I am not taking sides now, but I want to share with you a useful diagram to reason about typing support in Python.
I struggled to explain what Python offers until I came up with this diagram: https://standupdev.com/wiki/doku.php?id=python_protocols#the_typing_map The Typing Map has two orthogonal axis: - when are types checked: -- runtime checking -- static checking - how are type checked: -- structural types -- nominal types The quadrants are informally labeled with the terms in ALL CAPS below. Traditionally, mainstream languages supported one of two diagonally opposite quadrants: STATIC TYPING and DUCK TYPING. Now the situation is more complicated. - Java supports only STATIC TYPING: static checking of nominal types; Python started supporting nominal types with PEP 484 - Before ABCs, Python supported only DUCK TYPING: runtime checking of structural types; - With ABCs, Python started supporting GOOSE TYPING (a term invented by Alex Martelli, in cc because I just quoted him): runtime checking of nominal types (with subclass hook which is a backdoor to support explicit checks on structural types as well); - With PEP 544, Python started supporting STATIC DUCK TYPING: static checking of structural types; There are languages that support multiple quadrants: - TypeScript, like Python, supports all four quadrants. - Go supports STATIC TYPING, but it also famously popularized STATIC DUCK TYPING, and even supports GOOSE TYPING with features like type assertions and type switches [1] designed for explicit runtime checking of nominal or structural types. [1] https://tour.golang.org/methods/16 The Typing Map will be featured in my upcoming PyCon US talk [2] [2] https://us.pycon.org/2021/schedule/presentation/80/ Cheers, Luciano PS. If you are aware of other languages that support more than one of these quadrants, please let me know! On Tue, Apr 20, 2021 at 6:53 PM Eric Casteleijn <thisf...@gmail.com> wrote: > > > > On Tue, Apr 20, 2021 at 10:03 AM Mark Shannon <m...@hotpy.org> wrote: >> >> ... >> PEP 544 supports structural typing, but to declare a structural type you >> must inherit from Protocol. >> That smells a lot like nominal typing to me. > > > Note that to implement a protocol you do not have to inherit from anything. > You create a structural type that subclasses Protocol, but then any object > that satisfies that protocol can be passed where that type is expected, > without having to inherit anything, so I would argue that this really is > structural typing. > > > -- > - eric casteleijn (he/him) > _______________________________________________ > Python-Dev mailing list -- python-dev@python.org > To unsubscribe send an email to python-dev-le...@python.org > https://mail.python.org/mailman3/lists/python-dev.python.org/ > Message archived at > https://mail.python.org/archives/list/python-dev@python.org/message/R3VP4KORAWI6KK4CNFL6JNYCATWR47EV/ > Code of Conduct: http://python.org/psf/codeofconduct/ -- Luciano Ramalho | Author of Fluent Python (O'Reilly, 2015) | http://shop.oreilly.com/product/0636920032519.do | Technical Principal at ThoughtWorks | Twitter: @ramalhoorg _______________________________________________ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-le...@python.org https://mail.python.org/mailman3/lists/python-dev.python.org/ Message archived at https://mail.python.org/archives/list/python-dev@python.org/message/DYFZORZKWAB7BFCXMDPNTGSWH6IPPIRL/ Code of Conduct: http://python.org/psf/codeofconduct/
