Question

Why can b list be found in some_list even though it’s not in some_list?

>>> a = []
>>> b = []
>>> a is b
False
>>> some_list = [0, 1, 2, 3, a]
>>> some_list.index(b)
4

Answer 1

Python’s list.index searches tests for equality, not identity. a and b are not identical, but equal:

>>> a = []
>>> b = []
>>> a == b
True

Define __eq__ on your objects a,b if you want a different definition of equality, for example:

def __eq__(self, other):
  return self is other
def __hash__(self):
  return 0 # Insert a more sensible, object-specific hash function here