Hi Luciano,
On 20/04/2021 11:35 pm, Luciano Ramalho wrote:
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
That's really nice, thanks.
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/
_______________________________________________
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/UXBP7AII3Z4Z7COK7P4OETAR7J5F5XP5/
Code of Conduct: http://python.org/psf/codeofconduct/
