[FIXED] How to write a decorator for asynchronous functions

Issue

I wrote a decorator for asynchronous functions, but it does't work - an assertion is not thrown. Where is an error? The code in attachment.

import asyncio
import pytest

def tmi_freq_check(needed_value: int, tmi_freq_tolerance: int, tmi_freq_critical: bool, info_message: str = ""):
    def decorator(func):
        @functools.wraps(func)
        async def wrapper(*args, **kwargs):
            result = await func(*args, **kwargs)
            if tmi_freq_critical:
                assert needed_value == pytest.approx(result, tmi_freq_tolerance)
            return result

        return wrapper

    return decorator


@tmi_freq_check(600, 10, True)
async def my_function():
    return 500


async def main():
    await my_function()

asyncio.run(main())

Solution

Your decorator works and the underlying assertion passes. But the issue is in tolerance parameter. It's not really a simple diff between two arguments but a relative tolerance.

From pytest.approx doc:

Tolerances

By default, approx considers numbers within a relative tolerance of 1e-6 (i.e. one part in a million) of its expected value to be equal. This treatment would lead to surprising results if the expected value was 0.0, because nothing but 0.0 itself is relatively close to 0.0. To handle this case less surprisingly, approx also considers numbers within an absolute tolerance of 1e-12 of its expected value to be equal. Infinity and NaN are special cases. Infinity is only considered equal to itself, regardless of the relative tolerance. NaN is not considered equal to anything by default, but you can make it be equal to itself by setting the nan_ok argument to True. (This is meant to facilitate comparing arrays that use NaN to mean “no data”.)
...

In your case: 10 is 0.02 part of 500 and should be specified in that pattern:

@tmi_freq_check(600, 2e-2, True)
async def my_function():
    return 500

---

Now, the it's seen that the relative threshold is exceeded:

Traceback (most recent call last):
  ...
    assert needed_value == pytest.approx(result, tmi_freq_tolerance)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError

Answered By - RomanPerekhrest