[issue15351] Add to unittest.TestCase support for using context managers

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Opened issue45046 for completely different approach to this problem.

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[issue15351] Add to unittest.TestCase support for using context managers

[Martin Panter]

Changes by Martin Panter vadmium...@gmail.com:


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[issue15351] Add to unittest.TestCase support for using context managers

[Julian Berman]

Julian Berman added the comment:

Now that we have contextlib.ExitStack, I think we should consider that here.

I.e., I think ExitStack deserves a method that calls its __enter__ and 
__exit__, say .enter() and .exit(), and then the idiom for this wouldn't 
require anything on TestCase, it'd be:


class TestStuff(TestCase):
def setUp(self):
self.stack = ExitStack()

self.stack.enter_context(my_context_manager())
self.stack.enter_context(my_context_manager2())
self.stack.enter_context(my_context_manager3())

self.stack.enter()
self.addCleanup(self.stack.exit)

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[issue15351] Add to unittest.TestCase support for using context managers

[Chris Calloway]

Changes by Chris Calloway c...@chriscalloway.org:


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[issue15351] Add to unittest.TestCase support for using context managers

[Michael Foord]

Changes by Michael Foord mich...@voidspace.org.uk:


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[issue15351] Add to unittest.TestCase support for using context managers

[Eli Bendersky]

Changes by Eli Bendersky eli...@gmail.com:


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[issue15351] Add to unittest.TestCase support for using context managers

[Chris Jerdonek]

Chris Jerdonek added the comment:

Adding Éric because of the interest in test setup and tear down in issue 11664.

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[issue15351] Add to unittest.TestCase support for using context managers

[Chris Jerdonek]

Chris Jerdonek chris.jerdo...@gmail.com added the comment:

Attached is a patch illustrating the API I suggested for discussion.

To add custom setup and teardown context managers, the user can override the 
following method:

def executeTest(self):
with self.setUpContext():
self.runTestMethod()

The custom context managers can be placed either before or after the existing 
setUp/tearDown, or both.

The patch preserves the existing behavior that tearDown() should run only if 
setUp() was successful, and that doCleanups() should always run.  All tests 
continue to pass with the patch.

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Added file: http://bugs.python.org/file26405/issue-15351-concept.patch

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[issue15351] Add to unittest.TestCase support for using context managers

[Michael Foord]

Michael Foord mich...@voidspace.org.uk added the comment:

A method on TestCase that *just* executes the test method - allowing for 
overriding in subclasses - is an interesting idea. Including setUp and tearDown 
would be harder because TestCase necessarily does a bunch of bookkeeping 
between each of these steps.

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[issue15351] Add to unittest.TestCase support for using context managers

[Chris Jerdonek]

New submission from Chris Jerdonek chris.jerdo...@gmail.com:

The setUp() and tearDown() methods of unittest.TestCase are of course extremely 
useful.  But sometimes one has set up and tear down functionality that one 
would like to apply in the form of an existing context manager (and that may be 
from an outside source).

There is currently no clear or clean way to do this.  It would be nice if 
unittest exposed a way to do this.

The closest I have been able to come is overriding TestCase.run() as follows:

class MyTest(unittest.TestCase):

def run(self, result=None):
with my_context_manager() as foo:
# Do stuff.
super(MyTest, self).run(result)

But this is not ideal because the context manager is surrounding more than it 
should (various test initialization code internal to unittest, etc).  Also, 
ideally the API would let one apply a context manager either before or after 
setUp() and tearDown(), or both.

Here is one idea for an API.  unittest.TestCase could expose a setUpContext() 
context manager that wraps the user-defined setUp() and tearDown(), and also a 
runTestMethod() method that runs the test method code by itself (i.e. currently 
the following line of unittest/case.py: `self._executeTestPart(testMethod, 
outcome, isTest=True)`).

To use the API, the user could override a simple method called something like 
runTest():

def runTest(self):
with setUpContext():
self.runTestMethod()

The user would, in the override, be free to insert additional context managers 
before or after setUpContext(), or both.

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components: Library (Lib)
messages: 165454
nosy: cjerdonek
priority: normal
severity: normal
status: open
title: Add to unittest.TestCase support for using context managers
type: enhancement

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[issue15351] Add to unittest.TestCase support for using context managers

[R. David Murray]

R. David Murray rdmur...@bitdance.com added the comment:

Well, if you want to invoke the context in setup/teardown for some reason (as 
opposed to in the test methods themselves), you can do this:

  def setUp(self):
  self.foo = MyContextManager.__enter__()
  self.addCleanup(MyContextManager.__exit__())

Personally I rarely do this (except occasionally the mock equivalent, see 
below), preferring to call the context manager explicitly in the test method 
itself, often factored out into a test helper.

I think we as a community are still learning how best to use context managers 
and what the optimum design of context manager is.  There is some thought that 
a context manager should always provide a non-context way of getting at the 
functionality of the enter and exit methods.  For example, the mock context 
managers have start() and stop() methods.  There has also been a small amount 
of discussion of making more use of context managers in unittest itself, 
possibly along the lines you suggest.

I think this may be an area in which we are not yet ready to settle on an API.  
Michael may have a different opinion, of course ;)

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[issue15351] Add to unittest.TestCase support for using context managers

[R. David Murray]

R. David Murray rdmur...@bitdance.com added the comment:

That should have been

   self.addCleanup(MyContextManager.__exit__)

You could alternatively call __exit__() explicitly in tearDown, of course, but 
I believe addCleanup is a more reliable cleanup than tearDown.

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[issue15351] Add to unittest.TestCase support for using context managers

[Chris Jerdonek]

Chris Jerdonek chris.jerdo...@gmail.com added the comment:

Thanks for the interesting background and feedback. I was aware of the 
__enter__/__exit__ option but not the other information. And yes, I agree on 
the importance of trying and discussing any API before settling on it. The one 
I suggested was merely a point of departure. :)

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