Customizing module attribute access example raises RecursionError:

>>> import sys
>>> from types import ModuleType
>>> class VerboseModule(ModuleType):
...     def __repr__(self):
...         return f'Verbose {self.__name__}'
...     def __setattr__(self, attr, value):
...         print(f'Setting {attr}...')
...         setattr(self, attr, value)
...
>>> sys.modules[__name__].__class__ = VerboseModule
>>> sys.modules[__name__].a = 5
Setting a...
<...>
Setting a...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 6, in __setattr__
  File "<stdin>", line 6, in __setattr__
  File "<stdin>", line 6, in __setattr__
  [Previous line repeated 495 more times]
  File "<stdin>", line 5, in __setattr__
RecursionError: maximum recursion depth exceeded while calling a Python object
Setting a...>>>

Something like this can fix it:

def __setattr__(self, attr, value):
...         print(f'Setting {attr}...')
...         super().setattr(self, attr, value)

Thanks for the report Denis. Would you like to propose a PR for this? The file is at https://github.com/python/cpython/blob/master/Doc/reference/datamodel.rst . This was added as part of 5364b5cd757. I am adding Ivan who added the example for confirmation and review.

I'm not convinced that any change is needed, this is completely expected behaviour (and not special to modules).

The following code also raises RecursionError:

class VerboseObject:
    def __setattr__(self, nm, value):
        print(f"Setting {nm} to {value}")
        setattr(self, nm, value)

o = VerboseObject()
o.a = 42

This is because setattr() calls the __setattr__ method, which calls setattr() again, ... .


The fix is to call super().__setattr__ instead:

class VerboseObject:
    def __setattr__(self, nm, value):
        print(f"Setting {nm} to {value}")
        super().__setattr__(nm, value)

o = VerboseObject()
o.a = 42

Yes, this is expected, you should use ``super().__setattr__()``.

I understand that it's expected behavior. But why don't use completely working example in the docs, which one could just copy and paste? It requires to add just seven chars)

Ah, sorry, I didn't understand this was a documentation issue. Please feel free to submit a PR that fixes the example to use `super().__setattr__()`.

New changeset 0bee3c36d406e47fa9f99cfc1e07b701512c4f3f by Ivan Levkivskyi (Denis Osipov) in branch 'master':
bpo-35119: Fix RecursionError in example of customizing module attribute access. (GH-10323)
https://github.com/python/cpython/commit/0bee3c36d406e47fa9f99cfc1e07b701512c4f3f

New changeset 558dc8adbec0b85e0ff257fcedc85c5d89cd2825 by Miss Islington (bot) in branch '3.7':
bpo-35119: Fix RecursionError in example of customizing module attribute access. (GH-10323)
https://github.com/python/cpython/commit/558dc8adbec0b85e0ff257fcedc85c5d89cd2825