(Cross-posting from SO, https://stackoverflow.com/q/78206137/606576)
Python 3.10.
I have written two versions of a logging decorator, one for normal functions and one for async functions. Mypy is happy with these:
from functools import wraps
from inspect import Signature, signature
from logging import getLogger
from typing import Any, Awaitable, Callable, ParamSpec, TypeVar, Union, overload
# You heard it here first: type hinting for decorators is a pain in the arse.
Param = ParamSpec("Param")
RetType = TypeVar("RetType")
def _log_with_bound_arguments(
func_name: str, func_sig: Signature, *args: Any, **kwargs: Any
) -> None:
# Cf. <https://docs.python.org/3/library/inspect.html#inspect.BoundArguments>
bound_func = func_sig.bind_partial(*args, **kwargs)
func_params = ", ".join([k + "=" + repr(v) for k, v in bound_func.arguments.items()])
getLogger().debug("-> %s(%s)", func_name, func_params)
def atrace(func: Callable[Param, Awaitable[RetType]]) -> Callable[Param, Awaitable[RetType]]:
"""Async decorator that safely logs the function call at the debug level."""
@wraps(func)
async def async_wrapper(*args: Param.args, **kwargs: Param.kwargs) -> RetType:
_log_with_bound_arguments(func.__qualname__, signature(func), *args, **kwargs)
return await func(*args, **kwargs)
return async_wrapper
def trace(func: Callable[Param, RetType]) -> Callable[Param, RetType]:
"""Decorator that safely logs the function call at the debug level."""
@wraps(func)
def wrapper(*args: Param.args, **kwargs: Param.kwargs) -> RetType:
_log_with_bound_arguments(func.__qualname__, signature(func), *args, **kwargs)
return func(*args, **kwargs)
return wrapper
I would like to consolidate these into a single decorator that I can slap on all functions, async or not. Continuing:
from inspect import iscoroutinefunction
from typing import Union, overload
@overload
def ctrace(func: Callable[Param, RetType]) -> Callable[Param, RetType]: ...
@overload
def ctrace(func: Callable[Param, Awaitable[RetType]]) -> Callable[Param, Awaitable[RetType]]: ...
def ctrace(
func: Union[Callable[Param, RetType], Callable[Param, Awaitable[RetType]]],
) -> Union[Callable[Param, RetType], Callable[Param, Awaitable[RetType]]]:
if iscoroutinefunction(func):
@wraps(func)
async def async_wrapper(*args: Param.args, **kwargs: Param.kwargs) -> RetType:
_log_with_bound_arguments(func.__qualname__, signature(func), *args, **kwargs)
return await func(*args, **kwargs)
return async_wrapper
else:
@wraps(func)
def wrapper(*args: Param.args, **kwargs: Param.kwargs) -> RetType:
_log_with_bound_arguments(func.__qualname__, signature(func), *args, **kwargs)
return func(*args, **kwargs)
return wrapper
However, now Mypy is giving me four errors I can’t figure out how to fix:
src/my_package/log/__init__.py:109: error: Overloaded function implementation does not accept all possible arguments of signature 2 [misc]
src/my_package/log/__init__.py:109: error: Overloaded function implementation cannot produce return type of signature 2 [misc]
src/my_package/log/__init__.py:118: error: Returning Any from function declared to return "RetType" [no-any-return]
src/my_package/log/__init__.py:127: error: Incompatible return value type (got "RetType | Awaitable[RetType]", expected "RetType") [return-value]
The line numbers obviously don’t help a lot, so I’ll point out that line 109 is the def ctrace( line that starts the implementation of the overloaded function. 118 is return await func(*args, **kwargs) and 127 is return func(*args, **kwargs).
Pylance only shows a single error: on line 127, Expression of type “RetType@ctrace | Awaitable[RetType@ctrace]” cannot be assigned to return type “RetType@ctrace”
Type “RetType@ctrace | Awaitable[RetType@ctrace]” cannot be assigned to type “RetType@ctrace” This is the same as the fourth error from Mypy as far as I can tell.
Any help in hinting this properly would be greatly appreciated.