MutableSequence作为isinstance（）检查中的列表传递

Question

I built a custom list-like class based on collections.MutableSequence :

class MyList(collections.MutableSequence):
    etc... behave mostly like a list...

value = MyList([1,2,3])

Before processing list data, a third-party library runs a check:

def check_correct_type(value):
    assert isinstance(value, list)

I do not wish to convert my custom list-like object to a built-in list before passing it to the third-party library.

Is there an elegant way to make an instance of MyList appear as though it was an instance of list in the isinstance(MyList([1,2,3]), list) check?

Answer 1

No, there is no way instances of your class can pass that test without inheriting from list . You have to subclass list to pass that test.

You can try inheriting from both MutableSequence and list ; any method or attribute not implemented by your class or by MutableSequence will then be looked up on list , so you may get extra methods that you don't want this way and those may behave unexpectedly:

class MyList(collections.MutableSequence, list):

You could also monkeypatch the check_correct_type() function, provided it really is a stand-alone function like that:

def my_check_correct_type(value):
    assert isinstance(value, collections.MutableSequence)

third_party_library.check_correct_type = my_check_correct_type