NameError in class definition using list comprehension with if statement

Python Help
1

This code (a simplified example from the real code) works:

class A:
    data = ' '
    remains = [c for c in data]

The following code produces NameError: name 'data' is not defined:

class A:
    data = ' '
    remains = [c for c in data if c in data]

I created an issue on the Python bugtracker and the answer was a link to the “Naming and Binding” section of the docs where it says:

Class definition blocks and arguments to exec() and eval() are special in the context of name resolution. A class definition is an executable statement that may use and define names. These references follow the normal rules for name resolution with an exception that unbound local variables are looked up in the global namespace. The namespace of the class definition becomes the attribute dictionary of the class. The scope of names defined in a class block is limited to the class block; it does not extend to the code blocks of methods – this includes comprehensions and generator expressions since they are implemented using a function scope.

I understand this definition. But - and this is a big “but” - if it is like this, shouldn’t the first code example also trigger an error?

2

I answer my own question since I was pointed to another part of the documentation (Displays for lists, sets and dictionaries) where it says:

The iterable expression in the leftmost for clause is evaluated directly in the enclosing scope and then passed as an argument to the implicitly nested scope. Subsequent for clauses and any filter condition in the leftmost for clause cannot be evaluated in the enclosing scope as they may depend on the values obtained from the leftmost iterable.

This behaviour is unexpected (at least for me) and spread over the documentation, but it is documented and there is a reason why it is as it is. Case closed.