[Python-ideas] Delay evaluation of annotations

[Neil Girdhar]

I don't understand why that would work and this clearly doesn't?

Mutual2 = "Mutual2" # Pre-declare Mutual2

class Mutual1:
    def spam(self, x=Mutual2):
        print(type(x))

class Mutual2:
    def spam(self):
        pass

Mutual1().spam()

prints class "str" rather than "type".


On Tue, Sep 27, 2016 at 6:20 AM Stephen J. Turnbull <
turnbull.stephen.fw at u.tsukuba.ac.jp> wrote:

> Neil Girdhar writes:
>
>  > I agree that circularity should in general be avoided, but it's not
> always
>  > possible or elegant to do that.  Sometimes you really need two classes
> to
>  > refer to each other.  In that case, why not expose your placeholder
> idea to
>  > the user via a library?
>
> Why not just expose it through a simple assignment?
>
> https://mail.python.org/pipermail/python-ideas/2016-September/042563.html
>
> Note that this also works for typechecking in PEP 484 checkers that
> allow forward reference via the stringified class name.  See also
> https://mail.python.org/pipermail/python-ideas/2016-September/042544.html,
> which would allow eliding the assignment, but pollutes the class namespace.
>
> "Simple is better than complex."
>
> This feature is still looking for a persuasive use that needs it, and
> not something simpler.
>
> Steve
>
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