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<div class="section" id="proposal-parametrize-with-fixtures">
<h1>PROPOSAL: Parametrize with fixtures<a class="headerlink" href="#proposal-parametrize-with-fixtures" title="Permalink to this headline">¶</a></h1>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">This document outlines a proposal around using fixtures as input
of parametrized tests or fixtures.</p>
</div>
<div class="section" id="problem">
<h2>Problem<a class="headerlink" href="#problem" title="Permalink to this headline">¶</a></h2>
<p>As a user I have functional tests that I would like to run against various
scenarios.</p>
<p>In this particular example we want to generate a new project based on a
cookiecutter template. We want to test default values but also data that
emulates user input.</p>
<ul class="simple">
<li>use default values</li>
<li>emulate user input<ul>
<li>specify 'author'</li>
<li>specify 'project_slug'</li>
<li>specify 'author' and 'project_slug'</li>
</ul>
</li>
</ul>
<p>This is how a functional test could look like:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">pytest</span>
<span class="nd">@pytest.fixture</span>
<span class="k">def</span> <span class="nf">default_context</span><span class="p">():</span>
<span class="k">return</span> <span class="p">{</span><span class="s1">'extra_context'</span><span class="p">:</span> <span class="p">{}}</span>
<span class="nd">@pytest.fixture</span><span class="p">(</span><span class="n">params</span><span class="o">=</span><span class="p">[</span>
<span class="p">{</span><span class="s1">'author'</span><span class="p">:</span> <span class="s1">'alice'</span><span class="p">},</span>
<span class="p">{</span><span class="s1">'project_slug'</span><span class="p">:</span> <span class="s1">'helloworld'</span><span class="p">},</span>
<span class="p">{</span><span class="s1">'author'</span><span class="p">:</span> <span class="s1">'bob'</span><span class="p">,</span> <span class="s1">'project_slug'</span><span class="p">:</span> <span class="s1">'foobar'</span><span class="p">},</span>
<span class="p">])</span>
<span class="k">def</span> <span class="nf">extra_context</span><span class="p">(</span><span class="n">request</span><span class="p">):</span>
<span class="k">return</span> <span class="p">{</span><span class="s1">'extra_context'</span><span class="p">:</span> <span class="n">request</span><span class="o">.</span><span class="n">param</span><span class="p">}</span>
<span class="nd">@pytest.fixture</span><span class="p">(</span><span class="n">params</span><span class="o">=</span><span class="p">[</span><span class="s1">'default'</span><span class="p">,</span> <span class="s1">'extra'</span><span class="p">])</span>
<span class="k">def</span> <span class="nf">context</span><span class="p">(</span><span class="n">request</span><span class="p">):</span>
<span class="k">if</span> <span class="n">request</span><span class="o">.</span><span class="n">param</span> <span class="o">==</span> <span class="s1">'default'</span><span class="p">:</span>
<span class="k">return</span> <span class="n">request</span><span class="o">.</span><span class="n">getfuncargvalue</span><span class="p">(</span><span class="s1">'default_context'</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">return</span> <span class="n">request</span><span class="o">.</span><span class="n">getfuncargvalue</span><span class="p">(</span><span class="s1">'extra_context'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">test_generate_project</span><span class="p">(</span><span class="n">cookies</span><span class="p">,</span> <span class="n">context</span><span class="p">):</span>
<span class="sd">"""Call the cookiecutter API to generate a new project from a</span>
<span class="sd"> template.</span>
<span class="sd"> """</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">cookies</span><span class="o">.</span><span class="n">bake</span><span class="p">(</span><span class="n">extra_context</span><span class="o">=</span><span class="n">context</span><span class="p">)</span>
<span class="k">assert</span> <span class="n">result</span><span class="o">.</span><span class="n">exit_code</span> <span class="o">==</span> <span class="mi">0</span>
<span class="k">assert</span> <span class="n">result</span><span class="o">.</span><span class="n">exception</span> <span class="ow">is</span> <span class="bp">None</span>
<span class="k">assert</span> <span class="n">result</span><span class="o">.</span><span class="n">project</span><span class="o">.</span><span class="n">isdir</span><span class="p">()</span>
</pre></div>
</div>
</div>
<div class="section" id="issues">
<h2>Issues<a class="headerlink" href="#issues" title="Permalink to this headline">¶</a></h2>
<ul class="simple">
<li>By using <code class="docutils literal"><span class="pre">request.getfuncargvalue()</span></code> we rely on actual fixture function
execution to know what fixtures are involved, due to it's dynamic nature</li>
<li>More importantly, <code class="docutils literal"><span class="pre">request.getfuncargvalue()</span></code> cannot be combined with
parametrized fixtures, such as <code class="docutils literal"><span class="pre">extra_context</span></code></li>
<li>This is very inconvenient if you wish to extend an existing test suite by
certain parameters for fixtures that are already used by tests</li>
</ul>
<p>pytest version 3.0 reports an error if you try to run above code:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">Failed</span><span class="p">:</span> <span class="n">The</span> <span class="n">requested</span> <span class="n">fixture</span> <span class="n">has</span> <span class="n">no</span> <span class="n">parameter</span> <span class="n">defined</span> <span class="k">for</span> <span class="n">the</span> <span class="n">current</span>
<span class="n">test</span><span class="o">.</span>
<span class="n">Requested</span> <span class="n">fixture</span> <span class="s1">'extra_context'</span>
</pre></div>
</div>
</div>
<div class="section" id="proposed-solution">
<h2>Proposed solution<a class="headerlink" href="#proposed-solution" title="Permalink to this headline">¶</a></h2>
<p>A new function that can be used in modules can be used to dynamically define
fixtures from existing ones.</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="n">pytest</span><span class="o">.</span><span class="n">define_combined_fixture</span><span class="p">(</span>
<span class="n">name</span><span class="o">=</span><span class="s1">'context'</span><span class="p">,</span>
<span class="n">fixtures</span><span class="o">=</span><span class="p">[</span><span class="s1">'default_context'</span><span class="p">,</span> <span class="s1">'extra_context'</span><span class="p">],</span>
<span class="p">)</span>
</pre></div>
</div>
<p>The new fixture <code class="docutils literal"><span class="pre">context</span></code> inherits the scope from the used fixtures and yield
the following values.</p>
<ul class="simple">
<li><code class="docutils literal"><span class="pre">{}</span></code></li>
<li><code class="docutils literal"><span class="pre">{'author':</span> <span class="pre">'alice'}</span></code></li>
<li><code class="docutils literal"><span class="pre">{'project_slug':</span> <span class="pre">'helloworld'}</span></code></li>
<li><code class="docutils literal"><span class="pre">{'author':</span> <span class="pre">'bob',</span> <span class="pre">'project_slug':</span> <span class="pre">'foobar'}</span></code></li>
</ul>
</div>
<div class="section" id="alternative-approach">
<h2>Alternative approach<a class="headerlink" href="#alternative-approach" title="Permalink to this headline">¶</a></h2>
<p>A new helper function named <code class="docutils literal"><span class="pre">fixture_request</span></code> would tell pytest to yield
all parameters marked as a fixture.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The <a class="reference external" href="https://pypi.python.org/pypi/pytest-lazy-fixture">pytest-lazy-fixture</a> plugin implements a very
similar solution to the proposal below, make sure to check it out.</p>
</div>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="nd">@pytest.fixture</span><span class="p">(</span><span class="n">params</span><span class="o">=</span><span class="p">[</span>
<span class="n">pytest</span><span class="o">.</span><span class="n">fixture_request</span><span class="p">(</span><span class="s1">'default_context'</span><span class="p">),</span>
<span class="n">pytest</span><span class="o">.</span><span class="n">fixture_request</span><span class="p">(</span><span class="s1">'extra_context'</span><span class="p">),</span>
<span class="p">])</span>
<span class="k">def</span> <span class="nf">context</span><span class="p">(</span><span class="n">request</span><span class="p">):</span>
<span class="sd">"""Returns all values for ``default_context``, one-by-one before it</span>
<span class="sd"> does the same for ``extra_context``.</span>
<span class="sd"> request.param:</span>
<span class="sd"> - {}</span>
<span class="sd"> - {'author': 'alice'}</span>
<span class="sd"> - {'project_slug': 'helloworld'}</span>
<span class="sd"> - {'author': 'bob', 'project_slug': 'foobar'}</span>
<span class="sd"> """</span>
<span class="k">return</span> <span class="n">request</span><span class="o">.</span><span class="n">param</span>
</pre></div>
</div>
<p>The same helper can be used in combination with <code class="docutils literal"><span class="pre">pytest.mark.parametrize</span></code>.</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="nd">@pytest.mark.parametrize</span><span class="p">(</span>
<span class="s1">'context, expected_response_code'</span><span class="p">,</span>
<span class="p">[</span>
<span class="p">(</span><span class="n">pytest</span><span class="o">.</span><span class="n">fixture_request</span><span class="p">(</span><span class="s1">'default_context'</span><span class="p">),</span> <span class="mi">0</span><span class="p">),</span>
<span class="p">(</span><span class="n">pytest</span><span class="o">.</span><span class="n">fixture_request</span><span class="p">(</span><span class="s1">'extra_context'</span><span class="p">),</span> <span class="mi">0</span><span class="p">),</span>
<span class="p">],</span>
<span class="p">)</span>
<span class="k">def</span> <span class="nf">test_generate_project</span><span class="p">(</span><span class="n">cookies</span><span class="p">,</span> <span class="n">context</span><span class="p">,</span> <span class="n">exit_code</span><span class="p">):</span>
<span class="sd">"""Call the cookiecutter API to generate a new project from a</span>
<span class="sd"> template.</span>
<span class="sd"> """</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">cookies</span><span class="o">.</span><span class="n">bake</span><span class="p">(</span><span class="n">extra_context</span><span class="o">=</span><span class="n">context</span><span class="p">)</span>
<span class="k">assert</span> <span class="n">result</span><span class="o">.</span><span class="n">exit_code</span> <span class="o">==</span> <span class="n">exit_code</span>
</pre></div>
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<li><a class="reference internal" href="#">PROPOSAL: Parametrize with fixtures</a><ul>
<li><a class="reference internal" href="#problem">Problem</a></li>
<li><a class="reference internal" href="#issues">Issues</a></li>
<li><a class="reference internal" href="#proposed-solution">Proposed solution</a></li>
<li><a class="reference internal" href="#alternative-approach">Alternative approach</a></li>
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