Support recursive types

[o11c]

The following in particular would be useful:

Callback = Callable[[str], 'Callback']
Foo = Union[str, List['Foo']]

[JukkaL]

If (or when) mypy will have structural subtyping, recursive types would also be useful there.

[JukkaL]

My current plan is to postpone recursive types until simple structural subtyping is in and reconsider them later. After thinking about them more they are going to increase complexity a lot of I haven't seen much evidence for them being needed that often.

[oconnor663]

If I'm understanding this issue right, I'm running into it for classes that want a fluent interface. So for example, if I want callers to do something like this:

myfoo.add(bar).add(baz).finish()

Then the definition of the Foo class and the add method need to look something like this:

class Foo:
    def add(self, x) -> Foo:  # Python chokes on this line!
        # do stuff
        return self

Another place where Python commonly does return self is in the __enter__ method for context managers. Is mypy able to typecheck those right now?

[refi64]

@oconnor663 Try:

class Foo:
    def add(self, x) -> 'Foo':
        # do stuff
        return self

[oconnor663]

Ah, thank you.

[oconnor663]

@kirbyfan64, do you know if there are standard functions anywhere that understand this convention? Like, if I wanted to introspect a couple functions and compare the types of their arguments, should I handle the Foo == "Foo" case explicitly? That seems doable, but a string-aware version of say isinstance seems harder.

[refi64]

@oconnor663 I don't think there's anything like that. If you're introspecting the functions via a decorator, you could try accessing the caller's globals and locals.

[gvanrossum]

You're aware of typing.get_type_hints(obj)right? It is similar to obj.annotations` but expands forward references.
https://docs.python.org/3/library/typing.html?highlight=typing#typing.get_type_hints

There used to be an instance() implementation in typing.py but Mark Shannon
made me take it out. It's being deleted in this rev:
python/typing@ac7494f

On Fri, Oct 16, 2015 at 9:37 AM, Ryan Gonzalez notifications@github.com
wrote:

@oconnor663 https://github.com/oconnor663 I don't think there's
anything like that. If you're introspecting the functions via a decorator,
you could try accessing the caller's globals and locals.

—
Reply to this email directly or view it on GitHub
#731 (comment).

--Guido van Rossum (python.org/~guido)

[oconnor663]

@gvanrossum that's exactly what I was looking for, thanks. Sorry for the n00b questions today, but awesome that all this is supported.

[JukkaL]

Mypy should detect missing string literal escapes (see #948).

[dmoisset]

Going back to the original point on the issue, I found a case in the stdlib where this would be needed; the type for isinstance() is currently:

def isinstance(o: object, t: Union[type, Tuple[type, ...]]) -> bool: ...

but it should actually be:

ClassInfo = Union[type, Tuple['ClassInfo', ...]]
def isinstance(o: object, t: ClassInfo) -> bool: ...

Because according to https://docs.python.org/3/library/functions.html#isinstance the tuples can be nested. I found an actual example of this while typechecking django.http.response.HttpResponse.content