18.05.2015, 18:05, Guido van Rossum kirjoitti:
On Mon, May 18, 2015 at 12:14 AM, Alex Grönholm
<alex.gronh...@nextday.fi <mailto:alex.gronh...@nextday.fi>> wrote:
18.05.2015, 02:50, Guido van Rossum kirjoitti:
On Sun, May 17, 2015 at 3:07 PM, Alex Grönholm
<alex.gronh...@nextday.fi <mailto:alex.gronh...@nextday.fi>> wrote:
Looking at PEP 484, I came up with two use cases that I felt
were not catered for:
1. Specifying that a parameter should be a subclass of
another (example: Type[dict] would match dict or
OrderedDict; plain "Type" would equal "type" from builtins)
I don't understand. What is "Type"? Can you work this out in a
full example? This code is already okay:
def foo(a: dict):
...
foo(OrderedDict())
This code is passing an /instance/ of OrderedDict. But how can I
specify that foo() accepts a /subclass/ of dict, and not an
instance thereof?
A full example:
def foo(a: Type[dict]):
...
foo(dict) # ok
foo(OrderedDict) # ok
foo({'x': 1}) # error
You want the argument to be a *class*. We currently don't support that
beyond using 'type' as the annotation. We may get to this in a future
version; it is relatively uncommon. As to what notation to use,
perhaps it would make more sense to use Class and Class[dict], since
in the world of PEP 484, a class is a concrete thing that you can
instantiate, while a type is an abstraction used to describe the
possible values of a variable/argument/etc.
Also, what you gave is still not a full example, since you don't show
what you are going to do with that type. Not every class can be easily
instantiated (without knowing the specific signature). So if you were
planning to instantiate it, perhaps you should use Callable[..., dict]
as the type instead. (The ellipsis is not yet supported by mypy --
https://github.com/JukkaL/mypy/issues/393 -- but it is allowed by the
PEP.)
Here's one example, straight from the code of my new framework:
@typechecked
def register_extension_type(ext_type: str, extension_class: type,
replace: bool=False):
"""
Adds a new extension type that can be used with a dictionary based
configuration.
:param ext_type: the extension type identifier
:param extension_class: a class that implements IExtension
:param replace: ``True`` to replace an existing type
"""
assert_subclass('extension_class', extension_class, IExtension)
if ext_type in extension_types and not replace:
raise ValueError('Extension type "{}" already
exists'.format(ext_type))
extension_types[ext_type] = extension_class
I would like to declare the second argument as "extension_class:
Type[IExtension]" (or Class[IExtension], doesn't matter to me).
Likewise, the type hint for "extension_types" should be "Dict[str,
Type[IExtension]]".
1. Specifying that a callable should take at least the
specified arguments but would not be limited to them:
Callable[[str, int, ...], Any]
Case #2 works already (Callable[[str, int], Any] if the
unspecified arguments are optional, but not if they're
mandatory. Any thoughts?
For #2 we explicitly debated this and found that there aren't use
cases known that are strong enough to need additional flexibility
in the args of a callable. (How is the code calling the callable
going to know what arguments are safe to pass?) If there really
is a need we can address in a future revision.
Consider a framework where a request handler always takes a
Request object as its first argument, but the rest of the
arguments could be anything. If you want to only allow
registration of such callables, you could do this:
def calculate_sum(request: Request, *values):
return sum(values)
def register_request_handler(handler: Callable[[Request, ...], Any]):
...
Hm... Yeah, you'd be stuck with using Callable[..., Any] for now.
Maybe in a future version of the PEP. (We can't boil the ocean of
typing in one PEP. :-)
--
--Guido van Rossum (python.org/~guido <http://python.org/%7Eguido>)
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