Why Can't Python 3 List Comprehensions Directly Access Class Variables?

Accessing Class Variables from List Comprehensions

In Python 3, list comprehensions cannot directly access class variables defined within the class definition, as they operate within their own nested scope. This behavior differs from Python 2, where such access was possible.

Error Example

The following code demonstrates the issue in Python 3:

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

This code will raise a NameError for x, as the list comprehension cannot access the class variable.

Class Scope and List/Set/Dict Comprehensions

List, set, and dictionary comprehensions, as well as generator expressions, operate in a nested scope that does not inherit from the class scope. This is because such comprehensions are implemented as functions and run in a separate scope.

Workarounds

Despite the restriction, there are workarounds to access class variables from comprehensions:

1. Explicit Scope

Create a specific scope within the class definition to hold the variables needed by the comprehension:

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

2. Instance Variables

Use instance variables instead, by initializing them in the constructor:

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

3. Global Scope

If possible, move the variables and logic outside the class definition and use the global scope:

x = 5

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

Exception: Outermost Iterable

While accessing class variables directly in comprehensions is not possible, there is an exception for the outermost iterable. The expression that determines the outermost iterable can access class variables:

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

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