This puzzled me at first, but I think others have nailed it. It is not
to do with the 'with' statement, but with the way functions are defined.
When a class is instantiated, as in x=X():
the instance object gets (at least in effect), as attributes,
copies of functions defined *in the class* (using def or lambda) but
they become "bound methods", i.e. bound to the instance. Whenever they
are called, they will be called with the instance as the first argument,
aka self:
class X(object):
def func(*args, **kargs): pass
x = X()
y = ()
x.func and y.func are two *different" functions. When x.func is called,
x is added as the first argument. When y.func is called. y is added as
the first argument.
boundFunc = y.func
boundFunc() # Adds y as first argument.
Indeed, these functions have an attribute called __self__ whose value is
... you guessed it ... the object they are bound to
When a function is defined outside of a class, it remains a simple
function, not bound to any object. It does not have a __self__
attribute. Neither does a built-in type such as 'int'.
Nor for that matter does the class function X.func:
X.func() # Called with no arguments
Best wishes
Rob Cliffe
On 20/04/2023 23:44, 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?
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
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