Creating a class within a function and access a function defined in the containing function’s scope

by

That’s an artifact of Python’s name resolution rules: you only have access to the global and the local scopes, but not to the scopes in-between, e.g. not to your immediate outer scope.

EDIT: The above was poorly worded, you do have access to the variables defined in outer scopes, but by doing x = x or mymethod = mymethod from a non-global namespace, you’re actually masking the outer variable with the one you’re defining locally.

In example 2, your immediate outer scope is the global scope, so MyClass can see mymethod, but in example 4 your immediate outer scope is my_defining_func(), so it can’t, because the outer definition of mymethod is already masked by its local definition.

See PEP 3104 for more details about nonlocal name resolution.

Also note that, for the reasons explained above, I can’t get example 3 to run under either Python 2.6.5 or 3.1.2:

>>> def myfunc():
...     x = 3
...     class MyClass(object):
...         x = x
...     return MyClass
... 
>>> myfunc().x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in myfunc
  File "<stdin>", line 4, in MyClass
NameError: name 'x' is not defined

But the following would work:

>>> def myfunc():
...     x = 3
...     class MyClass(object):
...         y = x
...     return MyClass
... 
>>> myfunc().y
3

More Related Contents:

  1. Using global variables in a function
  2. What does __all__ mean in Python?
  3. Variable scopes in Python classes
  4. What exactly does “import *” import?
  5. How do I create a list of lambdas (in a list comprehension/for loop)?
  6. What’s the scope of a variable initialized in an if statement?
  7. Python ElementTree module: How to ignore the namespace of XML files to locate matching element when using the method “find”, “findall”
  8. Nested classes’ scope?
  9. Block scope in Python
  10. except-clause deletes local variable
  11. Advanced Nested List Comprehension Syntax
  12. type object ‘datetime.datetime’ has no attribute ‘datetime’
  13. Django: “projects” vs “apps”
  14. Is it bad practice to use a built-in function name as an attribute or method identifier?
  15. Namespaces with Module Imports
  16. Python – Why is this class variable not defined in the method?
  17. How to inject variable into scope with a decorator?
  18. Python: Sharing global variables between modules and classes therein
  19. Tensorflow: How to get a tensor by name?
  20. The scope of names defined in class block doesn’t extend to the methods’ blocks. Why is that?
  21. What are Python namespaces all about
  22. What is the scope of a defaulted parameter in Python?
  23. What does if __name__ == “__main__”: do in Python?
  24. Python: Why should ‘from import *’ be prohibited?
  25. Access self from decorator
  26. Why do I get a `NameError` (or `UnboundLocalError`) from using a named exception after the `except` block?
  27. Python: Name resolution; order of function def’s
  28. Python modules with identical names (i.e., reusing standard module names in packages)
  29. Why can functions in Python print variables in enclosing scope but cannot use them in assignment? [duplicate]
  30. Scope of variable within “with” statement?

Leave a Comment