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pythongenerator

Why does redefining a variable used in a generator give strange results?

One of my friends asked me about this piece of code:

array = [1, 8, 15]
gen = (x for x in array if array.count(x) > 0)
array = [2, 8, 22]
print(list(gen))

The output:

[8]

Where did the other elements go?

Solution

  • The answer is in the PEP of the generator expressions, in particular the session Early Binding vs Late biding:

    After much discussion, it was decided that the first (outermost) for-expression should be evaluated immediately and that the remaining expressions be evaluated when the generator is executed.

    So basically the array in:

    x for x in array

    is evaluated using the original list [1, 8, 15] (i.e. immediately), while the other one:

    if array.count(x) > 0

    is evaluated when the generator is executed using:

    print(list(gen))

    at which point array refers to a new list [2, 8, 22]