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Dispelling py.test magic


This short talk will look under the hood of how py.test uses assertion statement rewriting to give users a better, more pythonic testing experience.

Usually in Python, assertion statements are quite simple and tedious to work with, where a simple snippet of code like this:

def double(x):
    return x * 2
expected = 5
assert double(2) == expected

finishes with message that does not include any context:

Traceback (most recent call last):
  File "", line 5, in <module>
    assert double(2) == expected

With py.test, we get a lot more information with all intermittent values nicely described: in <module>
    assert double(2) == expected
E   assert 4 == 5
E    +  where 4 = <function double at 0x1033add08>(2)

During this talk you will learn about all the ingredients needed to reverse- engineer py.test behaviour, using import hooks described in PEP 302, and the ast module from standard library. We will try to use these hidden gems in a broader context, outside of testing.


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