Python List & for-each access (Find/Replace in built-in list)

Question

I originally thought Python was a pure pass-by-reference language.

Coming from C/C++ I can\'t help but think about memory management, and it\'s hard to put it out of my

Answer 1

Answering this has been good, as the comments have led to an improvement in my own understanding of Python variables.

As noted in the comments, when you loop over a list with something like for member in my_list the member variable is bound to each successive list element. However, re-assigning that variable within the loop doesn't directly affect the list itself. For example, this code won't change the list:

my_list = [1,2,3]
for member in my_list:
    member = 42
print my_list

Output:

[1, 2, 3]

If you want to change a list containing immutable types, you need to do something like:

my_list = [1,2,3]
for ndx, member in enumerate(my_list):
    my_list[ndx] += 42
print my_list

Output:

[43, 44, 45]

If your list contains mutable objects, you can modify the current member object directly:

class C:
    def __init__(self, n):
        self.num = n
    def __repr__(self):
        return str(self.num)

my_list = [C(i) for i in xrange(3)]
for member in my_list:
    member.num += 42
print my_list

[42, 43, 44]

Note that you are still not changing the list, simply modifying the objects in the list.

You might benefit from reading Naming and Binding.