function inside list comprehension - is it evaluated multiple times [duplicate]

Question

Which one's a better way of doing list comprehension in python (in terms of computation time & cpu cycles). In example (1) is the value f(r) evaluated in each iteration or is it evaluated once and cached ?

1. y = [x*f(r) for x in xlist]

2. c = f(r)

   y = [x*c for x in xlist]

where

def f(r):
    ... some arbitrary function ...

Answer 1

It evaluates for every iteration. Look at this:

>>> def f():
...     print("func")
... 
>>> [f() for i in range(4)]
func
func
func
func
[None, None, None, None]

As you say, if f() has no side effects, storing the return value on a variable and using that variable instead is a lot faster solution.

Answer 2

I would probably choose the latter because the Python compiler doesn't know if the function has side-effects so it is called for each element.

Answer 3

Here is an easy way to find out:

>>> def f():
...     print "called"
...     return 1
...
>>> [1+f() for x in xrange(5)]
called
called
called
called
called
[2, 2, 2, 2, 2]

so yes, if the function is going to be the same each time then it is better to call it once outside the list comprehension.