Name localization

Now that we are familiar with all types of comprehensions and generator expressions, let us talk about name localization within them. Python 3 localizes loop variables in all four forms of comprehensions: list, dictionary, set, and generator expressions. This behavior is different from that of the for loop. Let us look at some simple examples to show all the cases:

# scopes.py
A = 100
ex1 = [A for A in range(5)]
print(A)  # prints: 100
ex2 = list(A for A in range(5))
print(A)  # prints: 100
ex3 = {A: 2 * A for A in range(5)}
print(A)  # prints: 100
ex4 = {A for A in range(5)}
print(A)  # prints: 100
s = 0
for A in range(5):
    s += A
print(A)  # prints: 4

In the preceding code, we declare a global name, A = 100. We then have list, dictionary, and set comprehensions, and a generator expression. Even though they all use the name A, none of them alter the global name, A. On the other hand, the for loop at the end does modify the global A. The last print statement...