Why can't you assign to a list in a loop without enumerate?

[Danny Colligan]

In the following code snippet, I attempt to assign 10 to every index in
the list a and fail because when I try to assign number to 10, number
is a deep copy of the ith index (is this statement correct?).

>>> a = [1,2,3,4,5]
>>> for number in a:
...     number = 10
...
>>> a
[1, 2, 3, 4, 5]

So, I have to resort to using enumerate to assign to the list:

>>> for i, number in enumerate(a):
...     a[i] = 10
...
>>> a
[10, 10, 10, 10, 10]

My question is, what was the motivation for returning a deep copy of
the value at the ith index inside a for loop instead of the value
itself?  Also, is there any way to assign to a list in a for loop (with
as little code as used above) without using enumerate?

Thanks,

Danny