Question

How does Python __contains__ work for lists if I use arbitrary objects for elements?
Will it revert to the is operator? Or does it use __eq__ if provided?

A simple test gave

class Test: pass

print(Test() in [Test()]) # -> no
a=Test() 
print(a in [a])   # -> yes
print(a in [Test()])  # -> no

So can I assume it uses the comparison by references is?

Answer 1

The default __contains__ implementation for list just compares the object with the == (__eq__) operator.
It is this operator that, if not defined for a class, defaults to the behavior of is.

In short, “a in lst” is the equivalent to:

def contains(obj, lst):
   for item in lst:
       if obj == lst: 
          return True
   return False

contains(a, lst)

The behavior of == does change whether item implements __eq__ explicitly, and defaults to object identity (is) otherwise.