Why Does Python 2\'s List Comprehension Rebind Variables, and How Did Python 3 Change This?

List Comprehensions and Name Rebinding

List comprehensions are a concise syntax for creating lists in Python. However, they exhibit unexpected behavior in interaction with scoping.

Consider the following code:

x = "original value"
squares = [x**2 for x in range(5)]
print(x)  # Prints 4 in Python 2!

In this example, the list comprehension rebinds the variable x to the current value from the iteration of range(5). This behavior is known as name rebinding.

Reason for Name Rebinding

In Python 2, list comprehensions were implemented differently from generator expressions. List comprehensions were optimized for speed by leaking the loop control variable into the surrounding scope. Generator expressions, on the other hand, used a separate execution frame, preventing this leakage.

Python 3's Change

In Python 3, this distinction was removed. List comprehensions now use the same implementation as generator expressions. As a result, name rebinding no longer occurs in Python 3.

Consequences

Name rebinding can lead to unexpected behavior and errors, especially in cases where the same variable name is used in both the list comprehension and the surrounding scope. As mentioned in the question, it can be mitigated by using underscore prefixes for temporary variables in list comprehensions.

Guido van Rossum, the creator of Python, explained the history behind this change: In Python 2, list comprehensions leaked the loop control variable as an artifact of the initial implementation to optimize performance. However, in Python 3, this was deemed a dirty little secret that should be fixed by adopting the same implementation strategy as generator expressions.

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