Question regarding unexpected behavior in using __enter__ method

[Lorenzo Catoni]

Dear Python Mailing List members,

I am writing to seek your assistance in understanding an unexpected
behavior that I encountered while using the __enter__ method. I have
provided a code snippet below to illustrate the problem:

```
>>> class X:
...     __enter__ = int
...     __exit__ = lambda *_: None
...
>>> with X() as x:
...     pass
...
>>> x
0
```
As you can see, the __enter__ method does not throw any exceptions and
returns the output of "int()" correctly. However, one would normally expect
the input parameter "self" to be passed to the function.

On the other hand, when I implemented a custom function in place of the
__enter__ method, I encountered the following TypeError:

```
>>> def myint(*a, **kw):
...     return int(*a, **kw)
...
>>> class X:
...     __enter__ = myint
...     __exit__ = lambda *_: None
...
>>> with X() as x:
...     pass
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in myint
TypeError: int() argument must be a string, a bytes-like object or a real
number, not 'X'
```
Here, the TypeError occurred because "self" was passed as an input
parameter to "myint". Can someone explain why this unexpected behavior
occurs only in the latter case?

I tested this issue on the following Python versions, and the problem
persists on all of them:
- Python 3.8.10 (default, Nov 14 2022, 12:59:47) [GCC 9.4.0] on linux
- Python 3.10.10 (main, Feb  8 2023, 14:50:01) [GCC 9.4.0] on linux
- Python 3.10.7 (tags/v3.10.7:6cc6b13, Sep  5 2022, 14:08:36) [MSC v.1933
64 bit (AMD64)] on win32

I appreciate any input or insights that you might have on this matter.

Thank you for your help in advance!

Best regards,
Lorenzo Catoni