In Python 3.5, the code
>>> T = map(print, [1, 2, 3])
>>> type(T)
<class 'map'>
returns a map object. I would expect this map object T to contain the numbers 1, 2 and 3; all on separate lines.
In actuality, this does happen. The only problem is that it also outputs a list of Nonevalues the same length as the input list.
>>> list(T)
1
2
3
[None, None, None]
>>>
This is repeatable for any input I use, not just the arbitrary integer list shown above. Can anyone explain why this happens?
See also:
https://stackoverflow.com/a/7731274
https://stackoverflow.com/a/11768129
https://stackoverflow.com/a/42399676
Each None that you see is what print function returns. To understand what map does, try the following code:
>>> T = map(lambda x: x**2, [1, 2, 3])
>>> t = list(T)
>>> print(t)
[1, 4, 9]
When you use print instead:
>>> T = map(print, [1, 2, 3])
>>> t = list(T)
1
2
3
>>> print(t)
[None, None, None]
This is not surprising, because:
>>> a = print("anything")
anything
>>> print(a)
None
