Many functions within Python 3 return an Iterator instead of a list by default. This is in order to help conserve memory and avoid unnecessary computation.
For example the map(), filter() and range() functions should ideally return a sequence. Instead the return value is an iterable. To consume its values we have to pass it to the list() function or iterate over it.
This comes in handy if the input list if large. For example, processing a list with 1 million values in a single go might not be the most efficient approach. Iterators fix this issue by processing values when needed.
Note you can pass the filter iterator to map iterator without converting the values to a list. Most Python functions support an iterable as an input.