Printing using list comprehension

Question

From my Python console

>>> numbers = [1,2,3]
>>> [print(x) for x in numbers]
1
2
3
[None, None, None]

Why does this print three none's at the end?

Answer 1

Alex Taylor gave the correct answer.

The print function returns None. The list comprehension is applying the function to each element of the list (outputting the results), but collecting those None objects into the results array. The Python interpreter is printing that array.

The None output can be consumed:

>>> numbers = [1,2,3]
>>> _ = [print(x) for x in numbers]
1
2
3
>>> _
[None, None, None]
>>>

Answer 2

You get a list of None values because print function returns None and list comprehension uses the value of the expressions, not what is printed.

However, you can use the next code (with caution) to obtain the desired behavior inside the list comprehension:

>>> [i for i in range(3) if print(i) or True]
0
1
2
[0, 1, 2]

The i-values are printed when is evaluated the if statement. This if guarantees call the print function and return True, so it doesn't discriminate values from the list.

Answer 3

[print(x) for x in numbers] is a list without values. so it returns none. you can simply do the followings:

print(*numbers)

1 2 3 or

for x in numbers:
 print(x)

1 2 3

Answer 4

For completeness, the * operator can be used also in combination with f-Strings

print(*(f'this is {x}' for x in [1,2,3]), sep='\n')

this is 1
this is 2
this is 3

Answer 5

You can use the String Join() method and print the string. For example:

>>> print('\n'.join(numbers))
1
2
3
>>> print(', '.join(numbers))
1, 2, 3
>>> print(',\n'.join(numbers))
1,
2,
3

Answer 6

3 ways to print using list comps:

1. print outside

print([(i) or i for i in range(4)])

2. create a function

def printVal(val): print("val: ", val) return val

[printVal(i) or i for i in range(4)]

3. Use 'or'

[print(i) or i for i in range(4)]

Answer 7

If this behaviour of the Print function bothers, I suggest you don't use the Print function in the list comprehension, just use the following one-liner which is compact and more Pythonic:

>>> [x for x in [1,2,3]]
[1, 2, 3]

Answer 8

You should restructure your loop to send arguments to print():

>>> numbers = [1,2,3]
>>> print(*(x for x in numbers), sep='\n')

Note that you don't need the explicit generator. Just unpack the list itself:

>>> numbers = [1,2,3]
>>> print(*numbers, sep='\n')

Answer 9

List comprehensions always return a list.

Based on this information, your print() statement must wrap the whole list comprehension argument:

Numbers = [1, 2, 3]

print([x for x in Numbers])

If you want to print items of a list one by one, a for loop is more suitable for this matter.

Answer 10

An ugly way of doing this is _=[print(i) for i in somelist] It does work but is not encouraged:)

Answer 11

print is a function, it's just like

>>>def f(x):
...:   pass
>>>[f(x) for x in numbers]

Answer 12

A list comprehension is not the right tool for the job at hand. It'll always return a list, and given that print() evaluates to None, the list is filled with None values. A simple for loop works better when we're not interested in creating a list of values, only in evaluating a function with no returned value:

for x in numbers:
    print(x)

Answer 13

The print function returns None. The list comprehension is applying the function to each element of the list (outputting the results), but collecting those None objects into the results array. The Python interpreter is printing that array.

Answer 14

print is a function in Python 3, which returns a None. Since you are calling it three times, it constructs a list of three None elements.