When I convert a map object to list object in Python using list(), and apply list() again to this list, why does the list become empty?

Question

Please follow along this code:

def addition(n): 
    return n + n

numbers = (1, 2, 3, 4) 
result = map(addition, numbers) 
print(list(result))

Output: [2, 4, 6, 8]

Now when I apply list() again on result, which has already become a list, result turns out to be empty.

list(result)

Output: []

Why is this so?

Printing `list(list(result))` gives the original result, are you perhaps trying `list(print(list(result)))`? The return of `print()` is `None`. But running your code and just adding `print(list(list(result)))` gives the expected result list. — Collin Heist, Oct 23 '20 at 18:15

Prune · Accepted Answer · 2020-10-23T18:19:04.137

3

The map object is a generator.

list(result)

involves iterating through the iterator. The iterator is now finished. When you try again, there is no data to return, so you have an empty list. If you reset the iterator in some way, you can get the expected results.

This is something like reading an entire file, and then wondering why your next read loop doesn't return anything: you're at the end.

edited Oct 23 '20 at 18:19

answered Oct 23 '20 at 18:16

Prune

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To be pedantic, `map` objects are *not* generators, but the are iterators – juanpa.arrivillaga Oct 23 '20 at 18:18
Good point ... I chose an inappropriate "simplification". Not good; fixing ... You're being **accurate**, not pedantic. Thanks. – Prune Oct 23 '20 at 18:18

When I convert a map object to list object in Python using list(), and apply list() again to this list, why does the list become empty?

1 Answers1