[issue41232] Python `functools.wraps` doesn't deal with defaults correctly

[Thor Whalen]

New submission from Thor Whalen <thorwhalen1 at gmail.com>:

# PROBLEM

When using `functools.wraps`, the signature claims one set of defaults, but the (wrapped) function uses the original (wrappee) defaults.

Why might that be the desirable default?

# PROPOSED SOLUTION
Adding '__defaults__',  '__kwdefaults__' to `WRAPPER_ASSIGNMENTS` so that actual defaults can be consistent with signature defaults.

# Code Demo

```python
from functools import wraps
from inspect import signature

def g(a: float, b=10):
    return a * b

def f(a: int, b=1):
    return a * b

assert f(3) == 3

f = wraps(g)(f)

assert str(signature(f)) == '(a: float, b=10)'  # signature says that b defaults to 10
assert f.__defaults__ == (1,)  # ... but it's a lie!
assert f(3) == 3 != g(3) == 30  # ... and one that can lead to problems!
```

Why is this so? Because `functools.wraps` updates the `__signature__` (including annotations and defaults), `__annotations__`, but not `__defaults__`, which python apparently looks to in order to assign defaults.

One solution is to politely ask wraps to include these defaults.

```python
from functools import wraps, WRAPPER_ASSIGNMENTS, partial

my_wraps = partial(wraps, assigned=(list(WRAPPER_ASSIGNMENTS) + ['__defaults__', '__kwdefaults__']))

def g(a: float, b=10):
    return a * b

def f(a: int, b=1):
    return a * b

assert f(3) == 3

f = my_wraps(g)(f)

assert f(3) == 30 == g(3)
assert f.__defaults__ == (10,)  # ... because now got g defaults!
```

Wouldn't it be better to get this out of the box?

When would I want the defaults that are actually used be different than those mentioned in the signature?

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----------
messages: 373254
nosy: Thor Whalen2
priority: normal
severity: normal
status: open
title: Python `functools.wraps` doesn't deal with defaults correctly
type: enhancement
versions: Python 3.8

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue41232>
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