skyking skyking - 25 days ago 7
Python Question

Why can't a module be a context manager (to a 'with' statement)?

Suppose we have the following

mod.py
:

def __enter__():
print("__enter__<")

def __exit__(*exc):
print("__exit__< {0}".format(exc))

class cls:
def __enter__(self):
print("cls.__enter__<")


def __exit__(self, *exc):
print("cls.__exit__< {0}".format(exc))


and the following use of it:

import mod

with mod:
pass


I get an error:

Traceback (most recent call last):
File "./test.py", line 3, in <module>
with mod:
AttributeError: __exit__


According to the documentation the documentation the
with
statement should execute as follows (I believe it fails at step 2 and therefore truncate the list):



  1. The context expression (the expression given in the with_item) is evaluated to obtain a context manager.

  2. The context manager’s
    __exit__()
    is loaded for later use.

  3. The context manager’s
    __enter__()
    method is invoked.

  4. etc...




As I've understood it there is no reason why
__exit__
could not be found. Is there something I've missed that makes a module not able to work as a context manager?

Answer

__exit__ is a special method, so Python looks it up on the type. The module type has no such method, which is why this fails.

See the Special method lookup section of the Python datamodel documentation:

For custom classes, implicit invocations of special methods are only guaranteed to work correctly if defined on an object’s type, not in the object’s instance dictionary.

Note that this applies to all special methods. For example, if you added a __str__ or __repr__ function to a module it'll not be called when printing the module, either.

Python does this to make sure type objects are hashable and representable too; if Python didn't do this then trying to put a class object into a dictionary would fail when a __hash__ method was defined for that class (as that method would expect an instance to be passed in for self).