How to modify elements of iterables with iterators? I.e. how to get write-iterators in Python?

Question

I like the Python syntax a lot, but as I'm coming from C++ I don't get one thing about iterators in Python. In C++ there are 2 kinds of iterators - constant and modifying (non-const). In python it seems (for what I've seen) like there is only the first kind and if you want to modify the elements, you have to use the indexing, which doesn't feel comfortable and so generic to me. Let me illustrate with a simple example:

ab = ["a", "b"]
for (index, lett) in enumerate(ab):
    print "Firstly, lett is ab[index]?", lett is ab[index]
    lett = str(index)
    print lett
    print ab[index]
    print "But after, lett is ab[index]?", lett is ab[index]

So I wasn't able to modify the list with the iterator. It just makes Lazy copies (see Wikipedia) as I discovered with the is operator, so is there a way to say I want it to be a directly modifying iterator instead using the neat

for variable in iterable_object:

syntax?

Answer 1

The syntax

for x in iterable

does not create any lazy copies -- it assigns the exact objects in the list to x one after the other. If these objects are mutable, you can modify them:

a = [[1, 2], [3, 4]]
for x in a:
    x.append(5)
print a

prints

[[1, 2, 5], [3, 4, 5]]

Your example uses a list of strings. Strings are immutable in Python, so you can't modify them.