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

user_16897043 0
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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?

generator python
2 Answers

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]

user_40015920

This becomes more clear if you give each array a unique name instead of re-binding array:

array1 = [1, 8, 15]
gen = (x for x in array1 if array2.count(x) > 0)
array2 = [2, 8, 22]
print(list(gen))

x for x in array1 is evaluated at creation of the generator, but if array2.count(x) > 0 is evaluated lazily, which is why you can already reference a yet undefined variable

user_36886480
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