How to use a type defined later for function annotations?

Question

The problem is like this:

class A():
    def foo() -> B:
        pass

class B():
    def bar() -> A:
        pass

This will raise a NameError: name 'B' is not defined.

For the sake of type checking, I'm not willing to change -> B to -> "B". Is there any workaround?

Does this answer your question? [How do I type hint a method with the type of the enclosing class?](https://stackoverflow.com/questions/33533148/how-do-i-type-hint-a-method-with-the-type-of-the-enclosing-class) — mkrieger1, May 02 '23 at 08:39

score 5 · Answer 1 · edited May 02 '23 at 08:41

5

You can pass the first reference as a string (Python 3.5+):

class A():
    def foo() -> 'B':
        pass

class B():
    def bar() -> A:
        pass

It's called "forward reference" and was laid out in PEP484. See this answer for more ways and detailed information.

edited May 02 '23 at 08:41

mkrieger1

19,194
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answered May 29 '22 at 17:49

Karl Lorey

1,536
17
21

score 2 · Accepted Answer · edited May 02 '23 at 08:41

2

I found a workaround, which is much similar to C or C++.

Tested in Pycharm 3.

class A: pass
class B: pass

class A(object):

    def foo(self, b: B) -> B:

        #CAN auto complete
        b.bar()

        return B()

class B(object):

    def bar(self) -> A:

        return A()

#CAN auto complete
A().foo(B()).bar()

edited May 02 '23 at 08:41

mkrieger1

19,194
5
54
65

answered Oct 17 '13 at 12:25

Cauly

370
1
2
12

1

Actually, I would only pre-declare B, not both A and B. Like so: `class B: pass # pre-define name B`, so that the reader has less to wonder about. – Lutz Prechelt Mar 18 '14 at 11:25

score 1 · Answer 3 · answered Oct 16 '13 at 14:34

Well, there is no way to directly do it using the annotation syntax, but one can modify the __annotations__ dictionary of a function object.

class A():
    def foo():
        pass

class B():
    def bar() -> A:
        pass

A.foo.__annotations__["return"] = B

How to use a type defined later for function annotations?

3 Answers3