testing if this is a container type

[buran]

You can always register separate callback for str. It will take precedence over the callback for the abstract class. Also it will propagate for any custom class that inherit from str.

import collections.abc
import functools
 
@functools.singledispatch
def foo(arg):
    print("that's not container", type(arg))
 
@foo.register(collections.abc.Container)
def _(arg):
    print('Now that is container', type(arg))
    
@foo.register(str)
def _(arg):
    print("that's str", type(arg))
    
class Foo(str):
    pass
 
 
test_items = [int(), str(), dict(), list(), Foo()]
 
for item in test_items:
    foo(item)

Output:
that's not container <class 'int'>
that's str <class 'str'>
Now that is container <class 'dict'>
Now that is container <class 'list'>
that's str <class '__main__.Foo'>
>>>

Note that you can use different names for the registered callbacks so that you can distinguish and check which callback will be used for certain class.