CODEWITHSUNDEEP
pythonpython-2.7list-comprehension
Eli Rose
Eli Rose

Reputation: 6988

Calling print in the middle of a python list comprehension

So I'm trying to pull off a trick I actually first heard about on this site.

[i for i in range(0, 10) if True or print(i)]

The idea being that you can call an arbitrary function at every step of a listcomp by sticking it inside an "if" statement that will always return True. But that code gives a syntax error.

If I wrap the function I want to call like this, though:

def f(i):
    print i

[i for i in range(0, 10) if True or f(i)]

it produces the desired output. So I was wondering what the difference is, in Python's mind, between the two, because I can't tell what it could be -- both functions return "None", right?

Upvotes: 3

Views: 457

Answers (3)

Martijn Pieters
Martijn Pieters

Reputation: 1121356

You cannot mix statements (like print in Python 2) with a list comprehension.

However, you can make print() a function by adding:

from __future__ import print_function

at the top of your file. This turns print() into a function for the whole module.

However, you are using the statement True or something and that will never evaluate 'something' because Python boolean expressions short-circuit. You want to turn that around:

if print(something) or True

There is no point in evaluating the right-hand side of a or expression if the left-hand side already evaluated to True; nothing the right-hand side can come up with will make the whole expression False, ever.

You really want to avoid such side effects in a list comprehension though. Use a proper loop and keep such surprises out of your code, using if something or True is really a hack that will confuse future maintainers of your code (including you).

Upvotes: 6

Ashwini Chaudhary
Ashwini Chaudhary

Reputation: 250881

print is a statement in py2x not expression so it can't be used where an expression is expected.

You need to import print_function from __future__ in order to make it work.

In [105]: from __future__ import print_function

In [107]: [i for i in range(0, 10) if print(i) or True]
0
1
2
3
4
5
6
7
8
9
Out[107]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Note that as print() returns None so in your or condition it must be placed first or use an and condition : if True and not print(i)

Upvotes: 1

Peter DeGlopper
Peter DeGlopper

Reputation: 37319

In Python 2.x, print is not a function. It became a function in Python 3.

Upvotes: 5

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