On: Static types in Python

[voidiac]

Python's implementation of sum doesn't care about types, the function looks something like:

    def sum(iterable):
        r = 0
        for v in iterable:
            r += v
        return v

it always starts with an integer, and since int.__add__(str) returns NotImplemented it tries str.__radd__(int) instead, which raises an AttributeError because str has no method __radd__, so no summing of strings and integers.

[clusmore → voidiac]

Actually Python does explicitly reject strings.

  >>> sum(['hello', ' world'], '')
  TypeError: sum() can't sum strings [use ''.join(seq) instead]

But this is done explicitly against strings. They would _just work_ if not for this restriction, because based on usage they should work.