为什么zip对象消失了？

Question

Please see the code,why list(w) properly display ,and h display nothing ?

>>> x=[1,2,3]
>>> y=[4,5,6]
>>> w=zip(x,y)
>>> list(w)
[(1, 4), (2, 5), (3, 6)]
>>> h=list(w)
>>> h
[]

Answer 1

In Python 3 , zip returns an iterator ¹ .

Make an iterator that aggregates elements from each of the iterables.

An iterator remembers to where it has been iterated; at the h=list(w) line, the iterator is already "at the end" and thus results in an empty list/sequence.

Try with w = list(zip(x,y)) , which would force the iterator to a list once .

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¹ The zip from Python 2 returns a list, and thus this behavior is only exhibited in Python 3.