why we can not apply map function to print python?

Question

I wonder why 'print' can not be used in python map function? I assume print is also a function so this should work as map works on functions as documentation: map(function, iterable, ...)

a=[1,2,3,4]
map(print,a)

I think you cannot in python 2 because print is a function only in python 3. — Jean-François Fabre, Jan 31 '17 at 15:46
you should tag this question as "python 2" because it's very likely a python-2 only problem. — Jean-François Fabre, Jan 31 '17 at 15:51
@scott, print prints the items given to it in python. Adding dictionary was a mistake sorry for that. — curious, Jan 31 '17 at 15:57
@ScottHunter: if you want to tag it with `map`, it gives `dictionary`. Might be an explanation. — Willem Van Onsem, Jan 31 '17 at 16:13
@Jean-FrançoisFabre: `from __future__ import print_function` would make it work in Python 2 as well.. — Martijn Pieters, Jan 31 '17 at 16:29
@Willem, You are right about the tag. I first tried map but got a dictionary. I do not have reputation to add a tag. — curious, Jan 31 '17 at 16:37
Possible duplicate of [Why map(print, a_list) doesn't work?](https://stackoverflow.com/questions/11768095/why-mapprint-a-list-doesnt-work) (if Python 2) or [Print doesn't print when it's in map](https://stackoverflow.com/questions/7731213/print-doesnt-print-when-its-in-map-python) (if Python 3) — user202729, Aug 04 '18 at 02:54

score 2 · Answer 1 · answered Jan 31 '17 at 15:46

You are not iterating over the map() object, so no function calls are made. map() does not immediately apply the calls; only as you drab results from the iterator, are elements from the input sequence drawn and have the print() function applied.

If you looped over the map() object the code works; for example, by using the list() call:

>>> a = [1, 2, 3, 4]
>>> it = map(print, a)
>>> list(it)
1
2
3
4
[None, None, None, None]

Willem Van Onsem · Answer 2 · 2017-01-31T16:03:46.287

1

In python-3.x you actually can apply print in a map. The print will always return None but that's not really a problem.

The only problem with python-3.x is that map works lazy: it returns a generator that will only calculate elements if they are needed. In order to get Python to work, you need to materialize the outcome of map somehow. You can for instance do this with list(..):

$ python3
Python 3.5.2 (default, Nov 17 2016, 17:05:23) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> a=[1,2,3,4]
>>> list(map(print,a))
1
2
3
4
[None, None, None, None]

In pyton-2.7, print is not a function and thus you cannot provide it to map.

edited Jan 31 '17 at 16:03

answered Jan 31 '17 at 15:46

Willem Van Onsem

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Willem, Thanks for the reply. why is it printing none?Should it be printing elements in the list as defined by the definition of map? – curious Jan 31 '17 at 16:01
@curious: because the `print` function does **not `return`s** something and in that case Python will let it return `None`. So the last line is the result of the map. – Willem Van Onsem Jan 31 '17 at 16:02
Got it, It returns none because print does not return anything – curious Jan 31 '17 at 16:04
@curious: indeed, it prints on the stdout, but does not - in any case - generates a result. – Willem Van Onsem Jan 31 '17 at 16:04

Jean-François Fabre · Answer 3 · 2017-01-31T20:54:31.213

In Python 2, print is not a function so map(print,[]) triggers invalid syntax.

Python 2 workaround would be to use a real function to write to standard output. sys.stdout.write would almost be a candidate, but you have to add a linefeed + string conversion to roughly emulate what print does:

map(lambda x: sys.stdout.write(str(x)+"\n"),a)

It works in python 3 (but map doesn't unless you force iteration)

EDIT: from __future__ import print_function also allows to use print as a function

why we can not apply map function to print python?

3 Answers3