Test a difficult typing case. Python type annotations!

circular_types.py

"""Test a difficult typing case.

Inspired by biopython's Bio.PDB.Entity.

https://gist.github.com/sbliven/8fb593f005eeafc0fecef71063f5dc39
"""

import sys

from typing import TypeVar, Union

if sys.version_info >= (3, 8):
    from typing import Protocol
else:
    from typing_extensions import Protocol


T = TypeVar("T")
ParentT = TypeVar("ParentT", bound="ParentOf", covariant=True)
ChildT = TypeVar("ChildT", bound="ChildOf", covariant=True)
Offspring = Union["Daughter", "Son"]


class ChildOf(Protocol[ParentT]):
    def get_parent(self):
        # type: () -> ParentT
        pass


class ParentOf(Protocol[ChildT]):
    def get_child(self):
        # type: () -> ChildT
        pass


class Mom(ParentOf[Offspring]):
    def __init__(self, child):
        # type: (Offspring) -> None
        self.child = child  # type: Offspring

    def get_child(self):
        # type: () -> Offspring
        return self.child


class Daughter(ChildOf["Mom"]):
    def __init__(self, parent):
        # type: (Mom) -> None
        self.parent = parent  # type: Mom

    def get_parent(self):
        # type: () -> Mom
        return self.parent


class Son(ChildOf["Mom"]):
    def __init__(self, parent):
        # type: (Mom) -> None
        self.parent = parent  # type: Mom

    def get_parent(self):
        # type: () -> Mom
        return self.parent