filter object becomes empty after iteration? [duplicate]

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This is a classic python3 doh!.

A filter is a special iterable object you can iterate over. However, much like a generator, you can iterate over it only once. So, by calling list(people2), you are iterating over each element of the filter object to generate the list. At this point, you’ve reached the end of the iterable and nothing more to return.

So, when you call list(people2) again, you get an empty list.

Demo:

>>> l = range(10)
>>> k = filter(lambda x: x > 5, l)
>>> list(k)
[6, 7, 8, 9]
>>> list(k)
[]

I should mention that with python2, filter returns a list, so you don’t run into this issue. The problem arises when you bring py3’s lazy evaluation into the picture.

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