double convert map to list return null-CodePudding

I am trying to convert a map to list:

a=map(float, [1,2,3])
list(a)   #first
list(a)   #second

If I do list once(run to the row of #first), it returns [1,2,3] as I expect. However, if I do list twice, it returns []. I don't know what the problem is. Any comments are appreciated. Thanks.

CodePudding user response：

The whole reason to convert a map to a list is that a map is a single-use iterator; once you iterate over it once, it's exhausted ("empty") because it doesn't actually store its contents anywhere.

A list stores all of its values and can be iterated over multiple times (it creates new iterators on demand).

The solution is to turn the map into a list once and save a reference to the list:

a=map(float, [1,2,3])
b = list(a)
# now you can iterate over b as much as you want

Note also that in place of map you can simply do a list comprehension to create a list in a single step:

a = [float(i) for i in [1, 2, 3]]
# now you can iterate over a as much as you want