why does isinstance check for abc.Sequence return False for custom classes?

user_7161823 0
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I've read in the abc python module docs that a Sequence is something that implements the following: __getitem__, __len__, __contains__, __iter__, __reversed__, index, and count.

Yet, when I run the following example it yields false:

from collections import abc

class Sequence():
    def __getitem__(self):
        pass
    def __len__(self):
        pass
    def index(self):
        pass
    def count(self):
        pass
    def __contains__(self):
        pass
    def __iter__(self):
        pass
    def __reversed__(self):
        pass

print(isinstance(Sequence(), abc.Sequence)) # False

When I was doing similar stuff for abc.Collection or abc.Reversed to play around I got the results I expected, for example I created a dummy class that implements __contains__, __iter__, __len__ and it was in fact detected correctly as an instance of abc.Collection.

Do you have any idea what's wrong with Sequence?

EDIT 1:

from collections import abc
class CustomIterable:
  def __iter__(self):
    pass
print(isinstance(CustomIterable(), abc.Iterable)) # True

even though my custom iterable is not from abc it still is recognized as Iterable because it implements __iter__ special method.

sequence python-3.x class python types
2 Answers

I had a similar question today.

From what I can gather, collections.abc.Iterable implements a custom __subclasshook__() method, whereas collections.abc.Sequence does not:

# _collections_abc.py

class Iterable(metaclass=ABCMeta):

    __slots__ = ()

    @abstractmethod
    def __iter__(self):
        while False:
            yield None

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Iterable:
            return _check_methods(C, "__iter__") # True if __iter__ implemented
        return NotImplemented

What this means is that if a class Foo defines the required __iter__ method then isinstance(Foo(), collections.abc.Iterable) will return True:

from collections.abc import Iterable

class Foo:
    def __iter__(self):
        return []

assert isinstance(Foo(), Iterable)

I'm not sure why collections.abc.Iterable implements a custom __subclasshook__() method but collections.abc.Sequence does not.

user_146699120

Your custom Sequence class is different from the abc.Sequence class so isinstance will return false.

If you're looking for True, your custom class needs to inherit from abc.Sequence:

class Sequence(abc.Sequence):
   .....
user_138780340
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