Question

I have a for loop as follows:

a=[1,2,3,4,5]
for i in a:
    i=6

What I would like is for every element of a to become 6.

Now I know that this for loop won’t do it, because I am merely changing the what i refers to.

I instead could write:

a=[1,2,3,4,5]
for i in len(range(a)):
    a[i]=6

But that doesn’t seem very Pythonic. What are good ways of doing this sort of thing? Obviously, setting everything to 6 is contrived example and, in reality, I would be doing more complicated (and wonderful) things. Therefore, answers should be generalised.

Answer 1

The for-variable is always a simple value, not a reference; there is no way to know that it came from a list and thus write back to the list on changing.

The len(a) approach is the usual idiom, although you need range (or xrange) too:

for i in range(len(a)):

or, just as commonly, use the enumerate function to get indexes as well as values:

for i, v in enumerate(a):
    a[i]= v+1

a list comprehension might be a good alternative, eg:

a= [6 for v in a]