Lazy Built-ins
In Python 3, zip(), map(), and filter() all return lazy iterators, not lists. Wrap in list() to materialize.
Python
Beginner
3 min read
Lazy Built-ins: zip, map, filter
Example
nums = [1, 2, 3, 4, 5]
doubled = map(lambda x: x * 2, nums) # lazy iterator
print(type(doubled)) # <class "map">
print(list(doubled)) # [2, 4, 6, 8, 10] Pro Tip
map/filter return iterators in Python 3; in Python 2 they returned lists.