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 "t.py", line 5, in <module> assert double(2) == expected AssertionError
With py.test, we get a lot more information with all intermittent values nicely described:
t.py:5: 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.