pyduck is a Python utility framework for more effective usage of language's defining traits: duck typing. It enhances the language with several useful features that increase the readability and reliability of Python code.

from pyduck import Interface, expects, Any, overload

class Iterable(Interface):
    def __iter__(self): pass

@expects(Iterable)
def __iterable_as_json(value):
    json_list = map(as_json, value)
    return "[%s]" % str.join(",", json_list)

@expects(basestring)
def __string_as_json(value):
    return '"%s"' % str(value)

@expects(Any)
def __other_as_json(value):
    return str(value)

as_json = overload(__iterable_as_json, __string_as_json, __other_as_json)

• interfaces which do not need to be explicitly declared (similar to Go language)
• automatic interface/type checking for function arguments
• function overloading based on interfaces/types of arguments
• automatic interface/type checking for function return values

For more info & the docs, see the project's webpage.