Python Language Tutorial => Leaked variables in list comprehension

Example

Python 2.x2.3

x = 'hello world!'
vowels = [x for x in 'AEIOU'] 

print (vowels)
# Out: ['A', 'E', 'I', 'O', 'U']
print(x)
# Out: 'U'

Python 3.x3.0

x = 'hello world!'
vowels = [x for x in 'AEIOU']

print (vowels)
# Out: ['A', 'E', 'I', 'O', 'U']
print(x)
# Out: 'hello world!'

As can be seen from the example, in Python 2 the value of x was leaked: it masked hello world! and printed out U, since this was the last value of x when the loop ended.

However, in Python 3 x prints the originally defined hello world!, since the local variable from the list comprehension does not mask variables from the surrounding scope.

Additionally, neither generator expressions (available in Python since 2.5) nor dictionary or set comprehensions (which were backported to Python 2.7 from Python 3) leak variables in Python 2.

Note that in both Python 2 and Python 3, variables will leak into the surrounding scope when using a for loop:

x = 'hello world!'
vowels = []
for x in 'AEIOU':
    vowels.append(x)
print(x)
# Out: 'U'

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Python Language Incompatibilities moving from Python 2 to Python 3 Leaked variables in list comprehension

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