problems with mutations to async generators

[ulrikjohansson]

I ran into mutmut run stopping b.c pytest execution failed.

Basically, I have an async function with yield inside it. When I run mutmut python fails b.c that has suddenly become a non-async generator instead, so it can't be used in an async for statement.

I'm not sure if this is a proper bug, or just something I'm configuring wrong

I have a small repro repo here: https://github.com/ulrikjohansson/mutmut_test

[boxed]

[...me being stupid and misunderstanding, sorry...]

[Otto-AA]

Thanks for the repro, this is indeed an issue by mutmut. The tests pass when running pytest, but the tests of the mutated code fail.

The issue is likely that our trampoline setup (which forwards calls to the mutated functions) is not async. In your example we mutate:

async def generate():
    lst = list(range(10))
    for item in lst:
        yield item

to

def generate(*args, **kwargs):
    result = yield from _mutmut_yield_from_trampoline(x_generate__mutmut_orig, x_generate__mutmut_mutants, args, kwargs)
    return result 

def _mutmut_yield_from_trampoline(orig, mutants, call_args, call_kwargs, self_arg = None):
    """Forward call to original or mutated function, depending on the environment"""
    ...
        result = yield from orig(*call_args, **call_kwargs)
        return result  # for the yield case
    ...

async def x_generate__mutmut_orig():
    lst = list(range(10))
    for item in lst:
        yield item

I only worked superficially with async functions in python, but I guess for async functions our trampoline and placeholder both need to be async+await as well.

If you want to work on this, @ulrikjohansson feel free to create a PR or ask for more pointers. It probably could be solved by adding a new template at trampoline_templates.py and updating the build_trampoline function to take another is_async param. Checking if the function is async in "file_mutation.py" is as simple as _is_async = function.asynchronous is not None.

I don't think I will find time to implement this myself in the next weeks, as I'd also need to read up on the async topic.

[ulrikjohansson]

Hi! Thanks for taking a look. My own very simple workaround was to just test inside the is_generator function whether we also are in an async function, and just return False in that case.
I'm not sure if that is enough or would cause other problems though. I noticed you have a PR up too, I'll try that out.

[Otto-AA]

Hi! Thanks for taking a look. My own very simple workaround was to just test inside the is_generator function whether we also are in an async function, and just return False in that case.

My PR removes our special handling of generators, so you could say that it sets is_generator to False all the time. It's a similar approach :)