exec function: import unknown in class definition

I want to use Python files to configure a program. The files will be read via exec. Part of the configuration are class definitions:

config.py

import collections as col  # module alias does not matter

a = col.defaultdict()  # works as expected

class Foo:
    b = col.defaultdict()  # causes NameError: name 'col' is not defined

main.py

CONFIG = {}

with open('config.py') as f:
    exec(f.read(), None, CONFIG)

Now, when running main.py I'll have the problem that outside of the class definition the imported module is known but inside it is not. Why and is there a workaround?

Solution

According to exec's documentation,

If exec gets two separate objects as globals and locals, the code will be executed as if it were embedded in a class definition.

So your config.py code is equivalent to

class SomeClass:
    import collections as col
    a = col.defaultdict()
    class Foo:
        b = col.defaultdict()

This is a problem because, according to Resolution of Names,

The scope of names defined in a class block is limited to the class block[.]

col is defined in the class block of the implicit SomeClass. This variable's scope is limited to that class block only; even blocks inside that block, such as class Foo:, can't access it.

One possible solution is to pass the same object for globals and locals so that your code is not executed as if it were embedded in a class definition.

exec(f.read(), CONFIG, CONFIG)