Why print cannot be used with for on the same line in python 2/3?

Question

I would love to implement the following:

print(i) for i in xrange(10)

However, this does not work in neither python2 nor python3. What is the limitation of the language? Or what did I do wrong?

Answer 1

Like this:

for i in xrange(10): print(i)

You could also do something like this, but only in Python 3 where print is a function. This is an abuse of generator expressions, though.

any(print(i) for i in range(10))

Answer 2

Set builder notation (called generator expressions in Python) only work when you are building a set. The notation you are using is set builder notation (generator expressions) (something like x for x in range(10) if ... ). That being said, you can use a regular for loop

for i in xrange(10):
    print(i)

or set builder notation building an empty set:

>>> x = [print(i) for i in xrange(10)]
0
1
2
3
4
5
6
7
8
9
>>> x
[None, None, None, None, None, None, None, None, None, None]
>>> 

The reason the set has all the none values is because you are never adding to the set, but only printing stuff out.

Answer 3

The form x for x in <sequence> is meant to result in a list. It was originally this:

>>> [i for i in range(10)] 
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

introduced in Python 2.0, Python Enhancement Proposal 202 , coming from a variation of uses of map() and filter(). Those functions, and list comprehensions, originate with functional programming environments, which tend to lean towards no-side-effects and no-surprise-state-changes and so on.

In the case where list comprehensions support a function call, it's intended for something like this simple transformation:

>>> [math.sin(x) for x in whatever]
[0.1, 0.2, ...]

The function is intended to change one list item into a new list item - and the output is still a list.

This syntax lost the surrounding brackets [ ] a couple of years later in PEP289 as they became more memory-efficient Generator Comprehensions in Python 2.4. These are intended for when you don't need the full list output as a list because you are processing all the contents as soon as you get them - eg sum(i for i in range(10)) can add up all the numbers as they appear, without waiting for the full sequence to finish computing first.

The way you want to use it - print(x) for x in <sequence> is in the form of a generator comprehension, but you don't want the generator, the list, the sequence. You only want to do it for a side effect which happens when you call the function.

That's not impossible, you can do this in Python 2

import sys
[sys.stdout.write(str(i) + '\n') for i in range(10)]

and as @wim notes in a comment, this in Python 3:

[print(i) for i in range(10)]

However, print() doesn't return anything useful, the newly built list or generator is irrelevant and immediately thrown away.

This structure is confusing/misleading to people reading your code, you are explicitly using the "I want to build a list" format when you don't want to build a list.

It's also against the original ideas of list comprehensions and their origins in functional programming - it's much easier to reason about things when they don't have side effects.

From the Zen of Python :

• Explicit is better than implicit.

If you want to iterate over a sequence and print something, explicitly iterate over a list and print it ( for item in sequence: print(item) ). Don't start with "I'm going to build a new list, oh I sneakily printed something as a side effect, then threw the list away".

It's hard to read your intent - did you mean to do that? What did you want it to do?

It's harder to explain - what does this code do? "It builds a new list of None s which are the return values from printing the list items". That's putting the focus of what's happening in all the wrong places.

Answer 4

Your attempt is syntactically incorrect but you can do actually do it in one line using print as a function and do it without the need for the explicit for loop:

print(*range(10), sep="\n")

Would be equivalent to:

 for i in range(10):
    print(i)

and output:

In [32]: print(*range(10),sep="\n")
0
1
2
3
4
5
6
7
8
9
In [2]: print(*range(10))
0 1 2 3 4 5 6 7 8 9

And in python2 you just need to import print_function :

from __future__ import print_function

In [3]: print(*xrange(10),sep="\n")
0
1
2
3
4
5
6
7
8
In [4]: print(*xrange(10))
0 1 2 3 4 5 6 7 8 9

or using str.join :

print("\n".join(map(str,range(10))))