[issue44192] Annotations, Inheritance and Circular Reference

[ReOb]

New submission from ReOb <contact.reob at gmail.com>:

Basically, I have:

```
class Base:
    _sub: list[Sub]

class Sub:
    _parent: Base
```

What creates a circular reference.

It could be solved doing:

```
class Sub:
    pass

class Base:
    _sub: list[Sub]

class Sub:
    _parent: Base
```

But in the annotation, I will get the class Sub without his annotations, methods and properties (the first definition is not updated in annotation after the redefinition).

Then, I suggest one of three possible solutions:

1. Update the reference to class in annotations if the class was redefined
2. Add a way to forward declaration (it could be implemented by a new reserved word "forward")
3. Or, like Ruby, make that the redefinition of a class, open the class to change the definition of class, including new methods, properties and annotations.

It is very important to have the correct type in annotation, because we use this type to create new objects dinamically, but actually is impossible to solve this problem because we can not have the correct definition at both classes: Base and Sub class.

When we use the type annotated to create a new object, as it is, we loose the annotations, methods and properties defined at the redefinition of the class.

This problem occurs only if we use annotations.

Like Python added the possibility to define annotations, needs to provide one of these solutions pointed (or another) to solve problems like this.

If the decision was to implement a forward reserved word, it could be solved doing:

```
class Sub:
    forward

class Base:
    _sub: list[Sub]

class Sub:
    _parent: Base
```

If the first or third solution was preferred this code will work:

```
class Sub:
    pass

class Base:
    _sub: list[Sub]

class Sub:
    _parent: Base
```

I hope Python could implement one of these solutions to provide a better support for annotations, because actually what we have are two classes, with the same name, that walks like a duck, swim like a duck, sound like a duck but they are not the same duck, and they should be.

The framework that we are developping depends strongly that we could get the correct type from annotation to create the correct object based on this.

I hope it could be handled as soon as possible, because change the definition to another class that was a super class of both classes (Base and Sub, in this example) will not permit us to know the correct class to create another object dinamically and probabily we will need to create a hack to handle this, making the code less Pythonic.

Thanks.

----------
components: Parser
messages: 394038
nosy: lys.nikolaou, pablogsal, reob-info
priority: normal
severity: normal
status: open
title: Annotations, Inheritance and Circular Reference
type: resource usage
versions: Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue44192>
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