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Testing Python in PyCharm

From: andrew cooke <andrew@...>

Date: Wed, 29 Jun 2011 21:11:27 -0400

I am trying to switch from Eclipse to Intellij / PyCharm (ideally Intellij
Idea with the Python plugin, so that I can also work in Java and C, but
currently I am using PyCharm because it's a later release with more fixes).

It's not been as smooth a ride as I expected from my (very positive)
experience using Intellij Idea at Mule with Java.  However, I've finally
worked out how to get the tests to run in Lepl.

First, you have to realise that PyCharm doesn't have a test runner of its own.
It delegates the work to one of: unittest, nose, or pytest.  Of those, only
pytest seems to be reliable (see below), yet that is the least well supported
in PyCharm (again, see below).


Test Runners
============

Unittest
--------

So, unittest is the standard Python package.  You might think that is
reliable - and it is the default used by PyCharm.  But it doesn't find all my
tests.  In particular, it seems to have problems with tests that delegate
tests to non-TestCase classes.  That might seem odd, but it's a really useful
pattern when testing multiple implementations for the same API (like Lepl's
regular expression engines).  The idea is that you write tests against an API
in a class called MyApiTest (which does not subclass TestCase).  Then, for
each engine, you create a subclass of TestCase that *also* subclasses
MyApiTest (and which provides the extra plumbing necessary so that the tests
run against that engine).

Anyway, neat testing pattern, but doesn't work with unittest.

Nose
----

Nose is the most well known 3rd party test runner.  It's supposed to be easy
to use, extend, customise.  But it has a hard-wired filter that refuses to
run any tests in directories that start with an underscore. 

Because I don't consider the tests to be part of the Lepl API they are in
directories called "_test" (the Python convention being that an underscore
indicates "private data").  And so nose won't run those tests.

Pytest
------

Pytest works just fine from the command line.  It has excellent
documentation.  The output is verbose, but PyCharm tames that.  So thankfully
this is the option to use.


PyCharm Integration
===================

First, don't install pytest in your root Python if you're going to use a
virtualenv.  Because PyCharm won't find it.  Install it in your virtualenv if
you're using one.

Next, you can change the default test runner in PyCharm via the main settings
dialog - choose the "Python Integrated Tools" option.  However, doing so is
pointless because (at least for me) the option to run tests by right-clicking
on a file in the project tree disappears.  Yup.  It reappears if you select
unittest and goes away if you switch back to pytest.  No idea why.

But, luckily, you can configure tests explicitly.  Go to the drop-down menu in
the toolbar and select "Edit COnfigurations".  You can add a new test (click
on + then pytest; clicking on pytest then + doesn't work!) and configure it
there.  You may notice that, unlike for unittest and nose, the pytest
configuration dialog doesn't let you select a directory, only a "script".
Ignore this!  Enter a directory in the "script" box.  It will work - it will
run all tests in that directory.

Once that is defined you can run it from the toolbar.  And you see PyCharm's
nice summary with little red and green lights.  All is good.

Andrew

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