Add "Slice" type to typing module
Usually slices are created with integers such as `foo[1:5]`. However slices are not restricted to having integers for the `start, stop, step` parameters and thus can be used with any types. The most prominent example is probably `pandas` which allows slicing by index (and the index can be `str` for example): df = pd.DataFrame([[0], [1], [2]], index=list('abc'), columns=['x']) print(df.loc['a':'b']) # The first two items, 'a' and 'b' inclusive. In one of my projects I also employed the slice syntax in a similar way. Sometimes more fine-grained control is desired, e.g. only allowing `str` slices and not `int` slices. For that purpose it would be constructive to explicitly indicate the type: class Foo: def __getitem__(self, index: Slice[str]): pass foo = Foo() foo[1:5] # type checker complains foo['foo':'bar'] # no complaints here The syntax would be somewhat similar to `slice` itself, with the difference that `Slice[T]` is similar to `Slice[T, T]`, i.e. specifying both the `start` and `stop` type. Now with [PEP 585](https://www.python.org/dev/peps/pep-0585/#importing-of-typing) using `slice` directly in annotations seems to be the preferred way, so I'm not even sure whether it is realistic to ask for such a new type being added to `typing`? Then, what else would it require for this idea to become part of the language (if accepted)? Best regards, Dominik
Hi Dominik, There's an existing "typing" issue open for this: https://github.com/python/typing/issues/159 You may meet more people interested in discussing additions to the typing module in the typing-sig mailing list. Finally, be aware that PEP 585 is a draft and not even one that's submitted for review -- ATM it's just a bunch of ideas by one person. --Guido On Wed, Aug 28, 2019 at 12:13 PM Dominik Vilsmeier <dominik.vilsmeier@gmx.de> wrote:
Usually slices are created with integers such as `foo[1:5]`. However slices are not restricted to having integers for the `start, stop, step` parameters and thus can be used with any types. The most prominent example is probably `pandas` which allows slicing by index (and the index can be `str` for example):
df = pd.DataFrame([[0], [1], [2]], index=list('abc'), columns=['x']) print(df.loc['a':'b']) # The first two items, 'a' and 'b' inclusive.
In one of my projects I also employed the slice syntax in a similar way. Sometimes more fine-grained control is desired, e.g. only allowing `str` slices and not `int` slices. For that purpose it would be constructive to explicitly indicate the type:
class Foo: def __getitem__(self, index: Slice[str]): pass
foo = Foo() foo[1:5] # type checker complains foo['foo':'bar'] # no complaints here
The syntax would be somewhat similar to `slice` itself, with the difference that `Slice[T]` is similar to `Slice[T, T]`, i.e. specifying both the `start` and `stop` type.
Now with [PEP 585]( https://www.python.org/dev/peps/pep-0585/#importing-of-typing) using `slice` directly in annotations seems to be the preferred way, so I'm not even sure whether it is realistic to ask for such a new type being added to `typing`? Then, what else would it require for this idea to become part of the language (if accepted)?
Best regards, Dominik _______________________________________________ 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/P4UVHU... Code of Conduct: http://python.org/psf/codeofconduct/
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Dominik Vilsmeier -
Guido van Rossum