Such preconditions (checks of input parameters beyond their type at compile-tine) are a feature of DbC: Design by Contract. https://en.wikipedia.org/wiki/Design_by_contract Python is listed under "Languages with third-party support" :
Python, using packages like icontract, PyContracts, Decontractors, dpcontracts, zope.interface, PyDBC or Contracts for Python. A permanent change to Python to support Design by Contracts was proposed in PEP-316, but deferred
Second, icontract allows inheritance of the contracts and supports weakining of the preconditions as well as strengthening of the
https://www.python.org/dev/peps/pep-0316/ (2003) "Contracts in python -- a report & next steps" (2018) https://mail.python.org/archives/list/python-ideas@python.org/thread/3TDEJI4... ... https://github.com/Parquery/icontract : postconditions and invariants. Notably, weakining and strengthening of the contracts is a feature indispensable for modeling many non-trivial class hierarchies. Please see Section Inheritance. To the best of our knowledge, there is currently no other Python library that supports inheritance of the contracts in a correct way. On Sun, Jun 14, 2020, 12:29 PM Christopher Barker <pythonchb@gmail.com> wrote:
On Sun, Jun 14, 2020 at 8:41 AM Paul Moore <p.f.moore@gmail.com> wrote:
As Chris A says, I'd be inclined to see how far we can get with (extended) type hints before going for new syntax, though.
def foo() -> {1, 2, 3}: return 2
That is, of course, valid syntax right now. I wonder what a type checker could do with it? Similarly,
Well, the thing is that there is no way to know at compile time what Value is getting passed in -- this is really more a way to catch a ValueError than TypeError, so can't be done with static type checking.
Unless you do, as ChrisA suggests, crate a Type (and Enum) that you can then check for.
But while I like the idea of Enums, particularly for, say multi-valued flags, They require an extra name and extra typingthat I find annoying (annoying enough that I haven't used one yet in my code. That is, I prefer, so use Chris A's example:
some_function(1, 2) ...
to:
from some_module import Spam
some_function(Spam(1), 2) ...
That being said, if you want your API to be "safe" and type-chackable, then Enum is the way to go.
As for the OP, who was asking for a run-time check, if:
def fun(a, b): if a not in {1,2,3}: raise ValueError("a has to be one of these values: 1, 2, 3")
is too verbose, you can always write a utility function (or a callable class or closure that stores the options) to make it a simple one-liner:
def fun(a, b): value_check(a, options= {1, 2, 3})
-CHB
-- Christopher Barker, PhD
Python Language Consulting - Teaching - Scientific Software Development - Desktop GUI and Web Development - wxPython, numpy, scipy, Cython _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-leave@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/MPST6D... Code of Conduct: http://python.org/psf/codeofconduct/