Why Can't I Access Class Variables Directly in Python 3 List Comprehensions Within a Class Definition?

Accessing Class Variables from a List Comprehension in the Class Definition

In Python 3, accessing class variables from a list comprehension within the class definition is not allowed due to scoping limitations. The class scope is not considered a valid lookup scope for variables used in the list comprehension.

Error Example:

class Foo:
    x = 5
    y = [x for i in range(1)]

This code would raise a NameError: name 'x' is not defined in Python 3.

Cause: Scoping Restrictions

Python follows strict scoping rules, and the class scope is separate from the scope of functions, loops, and comprehensions. Comprensions always execute within their own scope, which includes any variables declared within the comprehension itself.

In Python 2, this was not an issue because list comprehensions were implemented using a shortcut that allowed access to the enclosing class scope. However, this behavior was considered an inconsistency and was changed in Python 3 to enforce proper scoping.

Exception: Outermost Iterable

While the innermost iterable of a list comprehension can't access class variables, the outermost iterable expression can. This is because the outermost iterable is evaluated in the surrounding scope:

class Foo:
    x = 5
    y = [i for i in range(x)]  # This works fine

Workarounds:

Explicit Function:

Create a function within the class that has access to class variables and uses a list comprehension:

class Foo:
    x = 5

    def get_y(self):
        return self.x, [x for i in range(self.x)]

Instance Variable:

Initialize an instance variable in the constructor using a list comprehension:

class Foo:
    def __init__(self):
        self.y = [self.x for i in range(1)]

Global Variable:

Declare the variable outside the class and use it in the list comprehension:

x = 5

class Foo:
    y = [x for i in range(1)]

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