On 21/04/2023 00:44, Lorenzo Catoni wrote:
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?
Cameron is right, it's the descriptor protocol. Technically
inst.attr
invokes attr.__get__(...) if it exists:
>>> class A:
def __get__(self, *args): return args
>>> class B: pass
>>> class X:
a = A()
b = B()
>>> x = X()
>>> x.b
<__main__.B object at 0x02C2E388>
>>> x.a
(<__main__.X object at 0x02C2E280>, <class '__main__.X'>)
Python functions support the descriptor protocol
>>> hasattr(lambda: None, "__get__")
True
while builtin functions don't:
>>> hasattr(ord, "__get__")
False
I'm unsure whether to regard int as a class or or function, but as there
is no __get__
>>> hasattr(int, "__get__")
False
it behaves like builtin functions in this case.
--
https://mail.python.org/mailman/listinfo/python-list
