For loop returning blank output when used in conjunction with map function

Question

I have defined a factorial function which looks as below:

def fact(n):
    return 1 if n < 2 else n * fact(n-1)

Now, I am running map function as below:

results = map(fact, range(1,5))
for x in results:
    print(x)

Output:
    1
    2
    6
    24

I am completely understanding the above output. However, if I run the same for loop again I am getting a blank output which I am not able to understand.

for x in results:
    print(x)

Output:

Answer 1

That's because in python3 map function returns a iterator. When you traverse generator it becomes exhausted and empty, so trying to traverse it again is like traverse empty list.

print(results) # <map object at 0x10a392310>

if you want to traverse it more than once, you can convert it to list

results = list(map(fact, range(1,5)))

Answer 2

results is a map object, which is a generator. Elements are generated as you iterate through the map, and it doesn't reset when you start a new iteration.

If you want it to behave like a list, you could explicitly convert the map to a list like this:

results = list(map(fact, range(1,5)))

or just use a list comprehension instead of map:

results = [fact(n) for n in range(1, 5)]