<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On Mon, May 18, 2015 at 12:14 AM, Alex Grönholm <span dir="ltr"><<a href="mailto:alex.gronholm@nextday.fi" target="_blank">alex.gronholm@nextday.fi</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<div>18.05.2015, 02:50, Guido van Rossum
kirjoitti:<br>
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<div class="gmail_quote">On Sun, May 17, 2015 at 3:07 PM, Alex
Grönholm <span dir="ltr"><<a href="mailto:alex.gronholm@nextday.fi" target="_blank">alex.gronholm@nextday.fi</a>></span>
wrote:<br>
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<div bgcolor="#FFFFFF" text="#000000"> Looking at PEP 484,
I came up with two use cases that I felt were not
catered for:<br>
<ol>
<li>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)</li>
</ol>
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<div>I don't understand. What is "Type"? Can you work this
out in a full example? This code is already okay:<br>
<br>
</div>
<div>def foo(a: dict):<br>
...<br>
<br>
</div>
<div>foo(OrderedDict())<br>
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This code is passing an <i>instance</i> of OrderedDict. But how can
I specify that foo() accepts a <i>subclass</i> of dict, and not an
instance thereof?<br>
<br>
A full example:<br>
<br>
def foo(a: Type[dict]):<br>
...<br> <br></div></blockquote><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000">
foo(dict) # ok<br>
foo(OrderedDict) # ok<br>
foo({'x': 1}) # error<span class=""><br></span></div></blockquote><div><br></div><div>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.<br><br>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 -- <a href="https://github.com/JukkaL/mypy/issues/393">https://github.com/JukkaL/mypy/issues/393</a> -- but it is allowed by the PEP.)<br></div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000"><span class="">
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<ol>
<li>Specifying that a callable should take at least
the specified arguments but would not be limited to
them: Callable[[str, int, ...], Any]</li>
</ol>
<p>Case #2 works already (Callable[[str, int], Any] if
the unspecified arguments are optional, but not if
they're mandatory. Any thoughts?<br>
</p>
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<div>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.<br>
</div>
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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:<br>
<br>
def calculate_sum(request: Request, *values):<br>
return sum(values)<br>
<br>
def register_request_handler(handler: Callable[[Request, ...],
Any]):<br>
... <span class=""></span></div></blockquote><div><br></div><div>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. :-) <br></div></div><br>-- <br><div class="gmail_signature">--Guido van Rossum (<a href="http://python.org/~guido" target="_blank">python.org/~guido</a>)</div>
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