul>
</li>
</ul>

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