Why Does Python 2\'s List Comprehension Scope Differ from Python 3?

List Comprehensions: A Source of Confusing Scoping

One peculiar aspect of list comprehensions in Python 2 is their unusual interaction with variable scoping. Specifically, the loop control variable of a list comprehension "leaks" into the surrounding scope, leading to potential errors and confusion.

Consider the following code:

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

In Python 2, this code will unexpectedly print 4 instead of "original value". This is because the loop control variable x used in the list comprehension temporarily shadows the x defined outside the comprehension. This shadowing persists even after the comprehension is completed.

This behavior can be a significant source of frustration, as it can introduce subtle and hard-to-debug errors. Moreover, it undermines the convenience and simplicity typically associated with list comprehensions.

In Python 3, however, this behavior has been addressed. List comprehensions no longer leak the loop control variable into the surrounding scope. This brings them in line with generator expressions, which have always behaved in this manner.

According to Guido van Rossum, the creator of Python, the original leaky implementation of list comprehensions was an "intentional compromise" made to optimize their performance. However, in Python 3, this compromise was deemed unnecessary due to performance improvements.

The elimination of this leaky behavior in Python 3 has greatly improved the reliability and consistency of list comprehensions. By eliminating the potential for unintended variable shadowing, programmers can now use them with greater confidence.

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