[Python-Dev] PEP 443 - Single-dispatch generic functions (original) (raw)

Ethan Furman ethan at stoneleaf.us
Thu May 23 19:44:13 CEST 2013

User API ========

To define a generic function, decorate it with the @singledispatch decorator. Note that the dispatch happens on the type of the first argument, create your function accordingly: .. code-block:: pycon >>> from functools import singledispatch >>> @singledispatch ... def fun(arg, verbose=False): ... if verbose: ... print("Let me just say,", end=" ") ... print(arg) To add overloaded implementations to the function, use the register() attribute of the generic function. It takes a type parameter: .. code-block:: pycon >>> @fun.register(int) ... def (arg, verbose=False): ... if verbose: ... print("Strength in numbers, eh?", end=" ") ... print(arg) ... >>> @fun.register(list) ... def (arg, verbose=False): ... if verbose: ... print("Enumerate this:") ... for i, elem in enumerate(arg): ... print(i, elem) To enable registering lambdas and pre-existing functions, the register() attribute can be used in a functional form: .. code-block:: pycon >>> def nothing(arg, verbose=False): ... print("Nothing.") ... >>> fun.register(type(None), nothing)

So to have a generic mapping function that worked on dicts, namedtuples, user-defined record types, etc., would look something like:

--> from functools import singledispatch

--> @singledispatch --> def mapping(d): ... new_d = {} ... new_d.update(d) ... return new_d ...

--> @mapping.register(tuple) ... def _(t): ... names = getattr(t, '_fields', ['f%d' % n for n in range(len(t))]) ... values = list(t) ... return dict(zip(names, values)) ...

--> @mapping.register(user_class): ... def _(uc): ... blah blah ... return dict(more blah) ...

Very cool. I'm looking forward to it!

Oh, the tuple example above is intended primarily for named tuples, but since there is no common base class besides tuple I had to also handle the case where a plain tuple is passed in, and personally I'd rather have generic field names than raise an exception.

-- Ethan

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