Name localization

Now that we are familiar with all types of comprehensions and generator expression, let's talk about name localization within them. Python 3.* localizes loop variables in all four forms of comprehensions: list, dict, set, and generator expressions. This behavior is therefore different from that of the for loop. Let's see a simple example 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 = dict((A, 2 * A) for A in range(5))
print(A)  # prints: 100

ex4 = set(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, and then we exercise the four comprehensions: list, generator expression, dictionary, and set. None of them alter the global name, A. Conversely, you can see at the end that the for loop modifies it. The last print statement prints 4.

Let's see what happens...