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<div class="section" id="pytest-fixtures-explicit-modular-scalable">
<span id="fixture-functions"></span><span id="fixtures"></span><span id="fixture"></span><h1>pytest fixtures: explicit, modular, scalable<a class="headerlink" href="#pytest-fixtures-explicit-modular-scalable" title="Permalink to this headline">¶</a></h1>
<div class="versionadded">
<p><span class="versionmodified">New in version 2.0/2.3/2.4.</span></p>
</div>
<p>The <a class="reference external" href="http://en.wikipedia.org/wiki/Test_fixture#Software">purpose of test fixtures</a> is to provide a fixed baseline
upon which tests can reliably and repeatedly execute. pytest fixtures
offer dramatic improvements over the classic xUnit style of setup/teardown
functions:</p>
<ul class="simple">
<li>fixtures have explicit names and are activated by declaring their use
from test functions, modules, classes or whole projects.</li>
<li>fixtures are implemented in a modular manner, as each fixture name
triggers a <em>fixture function</em> which can itself use other fixtures.</li>
<li>fixture management scales from simple unit to complex
functional testing, allowing to parametrize fixtures and tests according
to configuration and component options, or to re-use fixtures
across class, module or whole test session scopes.</li>
</ul>
<p>In addition, pytest continues to support <a class="reference internal" href="xunit_setup.html#xunitsetup"><em>classic xunit-style setup</em></a>. You can mix
both styles, moving incrementally from classic to new style, as you
prefer. You can also start out from existing <a class="reference internal" href="unittest.html#unittest-testcase"><em>unittest.TestCase
style</em></a> or <a class="reference internal" href="nose.html#nosestyle"><em>nose based</em></a> projects.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">pytest-2.4 introduced an additional experimental
<a class="reference internal" href="yieldfixture.html#yieldfixture"><em>yield fixture mechanism</em></a> for easier context manager
integration and more linear writing of teardown code.</p>
</div>
<div class="section" id="fixtures-as-function-arguments">
<span id="id1"></span><span id="pytest-fixture"></span><span id="fixture-function"></span><span id="funcarg-mechanism"></span><span id="funcargs"></span><h2>Fixtures as Function arguments<a class="headerlink" href="#fixtures-as-function-arguments" title="Permalink to this headline">¶</a></h2>
<p>Test functions can receive fixture objects by naming them as an input
argument. For each argument name, a fixture function with that name provides
the fixture object. Fixture functions are registered by marking them with
<a class="reference internal" href="builtin.html#_pytest.python.fixture" title="_pytest.python.fixture"><tt class="xref py py-func docutils literal"><span class="pre">@pytest.fixture</span></tt></a>. Let’s look at a simple
self-contained test module containing a fixture and a test function
using it:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># content of ./test_smtpsimple.py</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">smtp</span><span class="p">():</span>
<span class="kn">import</span> <span class="nn">smtplib</span>
<span class="k">return</span> <span class="n">smtplib</span><span class="o">.</span><span class="n">SMTP</span><span class="p">(</span><span class="s">"merlinux.eu"</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">test_ehlo</span><span class="p">(</span><span class="n">smtp</span><span class="p">):</span>
<span class="n">response</span><span class="p">,</span> <span class="n">msg</span> <span class="o">=</span> <span class="n">smtp</span><span class="o">.</span><span class="n">ehlo</span><span class="p">()</span>
<span class="k">assert</span> <span class="n">response</span> <span class="o">==</span> <span class="mi">250</span>
<span class="k">assert</span> <span class="s">"merlinux"</span> <span class="ow">in</span> <span class="n">msg</span>
<span class="k">assert</span> <span class="mi">0</span> <span class="c"># for demo purposes</span>
</pre></div>
</div>
<p>Here, the <tt class="docutils literal"><span class="pre">test_ehlo</span></tt> needs the <tt class="docutils literal"><span class="pre">smtp</span></tt> fixture value. pytest
will discover and call the <a class="reference internal" href="builtin.html#_pytest.python.fixture" title="_pytest.python.fixture"><tt class="xref py py-func docutils literal"><span class="pre">@pytest.fixture</span></tt></a>
marked <tt class="docutils literal"><span class="pre">smtp</span></tt> fixture function. Running the test looks like this:</p>
<div class="highlight-python"><div class="highlight"><pre>$ py.test test_smtpsimple.py
=========================== test session starts ============================
platform linux -- Python 3.4.0 -- py-1.4.25 -- pytest-2.6.3
collected 1 items
test_smtpsimple.py F
================================= FAILURES =================================
________________________________ test_ehlo _________________________________
smtp = <smtplib.SMTP object at 0x2b040fa62128>
def test_ehlo(smtp):
response, msg = smtp.ehlo()
assert response == 250
> assert "merlinux" in msg
E TypeError: Type str doesn't support the buffer API
test_smtpsimple.py:11: TypeError
========================= 1 failed in 0.20 seconds =========================
</pre></div>
</div>
<p>In the failure traceback we see that the test function was called with a
<tt class="docutils literal"><span class="pre">smtp</span></tt> argument, the <tt class="docutils literal"><span class="pre">smtplib.SMTP()</span></tt> instance created by the fixture
function. The test function fails on our deliberate <tt class="docutils literal"><span class="pre">assert</span> <span class="pre">0</span></tt>. Here is
the exact protocol used by <tt class="docutils literal"><span class="pre">pytest</span></tt> to call the test function this way:</p>
<ol class="arabic simple">
<li>pytest <a class="reference internal" href="goodpractises.html#test-discovery"><em>finds</em></a> the <tt class="docutils literal"><span class="pre">test_ehlo</span></tt> because
of the <tt class="docutils literal"><span class="pre">test_</span></tt> prefix. The test function needs a function argument
named <tt class="docutils literal"><span class="pre">smtp</span></tt>. A matching fixture function is discovered by
looking for a fixture-marked function named <tt class="docutils literal"><span class="pre">smtp</span></tt>.</li>
<li><tt class="docutils literal"><span class="pre">smtp()</span></tt> is called to create an instance.</li>
<li><tt class="docutils literal"><span class="pre">test_ehlo(<SMTP</span> <span class="pre">instance>)</span></tt> is called and fails in the last
line of the test function.</li>
</ol>
<p>Note that if you misspell a function argument or want
to use one that isn’t available, you’ll see an error
with a list of available function arguments.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p>You can always issue:</p>
<div class="highlight-python"><div class="highlight"><pre>py.test --fixtures test_simplefactory.py
</pre></div>
</div>
<p>to see available fixtures.</p>
<p class="last">In versions prior to 2.3 there was no <tt class="docutils literal"><span class="pre">@pytest.fixture</span></tt> marker
and you had to use a magic <tt class="docutils literal"><span class="pre">pytest_funcarg__NAME</span></tt> prefix
for the fixture factory. This remains and will remain supported
but is not anymore advertised as the primary means of declaring fixture
functions.</p>
</div>
</div>
<div class="section" id="funcargs-a-prime-example-of-dependency-injection">
<h2>“Funcargs” a prime example of dependency injection<a class="headerlink" href="#funcargs-a-prime-example-of-dependency-injection" title="Permalink to this headline">¶</a></h2>
<p>When injecting fixtures to test functions, pytest-2.0 introduced the
term “funcargs” or “funcarg mechanism” which continues to be present
also in docs today. It now refers to the specific case of injecting
fixture values as arguments to test functions. With pytest-2.3 there are
more possibilities to use fixtures but “funcargs” remain as the main way
as they allow to directly state the dependencies of a test function.</p>
<p>As the following examples show in more detail, funcargs allow test
functions to easily receive and work against specific pre-initialized
application objects without having to care about import/setup/cleanup
details. It’s a prime example of <a class="reference external" href="http://en.wikipedia.org/wiki/Dependency_injection#Definition">dependency injection</a> where fixture
functions take the role of the <em>injector</em> and test functions are the
<em>consumers</em> of fixture objects.</p>
</div>
<div class="section" id="sharing-a-fixture-across-tests-in-a-module-or-class-session">
<span id="smtpshared"></span><h2>Sharing a fixture across tests in a module (or class/session)<a class="headerlink" href="#sharing-a-fixture-across-tests-in-a-module-or-class-session" title="Permalink to this headline">¶</a></h2>
<p>Fixtures requiring network access depend on connectivity and are
usually time-expensive to create. Extending the previous example, we
can add a <tt class="docutils literal"><span class="pre">scope='module'</span></tt> parameter to the
<a class="reference internal" href="builtin.html#_pytest.python.fixture" title="_pytest.python.fixture"><tt class="xref py py-func docutils literal"><span class="pre">@pytest.fixture</span></tt></a> invocation
to cause the decorated <tt class="docutils literal"><span class="pre">smtp</span></tt> fixture function to only be invoked once
per test module. Multiple test functions in a test module will thus
each receive the same <tt class="docutils literal"><span class="pre">smtp</span></tt> fixture instance. The next example puts
the fixture function into a separate <tt class="docutils literal"><span class="pre">conftest.py</span></tt> file so
that tests from multiple test modules in the directory can
access the fixture function:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># content of conftest.py</span>
<span class="kn">import</span> <span class="nn">pytest</span>
<span class="kn">import</span> <span class="nn">smtplib</span>
<span class="nd">@pytest.fixture</span><span class="p">(</span><span class="n">scope</span><span class="o">=</span><span class="s">"module"</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">smtp</span><span class="p">():</span>
<span class="k">return</span> <span class="n">smtplib</span><span class="o">.</span><span class="n">SMTP</span><span class="p">(</span><span class="s">"merlinux.eu"</span><span class="p">)</span>
</pre></div>
</div>
<p>The name of the fixture again is <tt class="docutils literal"><span class="pre">smtp</span></tt> and you can access its result by
listing the name <tt class="docutils literal"><span class="pre">smtp</span></tt> as an input parameter in any test or fixture
function (in or below the directory where <tt class="docutils literal"><span class="pre">conftest.py</span></tt> is located):</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># content of test_module.py</span>
<span class="k">def</span> <span class="nf">test_ehlo</span><span class="p">(</span><span class="n">smtp</span><span class="p">):</span>
<span class="n">response</span> <span class="o">=</span> <span class="n">smtp</span><span class="o">.</span><span class="n">ehlo</span><span class="p">()</span>
<span class="k">assert</span> <span class="n">response</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">==</span> <span class="mi">250</span>
<span class="k">assert</span> <span class="s">"merlinux"</span> <span class="ow">in</span> <span class="n">response</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span>
<span class="k">assert</span> <span class="mi">0</span> <span class="c"># for demo purposes</span>
<span class="k">def</span> <span class="nf">test_noop</span><span class="p">(</span><span class="n">smtp</span><span class="p">):</span>
<span class="n">response</span> <span class="o">=</span> <span class="n">smtp</span><span class="o">.</span><span class="n">noop</span><span class="p">()</span>
<span class="k">assert</span> <span class="n">response</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">==</span> <span class="mi">250</span>
<span class="k">assert</span> <span class="mi">0</span> <span class="c"># for demo purposes</span>
</pre></div>
</div>
<p>We deliberately insert failing <tt class="docutils literal"><span class="pre">assert</span> <span class="pre">0</span></tt> statements in order to
inspect what is going on and can now run the tests:</p>
<div class="highlight-python"><div class="highlight"><pre>$ py.test test_module.py
=========================== test session starts ============================
platform linux -- Python 3.4.0 -- py-1.4.25 -- pytest-2.6.3
collected 2 items
test_module.py FF
================================= FAILURES =================================
________________________________ test_ehlo _________________________________
smtp = <smtplib.SMTP object at 0x2abe3c757208>
def test_ehlo(smtp):
response = smtp.ehlo()
assert response[0] == 250
> assert "merlinux" in response[1]
E TypeError: Type str doesn't support the buffer API
test_module.py:5: TypeError
________________________________ test_noop _________________________________
smtp = <smtplib.SMTP object at 0x2abe3c757208>
def test_noop(smtp):
response = smtp.noop()
assert response[0] == 250
> assert 0 # for demo purposes
E assert 0
test_module.py:11: AssertionError
========================= 2 failed in 0.17 seconds =========================
</pre></div>
</div>
<p>You see the two <tt class="docutils literal"><span class="pre">assert</span> <span class="pre">0</span></tt> failing and more importantly you can also see
that the same (module-scoped) <tt class="docutils literal"><span class="pre">smtp</span></tt> object was passed into the two
test functions because pytest shows the incoming argument values in the
traceback. As a result, the two test functions using <tt class="docutils literal"><span class="pre">smtp</span></tt> run as
quick as a single one because they reuse the same instance.</p>
<p>If you decide that you rather want to have a session-scoped <tt class="docutils literal"><span class="pre">smtp</span></tt>
instance, you can simply declare it:</p>
<div class="highlight-python"><div class="highlight"><pre>@pytest.fixture(scope="session")
def smtp(...):
# the returned fixture value will be shared for
# all tests needing it
</pre></div>
</div>
</div>
<div class="section" id="fixture-finalization-executing-teardown-code">
<span id="finalization"></span><h2>fixture finalization / executing teardown code<a class="headerlink" href="#fixture-finalization-executing-teardown-code" title="Permalink to this headline">¶</a></h2>
<p>pytest supports execution of fixture specific finalization code
when the fixture goes out of scope. By accepting a <tt class="docutils literal"><span class="pre">request</span></tt> object
into your fixture function you can call its <tt class="docutils literal"><span class="pre">request.addfinalizer</span></tt> one
or multiple times:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># content of conftest.py</span>
<span class="kn">import</span> <span class="nn">smtplib</span>
<span class="kn">import</span> <span class="nn">pytest</span>
<span class="nd">@pytest.fixture</span><span class="p">(</span><span class="n">scope</span><span class="o">=</span><span class="s">"module"</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">smtp</span><span class="p">(</span><span class="n">request</span><span class="p">):</span>
<span class="n">smtp</span> <span class="o">=</span> <span class="n">smtplib</span><span class="o">.</span><span class="n">SMTP</span><span class="p">(</span><span class="s">"merlinux.eu"</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">fin</span><span class="p">():</span>
<span class="k">print</span> <span class="p">(</span><span class="s">"teardown smtp"</span><span class="p">)</span>
<span class="n">smtp</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="n">request</span><span class="o">.</span><span class="n">addfinalizer</span><span class="p">(</span><span class="n">fin</span><span class="p">)</span>
<span class="k">return</span> <span class="n">smtp</span> <span class="c"># provide the fixture value</span>
</pre></div>
</div>
<p>The <tt class="docutils literal"><span class="pre">fin</span></tt> function will execute when the last test using
the fixture in the module has finished execution.</p>
<p>Let’s execute it:</p>
<div class="highlight-python"><div class="highlight"><pre>$ py.test -s -q --tb=no
FFteardown smtp
2 failed in 0.16 seconds
</pre></div>
</div>
<p>We see that the <tt class="docutils literal"><span class="pre">smtp</span></tt> instance is finalized after the two
tests finished execution. Note that if we decorated our fixture
function with <tt class="docutils literal"><span class="pre">scope='function'</span></tt> then fixture setup and cleanup would
occur around each single test. In either case the test
module itself does not need to change or know about these details
of fixture setup.</p>
</div>
<div class="section" id="fixtures-can-introspect-the-requesting-test-context">
<span id="request-context"></span><h2>Fixtures can introspect the requesting test context<a class="headerlink" href="#fixtures-can-introspect-the-requesting-test-context" title="Permalink to this headline">¶</a></h2>
<p>Fixture function can accept the <a class="reference internal" href="builtin.html#_pytest.python.FixtureRequest" title="_pytest.python.FixtureRequest"><tt class="xref py py-class docutils literal"><span class="pre">request</span></tt></a> object
to introspect the “requesting” test function, class or module context.
Further extending the previous <tt class="docutils literal"><span class="pre">smtp</span></tt> fixture example, let’s
read an optional server URL from the test module which uses our fixture:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># content of conftest.py</span>
<span class="kn">import</span> <span class="nn">pytest</span>
<span class="kn">import</span> <span class="nn">smtplib</span>
<span class="nd">@pytest.fixture</span><span class="p">(</span><span class="n">scope</span><span class="o">=</span><span class="s">"module"</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">smtp</span><span class="p">(</span><span class="n">request</span><span class="p">):</span>
<span class="n">server</span> <span class="o">=</span> <span class="nb">getattr</span><span class="p">(</span><span class="n">request</span><span class="o">.</span><span class="n">module</span><span class="p">,</span> <span class="s">"smtpserver"</span><span class="p">,</span> <span class="s">"merlinux.eu"</span><span class="p">)</span>
<span class="n">smtp</span> <span class="o">=</span> <span class="n">smtplib</span><span class="o">.</span><span class="n">SMTP</span><span class="p">(</span><span class="n">server</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">fin</span><span class="p">():</span>
<span class="k">print</span> <span class="p">(</span><span class="s">"finalizing </span><span class="si">%s</span><span class="s"> (</span><span class="si">%s</span><span class="s">)"</span> <span class="o">%</span> <span class="p">(</span><span class="n">smtp</span><span class="p">,</span> <span class="n">server</span><span class="p">))</span>
<span class="n">smtp</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="k">return</span> <span class="n">smtp</span>
</pre></div>
</div>
<p>We use the <tt class="docutils literal"><span class="pre">request.module</span></tt> attribute to optionally obtain an
<tt class="docutils literal"><span class="pre">smtpserver</span></tt> attribute from the test module. If we just execute
again, nothing much has changed:</p>
<div class="highlight-python"><div class="highlight"><pre>$ py.test -s -q --tb=no
FF
2 failed in 0.16 seconds
</pre></div>
</div>
<p>Let’s quickly create another test module that actually sets the
server URL in its module namespace:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># content of test_anothersmtp.py</span>
<span class="n">smtpserver</span> <span class="o">=</span> <span class="s">"mail.python.org"</span> <span class="c"># will be read by smtp fixture</span>
<span class="k">def</span> <span class="nf">test_showhelo</span><span class="p">(</span><span class="n">smtp</span><span class="p">):</span>
<span class="k">assert</span> <span class="mi">0</span><span class="p">,</span> <span class="n">smtp</span><span class="o">.</span><span class="n">helo</span><span class="p">()</span>
</pre></div>
</div>
<p>Running it:</p>
<div class="highlight-python"><div class="highlight"><pre>$ py.test -qq --tb=short test_anothersmtp.py
F
================================= FAILURES =================================
______________________________ test_showhelo _______________________________
test_anothersmtp.py:5: in test_showhelo
assert 0, smtp.helo()
E AssertionError: (250, b'mail.python.org')
E assert 0
</pre></div>
</div>
<p>voila! The <tt class="docutils literal"><span class="pre">smtp</span></tt> fixture function picked up our mail server name
from the module namespace.</p>
</div>
<div class="section" id="parametrizing-a-fixture">
<span id="fixture-parametrize"></span><h2>Parametrizing a fixture<a class="headerlink" href="#parametrizing-a-fixture" title="Permalink to this headline">¶</a></h2>
<p>Fixture functions can be parametrized in which case they will be called
multiple times, each time executing the set of dependent tests, i. e. the
tests that depend on this fixture. Test functions do usually not need
to be aware of their re-running. Fixture parametrization helps to
write exhaustive functional tests for components which themselves can be
configured in multiple ways.</p>
<p>Extending the previous example, we can flag the fixture to create two
<tt class="docutils literal"><span class="pre">smtp</span></tt> fixture instances which will cause all tests using the fixture
to run twice. The fixture function gets access to each parameter
through the special <a class="reference internal" href="builtin.html#_pytest.python.FixtureRequest" title="_pytest.python.FixtureRequest"><tt class="xref py py-class docutils literal"><span class="pre">request</span></tt></a> object:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># content of conftest.py</span>
<span class="kn">import</span> <span class="nn">pytest</span>
<span class="kn">import</span> <span class="nn">smtplib</span>
<span class="nd">@pytest.fixture</span><span class="p">(</span><span class="n">scope</span><span class="o">=</span><span class="s">"module"</span><span class="p">,</span>
<span class="n">params</span><span class="o">=</span><span class="p">[</span><span class="s">"merlinux.eu"</span><span class="p">,</span> <span class="s">"mail.python.org"</span><span class="p">])</span>
<span class="k">def</span> <span class="nf">smtp</span><span class="p">(</span><span class="n">request</span><span class="p">):</span>
<span class="n">smtp</span> <span class="o">=</span> <span class="n">smtplib</span><span class="o">.</span><span class="n">SMTP</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="k">def</span> <span class="nf">fin</span><span class="p">():</span>
<span class="k">print</span> <span class="p">(</span><span class="s">"finalizing </span><span class="si">%s</span><span class="s">"</span> <span class="o">%</span> <span class="n">smtp</span><span class="p">)</span>
<span class="n">smtp</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="n">request</span><span class="o">.</span><span class="n">addfinalizer</span><span class="p">(</span><span class="n">fin</span><span class="p">)</span>
<span class="k">return</span> <span class="n">smtp</span>
</pre></div>
</div>
<p>The main change is the declaration of <tt class="docutils literal"><span class="pre">params</span></tt> with
<a class="reference internal" href="builtin.html#_pytest.python.fixture" title="_pytest.python.fixture"><tt class="xref py py-func docutils literal"><span class="pre">@pytest.fixture</span></tt></a>, a list of values
for each of which the fixture function will execute and can access
a value via <tt class="docutils literal"><span class="pre">request.param</span></tt>. No test function code needs to change.
So let’s just do another run:</p>
<div class="highlight-python"><div class="highlight"><pre>$ py.test -q test_module.py
FFFF
================================= FAILURES =================================
__________________________ test_ehlo[merlinux.eu] __________________________
smtp = <smtplib.SMTP object at 0x2afbf4496828>
def test_ehlo(smtp):
response = smtp.ehlo()
assert response[0] == 250
> assert "merlinux" in response[1]
E TypeError: Type str doesn't support the buffer API
test_module.py:5: TypeError
__________________________ test_noop[merlinux.eu] __________________________
smtp = <smtplib.SMTP object at 0x2afbf4496828>
def test_noop(smtp):
response = smtp.noop()
assert response[0] == 250
> assert 0 # for demo purposes
E assert 0
test_module.py:11: AssertionError
________________________ test_ehlo[mail.python.org] ________________________
smtp = <smtplib.SMTP object at 0x2afbf4496d68>
def test_ehlo(smtp):
response = smtp.ehlo()
assert response[0] == 250
> assert "merlinux" in response[1]
E TypeError: Type str doesn't support the buffer API
test_module.py:5: TypeError
-------------------------- Captured stdout setup ---------------------------
finalizing <smtplib.SMTP object at 0x2afbf4496828>
________________________ test_noop[mail.python.org] ________________________
smtp = <smtplib.SMTP object at 0x2afbf4496d68>
def test_noop(smtp):
response = smtp.noop()
assert response[0] == 250
> assert 0 # for demo purposes
E assert 0
test_module.py:11: AssertionError
4 failed in 6.02 seconds
</pre></div>
</div>
<p>We see that our two test functions each ran twice, against the different
<tt class="docutils literal"><span class="pre">smtp</span></tt> instances. Note also, that with the <tt class="docutils literal"><span class="pre">mail.python.org</span></tt>
connection the second test fails in <tt class="docutils literal"><span class="pre">test_ehlo</span></tt> because a
different server string is expected than what arrived.</p>
</div>
<div class="section" id="modularity-using-fixtures-from-a-fixture-function">
<span id="interdependent-fixtures"></span><h2>Modularity: using fixtures from a fixture function<a class="headerlink" href="#modularity-using-fixtures-from-a-fixture-function" title="Permalink to this headline">¶</a></h2>
<p>You can not only use fixtures in test functions but fixture functions
can use other fixtures themselves. This contributes to a modular design
of your fixtures and allows re-use of framework-specific fixtures across
many projects. As a simple example, we can extend the previous example
and instantiate an object <tt class="docutils literal"><span class="pre">app</span></tt> where we stick the already defined
<tt class="docutils literal"><span class="pre">smtp</span></tt> resource into it:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># content of test_appsetup.py</span>
<span class="kn">import</span> <span class="nn">pytest</span>
<span class="k">class</span> <span class="nc">App</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">smtp</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">smtp</span> <span class="o">=</span> <span class="n">smtp</span>
<span class="nd">@pytest.fixture</span><span class="p">(</span><span class="n">scope</span><span class="o">=</span><span class="s">"module"</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">app</span><span class="p">(</span><span class="n">smtp</span><span class="p">):</span>
<span class="k">return</span> <span class="n">App</span><span class="p">(</span><span class="n">smtp</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">test_smtp_exists</span><span class="p">(</span><span class="n">app</span><span class="p">):</span>
<span class="k">assert</span> <span class="n">app</span><span class="o">.</span><span class="n">smtp</span>
</pre></div>
</div>
<p>Here we declare an <tt class="docutils literal"><span class="pre">app</span></tt> fixture which receives the previously defined
<tt class="docutils literal"><span class="pre">smtp</span></tt> fixture and instantiates an <tt class="docutils literal"><span class="pre">App</span></tt> object with it. Let’s run it:</p>
<div class="highlight-python"><div class="highlight"><pre>$ py.test -v test_appsetup.py
=========================== test session starts ============================
platform linux -- Python 3.4.0 -- py-1.4.25 -- pytest-2.6.3 -- /home/hpk/p/pytest/.tox/regen/bin/python3.4
collecting ... collected 2 items
test_appsetup.py::test_smtp_exists[merlinux.eu] PASSED
test_appsetup.py::test_smtp_exists[mail.python.org] PASSED
========================= 2 passed in 5.45 seconds =========================
</pre></div>
</div>
<p>Due to the parametrization of <tt class="docutils literal"><span class="pre">smtp</span></tt> the test will run twice with two
different <tt class="docutils literal"><span class="pre">App</span></tt> instances and respective smtp servers. There is no
need for the <tt class="docutils literal"><span class="pre">app</span></tt> fixture to be aware of the <tt class="docutils literal"><span class="pre">smtp</span></tt> parametrization
as pytest will fully analyse the fixture dependency graph.</p>
<p>Note, that the <tt class="docutils literal"><span class="pre">app</span></tt> fixture has a scope of <tt class="docutils literal"><span class="pre">module</span></tt> and uses a
module-scoped <tt class="docutils literal"><span class="pre">smtp</span></tt> fixture. The example would still work if <tt class="docutils literal"><span class="pre">smtp</span></tt>
was cached on a <tt class="docutils literal"><span class="pre">session</span></tt> scope: it is fine for fixtures to use
“broader” scoped fixtures but not the other way round:
A session-scoped fixture could not use a module-scoped one in a
meaningful way.</p>
</div>
<div class="section" id="automatic-grouping-of-tests-by-fixture-instances">
<span id="automatic-per-resource-grouping"></span><h2>Automatic grouping of tests by fixture instances<a class="headerlink" href="#automatic-grouping-of-tests-by-fixture-instances" title="Permalink to this headline">¶</a></h2>
<p>pytest minimizes the number of active fixtures during test runs.
If you have a parametrized fixture, then all the tests using it will
first execute with one instance and then finalizers are called
before the next fixture instance is created. Among other things,
this eases testing of applications which create and use global state.</p>
<p>The following example uses two parametrized funcargs, one of which is
scoped on a per-module basis, and all the functions perform <tt class="docutils literal"><span class="pre">print</span></tt> calls
to show the setup/teardown flow:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># content of test_module.py</span>
<span class="kn">import</span> <span class="nn">pytest</span>
<span class="nd">@pytest.fixture</span><span class="p">(</span><span class="n">scope</span><span class="o">=</span><span class="s">"module"</span><span class="p">,</span> <span class="n">params</span><span class="o">=</span><span class="p">[</span><span class="s">"mod1"</span><span class="p">,</span> <span class="s">"mod2"</span><span class="p">])</span>
<span class="k">def</span> <span class="nf">modarg</span><span class="p">(</span><span class="n">request</span><span class="p">):</span>
<span class="n">param</span> <span class="o">=</span> <span class="n">request</span><span class="o">.</span><span class="n">param</span>
<span class="k">print</span> <span class="p">(</span><span class="s">"create"</span><span class="p">,</span> <span class="n">param</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">fin</span><span class="p">():</span>
<span class="k">print</span> <span class="p">(</span><span class="s">"fin </span><span class="si">%s</span><span class="s">"</span> <span class="o">%</span> <span class="n">param</span><span class="p">)</span>
<span class="k">return</span> <span class="n">param</span>
<span class="nd">@pytest.fixture</span><span class="p">(</span><span class="n">scope</span><span class="o">=</span><span class="s">"function"</span><span class="p">,</span> <span class="n">params</span><span class="o">=</span><span class="p">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">])</span>
<span class="k">def</span> <span class="nf">otherarg</span><span class="p">(</span><span class="n">request</span><span class="p">):</span>
<span class="k">return</span> <span class="n">request</span><span class="o">.</span><span class="n">param</span>
<span class="k">def</span> <span class="nf">test_0</span><span class="p">(</span><span class="n">otherarg</span><span class="p">):</span>
<span class="k">print</span> <span class="p">(</span><span class="s">" test0"</span><span class="p">,</span> <span class="n">otherarg</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">test_1</span><span class="p">(</span><span class="n">modarg</span><span class="p">):</span>
<span class="k">print</span> <span class="p">(</span><span class="s">" test1"</span><span class="p">,</span> <span class="n">modarg</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">test_2</span><span class="p">(</span><span class="n">otherarg</span><span class="p">,</span> <span class="n">modarg</span><span class="p">):</span>
<span class="k">print</span> <span class="p">(</span><span class="s">" test2"</span><span class="p">,</span> <span class="n">otherarg</span><span class="p">,</span> <span class="n">modarg</span><span class="p">)</span>
</pre></div>
</div>
<p>Let’s run the tests in verbose mode and with looking at the print-output:</p>
<div class="highlight-python"><div class="highlight"><pre>$ py.test -v -s test_module.py
=========================== test session starts ============================
platform linux -- Python 3.4.0 -- py-1.4.25 -- pytest-2.6.3 -- /home/hpk/p/pytest/.tox/regen/bin/python3.4
collecting ... collected 8 items
test_module.py::test_0[1] test0 1
PASSED
test_module.py::test_0[2] test0 2
PASSED
test_module.py::test_1[mod1] create mod1
test1 mod1
PASSED
test_module.py::test_2[1-mod1] test2 1 mod1
PASSED
test_module.py::test_2[2-mod1] test2 2 mod1
PASSED
test_module.py::test_1[mod2] create mod2
test1 mod2
PASSED
test_module.py::test_2[1-mod2] test2 1 mod2
PASSED
test_module.py::test_2[2-mod2] test2 2 mod2
PASSED
========================= 8 passed in 0.01 seconds =========================
</pre></div>
</div>
<p>You can see that the parametrized module-scoped <tt class="docutils literal"><span class="pre">modarg</span></tt> resource caused
an ordering of test execution that lead to the fewest possible “active” resources. The finalizer for the <tt class="docutils literal"><span class="pre">mod1</span></tt> parametrized resource was executed
before the <tt class="docutils literal"><span class="pre">mod2</span></tt> resource was setup.</p>
</div>
<div class="section" id="using-fixtures-from-classes-modules-or-projects">
<span id="usefixtures"></span><h2>using fixtures from classes, modules or projects<a class="headerlink" href="#using-fixtures-from-classes-modules-or-projects" title="Permalink to this headline">¶</a></h2>
<p>Sometimes test functions do not directly need access to a fixture object.
For example, tests may require to operate with an empty directory as the
current working directory but otherwise do not care for the concrete
directory. Here is how you can can use the standard <a class="reference external" href="http://docs.python.org/library/tempfile.html">tempfile</a> and pytest fixtures to
achieve it. We separate the creation of the fixture into a conftest.py
file:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># content of conftest.py</span>
<span class="kn">import</span> <span class="nn">pytest</span>
<span class="kn">import</span> <span class="nn">tempfile</span>
<span class="kn">import</span> <span class="nn">os</span>
<span class="nd">@pytest.fixture</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">cleandir</span><span class="p">():</span>
<span class="n">newpath</span> <span class="o">=</span> <span class="n">tempfile</span><span class="o">.</span><span class="n">mkdtemp</span><span class="p">()</span>
<span class="n">os</span><span class="o">.</span><span class="n">chdir</span><span class="p">(</span><span class="n">newpath</span><span class="p">)</span>
</pre></div>
</div>
<p>and declare its use in a test module via a <tt class="docutils literal"><span class="pre">usefixtures</span></tt> marker:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># content of test_setenv.py</span>
<span class="kn">import</span> <span class="nn">os</span>
<span class="kn">import</span> <span class="nn">pytest</span>
<span class="nd">@pytest.mark.usefixtures</span><span class="p">(</span><span class="s">"cleandir"</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">TestDirectoryInit</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">test_cwd_starts_empty</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">assert</span> <span class="n">os</span><span class="o">.</span><span class="n">listdir</span><span class="p">(</span><span class="n">os</span><span class="o">.</span><span class="n">getcwd</span><span class="p">())</span> <span class="o">==</span> <span class="p">[]</span>
<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s">"myfile"</span><span class="p">,</span> <span class="s">"w"</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
<span class="n">f</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s">"hello"</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">test_cwd_again_starts_empty</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">assert</span> <span class="n">os</span><span class="o">.</span><span class="n">listdir</span><span class="p">(</span><span class="n">os</span><span class="o">.</span><span class="n">getcwd</span><span class="p">())</span> <span class="o">==</span> <span class="p">[]</span>
</pre></div>
</div>
<p>Due to the <tt class="docutils literal"><span class="pre">usefixtures</span></tt> marker, the <tt class="docutils literal"><span class="pre">cleandir</span></tt> fixture
will be required for the execution of each test method, just as if
you specified a “cleandir” function argument to each of them. Let’s run it
to verify our fixture is activated and the tests pass:</p>
<div class="highlight-python"><div class="highlight"><pre>$ py.test -q
..
2 passed in 0.01 seconds
</pre></div>
</div>
<p>You can specify multiple fixtures like this:</p>
<div class="highlight-python"><div class="highlight"><pre>@pytest.mark.usefixtures("cleandir", "anotherfixture")
</pre></div>
</div>
<p>and you may specify fixture usage at the test module level, using
a generic feature of the mark mechanism:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">pytestmark</span> <span class="o">=</span> <span class="n">pytest</span><span class="o">.</span><span class="n">mark</span><span class="o">.</span><span class="n">usefixtures</span><span class="p">(</span><span class="s">"cleandir"</span><span class="p">)</span>
</pre></div>
</div>
<p>Lastly you can put fixtures required by all tests in your project
into an ini-file:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># content of pytest.ini</span>
<span class="p">[</span><span class="n">pytest</span><span class="p">]</span>
<span class="n">usefixtures</span> <span class="o">=</span> <span class="n">cleandir</span>
</pre></div>
</div>
</div>
<div class="section" id="autouse-fixtures-xunit-setup-on-steroids">
<span id="autouse-fixtures"></span><span id="autouse"></span><h2>autouse fixtures (xUnit setup on steroids)<a class="headerlink" href="#autouse-fixtures-xunit-setup-on-steroids" title="Permalink to this headline">¶</a></h2>
<p>Occasionally, you may want to have fixtures get invoked automatically
without a <a class="reference internal" href="#usefixtures">usefixtures</a> or <a class="reference internal" href="#funcargs">funcargs</a> reference. As a practical
example, suppose we have a database fixture which has a
begin/rollback/commit architecture and we want to automatically surround
each test method by a transaction and a rollback. Here is a dummy
self-contained implementation of this idea:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># content of test_db_transact.py</span>
<span class="kn">import</span> <span class="nn">pytest</span>
<span class="k">class</span> <span class="nc">DB</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">intransaction</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">def</span> <span class="nf">begin</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">intransaction</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">name</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">rollback</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">intransaction</span><span class="o">.</span><span class="n">pop</span><span class="p">()</span>
<span class="nd">@pytest.fixture</span><span class="p">(</span><span class="n">scope</span><span class="o">=</span><span class="s">"module"</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">db</span><span class="p">():</span>
<span class="k">return</span> <span class="n">DB</span><span class="p">()</span>
<span class="k">class</span> <span class="nc">TestClass</span><span class="p">:</span>
<span class="nd">@pytest.fixture</span><span class="p">(</span><span class="n">autouse</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">transact</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">request</span><span class="p">,</span> <span class="n">db</span><span class="p">):</span>
<span class="n">db</span><span class="o">.</span><span class="n">begin</span><span class="p">(</span><span class="n">request</span><span class="o">.</span><span class="n">function</span><span class="o">.</span><span class="n">__name__</span><span class="p">)</span>
<span class="n">request</span><span class="o">.</span><span class="n">addfinalizer</span><span class="p">(</span><span class="n">db</span><span class="o">.</span><span class="n">rollback</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">test_method1</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">db</span><span class="p">):</span>
<span class="k">assert</span> <span class="n">db</span><span class="o">.</span><span class="n">intransaction</span> <span class="o">==</span> <span class="p">[</span><span class="s">"test_method1"</span><span class="p">]</span>
<span class="k">def</span> <span class="nf">test_method2</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">db</span><span class="p">):</span>
<span class="k">assert</span> <span class="n">db</span><span class="o">.</span><span class="n">intransaction</span> <span class="o">==</span> <span class="p">[</span><span class="s">"test_method2"</span><span class="p">]</span>
</pre></div>
</div>
<p>The class-level <tt class="docutils literal"><span class="pre">transact</span></tt> fixture is marked with <em>autouse=true</em>
which implies that all test methods in the class will use this fixture
without a need to state it in the test function signature or with a
class-level <tt class="docutils literal"><span class="pre">usefixtures</span></tt> decorator.</p>
<p>If we run it, we get two passing tests:</p>
<div class="highlight-python"><div class="highlight"><pre>$ py.test -q
..
2 passed in 0.01 seconds
</pre></div>
</div>
<p>Here is how autouse fixtures work in other scopes:</p>
<ul class="simple">
<li>if an autouse fixture is defined in a test module, all its test
functions automatically use it.</li>
<li>if an autouse fixture is defined in a conftest.py file then all tests in
all test modules belows its directory will invoke the fixture.</li>
<li>lastly, and <strong>please use that with care</strong>: if you define an autouse
fixture in a plugin, it will be invoked for all tests in all projects
where the plugin is installed. This can be useful if a fixture only
anyway works in the presence of certain settings e. g. in the ini-file. Such
a global fixture should always quickly determine if it should do
any work and avoid expensive imports or computation otherwise.</li>
</ul>
<p>Note that the above <tt class="docutils literal"><span class="pre">transact</span></tt> fixture may very well be a fixture that
you want to make available in your project without having it generally
active. The canonical way to do that is to put the transact definition
into a conftest.py file <strong>without</strong> using <tt class="docutils literal"><span class="pre">autouse</span></tt>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># content of conftest.py</span>
<span class="nd">@pytest.fixture</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">transact</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">request</span><span class="p">,</span> <span class="n">db</span><span class="p">):</span>
<span class="n">db</span><span class="o">.</span><span class="n">begin</span><span class="p">()</span>
<span class="n">request</span><span class="o">.</span><span class="n">addfinalizer</span><span class="p">(</span><span class="n">db</span><span class="o">.</span><span class="n">rollback</span><span class="p">)</span>
</pre></div>
</div>
<p>and then e.g. have a TestClass using it by declaring the need:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="nd">@pytest.mark.usefixtures</span><span class="p">(</span><span class="s">"transact"</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">TestClass</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">test_method1</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="o">...</span>
</pre></div>
</div>
<p>All test methods in this TestClass will use the transaction fixture while
other test classes or functions in the module will not use it unless
they also add a <tt class="docutils literal"><span class="pre">transact</span></tt> reference.</p>
</div>
<div class="section" id="shifting-visibility-of-fixture-functions">
<h2>Shifting (visibility of) fixture functions<a class="headerlink" href="#shifting-visibility-of-fixture-functions" title="Permalink to this headline">¶</a></h2>
<p>If during implementing your tests you realize that you
want to use a fixture function from multiple test files you can move it
to a <a class="reference internal" href="plugins.html#conftest-py"><em>conftest.py</em></a> file or even separately installable
<a class="reference internal" href="plugins.html#plugins"><em>plugins</em></a> without changing test code. The discovery of
fixtures functions starts at test classes, then test modules, then
<tt class="docutils literal"><span class="pre">conftest.py</span></tt> files and finally builtin and third party plugins.</p>
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<li><a class="reference internal" href="#fixtures-as-function-arguments">Fixtures as Function arguments</a></li>
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<li><a class="reference internal" href="#fixtures-can-introspect-the-requesting-test-context">Fixtures can introspect the requesting test context</a></li>
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<li><a class="reference internal" href="#modularity-using-fixtures-from-a-fixture-function">Modularity: using fixtures from a fixture function</a></li>
<li><a class="reference internal" href="#automatic-grouping-of-tests-by-fixture-instances">Automatic grouping of tests by fixture instances</a></li>
<li><a class="reference internal" href="#using-fixtures-from-classes-modules-or-projects">using fixtures from classes, modules or projects</a></li>
<li><a class="reference internal" href="#autouse-fixtures-xunit-setup-on-steroids">autouse fixtures (xUnit setup on steroids)</a></li>
<li><a class="reference internal" href="#shifting-visibility-of-fixture-functions">Shifting (visibility of) fixture functions</a></li>
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