Question

I’m playing with for loops in Python and trying to get used to the way they handle variables.

Take the following piece for code:

a=[1,2,3,4,5]
b=a
b[0]=6

After doing this, the zeroth element of both b and a should be 6. The = sign points a reference at the array, yes?

Now, I take a for loop:

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

My expectation would be that every element of a is now 6, because I would imagine that i points to the elements in a rather than copying them; however, this doesn’t seem to be the case.

Clarification would be appreciated, thanks!

Answer 1

Everything in python is treated like a reference. What happens when you do b[0] = 6 is that you assign the 6 to an appropriate place defined by LHS of that expression.

In the second example, you assign the references from the array to i, so that i is 1, then 2, then 3, … but i never is an element of the array. So when you assign 6 to it, you just change the thing i represents.

http://docs.python.org/reference/datamodel.html is an interesting read if you want to know more about the details.