Pytest AST-modification : getting the tests final code

Pytest is a very complete test framework for Python. I like how you can write a basic unittest.TestCase and the py.test test runner command will inject all its magic at runtime, without you having to directly import anything: awesome separation of concerns.

This modularity comes at a cost though: py.test actually preprocess tests before running them, by parsing their AST tree and replacing assert calls by custom exceptions "manually "raised. The final compiled .pyc binaries are then cached in a __pycache__/ directory.

My curiosity got aroused by this blog post from 2011 : wouldn't that be nice to peek into this process and check what the modified code look like exactly ?

I considered 2 solutions:

  • either decompile the cached .pyc files, but I couldn't feed them to uncompyle2 nor pycdc without raising bytecode format errors.
  • take a glance at Pytest code base and find a way to invoke its custom AST-parsing method, then "AST-unparse" the resulting AST tree instead of compiling it down to bytecode.

This second solution revealed to be very easy to implement. I hesitated for a moment between two good-looking AST-unparser, namely astor and astunparse, and ended up with the following code:

from _pytest.assertion.rewrite import rewrite_asserts
import ast, astunparse, sys

with open(sys.argv[1], 'r') as open_file:
    ast_tree = ast.parse(
print astunparse.unparse(ast_tree)

And that's it ! To test it, just write a dummy file with:

def dummy_test():
    assert False

And then python :

import __builtin__ as @py_builtins
import _pytest.assertion.rewrite as @pytest_ar

def dummy_test():
    if (not False):
        @py_format1 = (('' + 'assert %(py0)s') % {'py0': (@pytest_ar._saferepr(False) if (('False' in @py_builtins.locals()) or @pytest_ar._should_repr_global_name(False)) else 'False')})
        raise AssertionError(@pytest_ar._format_explanation(@py_format1))

Mowgli hypnotized by Kaa

Now, I want to conclude on a more nuanced tone: not everything is perfect in the Pytest world, and I have a few pain points to mention:

  • Pytest terminal reports are rendered character by character, making it impossible to write log messages to stdout without messing everything
  • Pytest wraps every module/class/object in your tests into custom wrappers and use generic callback hooks on at least 3 invocation levels : I had to scratch my head for some time to debug minor errors stacktraces and hack around it
  • Pytest code base is very dense and not always following PEP code standards, making it very difficult to understand and contribute
  • finally, I have a last minor complaint: when using pytest-xdist to parallelize tests, you cannot write to stdout. I guess it'd be difficult to collect the standard outputs of every process spawned by pytest-xdist, but it's still a PITA.