Fix Python – List comprehension rebinds names even after scope of comprehension. Is this right?


Asked By – Jabavu Adams

Comprehensions show unusual interactions with scoping. Is this the expected behavior?

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

At the risk of whining, this is a brutal source of errors. As I write new code, I just occasionally find very weird errors due to rebinding — even now that I know it’s a problem. I need to make a rule like “always preface temp vars in list comprehensions with underscore”, but even that’s not foolproof.
The fact that there’s this random time-bomb waiting kind of negates all the nice “ease of use” of list comprehensions.

Now we will see solution for issue: List comprehension rebinds names even after scope of comprehension. Is this right?


List comprehensions leak the loop control variable in Python 2 but not in Python 3. Here’s Guido van Rossum (creator of Python) explaining the history behind this:

We also made another change in Python
3, to improve equivalence between list
comprehensions and generator
expressions. In Python 2, the list
comprehension “leaks” the loop control
variable into the surrounding scope:

x = 'before'
a = [x for x in 1, 2, 3]
print x # this prints '3', not 'before'

This was an artifact of the original
implementation of list comprehensions;
it was one of Python’s “dirty little
secrets” for years. It started out as
an intentional compromise to make list
comprehensions blindingly fast, and
while it was not a common pitfall for
beginners, it definitely stung people
occasionally. For generator
expressions we could not do this.
Generator expressions are implemented
using generators, whose execution
requires a separate execution frame.
Thus, generator expressions
(especially if they iterate over a
short sequence) were less efficient
than list comprehensions.

However, in Python 3, we decided to
fix the “dirty little secret” of list
comprehensions by using the same
implementation strategy as for
generator expressions. Thus, in Python
3, the above example (after
modification to use print(x) 🙂 will
print ‘before’, proving that the ‘x’
in the list comprehension temporarily
shadows but does not override the ‘x’
in the surrounding scope.

This question is answered By – Steven Rumbalski

This answer is collected from stackoverflow and reviewed by FixPython community admins, is licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0