Function singledispatch

Lib/functools.py:896–1012

Single-dispatch generic function decorator. Transforms a function into a generic function, which can have different behaviours depending upon the type of its first argument. The decorated function acts as the default implementation, and additional implementations can be registered u

(func)

    return registry.get(match)

def singledispatch(func):
    """Single-dispatch generic function decorator.

    Transforms a function into a generic function, which can have different
    behaviours depending upon the type of its first argument. The decorated
    function acts as the default implementation, and additional
    implementations can be registered using the register() attribute of the
    generic function.
    """
    # There are many programs that use functools without singledispatch, so we
    # trade-off making singledispatch marginally slower for the benefit of
    # making start-up of such applications slightly faster.
    import weakref

    registry = {}
    dispatch_cache = weakref.WeakKeyDictionary()
    cache_token = None

    def dispatch(cls):
        """generic_func.dispatch(cls) -> <function implementation>

        Runs the dispatch algorithm to return the best available implementation
        for the given *cls* registered on *generic_func*.

        """
        nonlocal cache_token
        if cache_token is not None:
            current_token = get_cache_token()
            if cache_token != current_token:
                dispatch_cache.clear()
                cache_token = current_token
        try:
            impl = dispatch_cache[cls]
        except KeyError:
            try:
                impl = registry[cls]
            except KeyError:
                impl = _find_impl(cls, registry)
            dispatch_cache[cls] = impl
        return impl

    def _is_valid_dispatch_type(cls):
        if isinstance(cls, type):
            return True
        return (isinstance(cls, UnionType) and
                all(isinstance(arg, type) for arg in cls.__args__))

    def register(cls, func=None):
        """generic_func.register(cls, func) -> func

        Registers a new implementation for the given *cls* on a *generic_func*.

        """
        nonlocal cache_token
        if _is_valid_dispatch_type(cls):
            if func is None:
                return lambda f: register(cls, f)
        else: