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[EDIT] FILE: _abc.py

import abc
import sys
import types
from collections.abc import Mapping, MutableMapping

class _TypingMeta(abc.ABCMeta):
    # A fake metaclass to satisfy typing deps in runtime
    # basically MultiMapping[str] and other generic-like type instantiations
    # are emulated.
    # Note: real type hints are provided by __init__.pyi stub file
    if sys.version_info >= (3, 9):

def __getitem__(self, key):
            return types.GenericAlias(self, key)

else:

def __getitem__(self, key):
            return self

class MultiMapping(Mapping, metaclass=_TypingMeta):
    @abc.abstractmethod
    def getall(self, key, default=None):
        raise KeyError

@abc.abstractmethod
    def getone(self, key, default=None):
        raise KeyError

class MutableMultiMapping(MultiMapping, MutableMapping):
    @abc.abstractmethod
    def add(self, key, value):
        raise NotImplementedError

@abc.abstractmethod
    def extend(self, *args, **kwargs):
        raise NotImplementedError

@abc.abstractmethod
    def popone(self, key, default=None):
        raise KeyError

@abc.abstractmethod
    def popall(self, key, default=None):
        raise KeyError

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