At first glance, your statement is incorrect: there is no Python code to create a list here, and no list comprehensions. On further examination, though, string_join in stringobject.c invokes PySequence_Fast to convert its iterable argument into a sequence --- a temporary list, in this case --- so your statement that this code generates a temporary list is correct, although I suspect that this is more by accident than because you have a deep knowledge of the implementation of the Python standard library.

The temporary list of temporary strings probably will use less memory than the original dict did, though, so it's only a very mild sort of "explosion". It will, however, be several times bigger than the final output string, which also has to be huge if the dictionary is huge.

Python doesn't have anything like a StringBuffer. If it did, it would be reasonable for string_join to use it instead of generating a temporary list. The Python code above would look the same.

But hey, if you just want to print the values, you could say

    sys.stdout.printlines("%s: %s\n" % (k, v) for k, v in mydict.items())

but frankly I think I would write instead

    for k, v in mydict.items():
        print "%r: %r" % (k, v)

He is using parentheses instead of square brackets, so the code is actually using a generator expression which will produce each value lazily.

Aside from the fact that I'm using a generator, where does it iterate twice? I don't see it.