MutableSequence to pass as a list in isinstance() check

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?

Sounds like a fork or at least issue report on that 3rd-party lib would be needed ;) — Antti Haapala -- Слава Україні, Mar 08 '15 at 13:10

score 2 · Accepted Answer · answered Mar 08 '15 at 12:52

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

MutableSequence to pass as a list in isinstance() check

1 Answers1