Is func_globals mutable?

Question

The Python documentation states:

__globals__ func_globals

A reference to the dictionary that holds the function’s gl

Answer 1

First, note that func.func_globals and func.__globals__ are two references to the same thing:

Changed in version 2.6: The double-underscore attributes __closure__, __code__, __defaults__, and __globals__ were introduced as aliases for the corresponding func_* attributes for forwards compatibility with Python 3.

In other words:

>>> def foo(bar):
    pass

>>> foo.__globals__ is foo.func_globals
True

When they are referred to as "read-only" attributes, this is because you cannot reassign the attribute:

>>> foo.__globals__ = {}

Traceback (most recent call last):
  File "<pyshell#3>", line 1, in <module>
    foo.__globals__ = {}
TypeError: readonly attribute

However, the object returned is mutable, as it's a dictionary. Therefore you can add keys to the dictionary and remove them from it (although I would recommend against removing any until you're sure you know what you're doing!):

>>> foo.func_globals
{'__builtins__': <module '__builtin__' (built-in)>, 
 '__package__': None, 
 '__name__': '__main__', 
 'foo': <function foo at 0x02ED15F0>, 
 '__doc__': None}
>>> foo.func_globals['bar'] = 'baz'
>>> del foo.func_globals['__doc__']
>>> foo.func_globals
{'bar': 'baz', 
 '__builtins__': <module '__builtin__' (built-in)>, 
 '__package__': None, 
 '__name__': '__main__', 
 'foo': <function foo at 0x02ED15F0>}