What is the order of execution of __eq__ if one side inherits from the other? [duplicate]

Question

I recently stumbled upon a seemingly strange behavior regarding the order in which __eq__ methods are executed, if one side of the comparison is an object which inherits from the other.

I've tried this in Python 3.7.2 in an admittedly academic example. Usually, given an equality comparison a == b, I expect a.__eq__ to be called first, followed by b.__eq__, if the first call returned NotImplemented. However, this doesn't seem to be the case, if a and b are part of the same class hierarchy. Consider the following example:

class A(object):
  def __eq__(self, other):
    print("Calling A({}).__eq__".format(self))
    return NotImplemented

class B(A):
  def __eq__(self, other):
    print("Calling B({}).__eq__".format(self))
    return True

class C(object):
  def __eq__(self, other):
    print("Calling C({}).__eq__".format(self))
    return False

a = A()
b = B()
c = C()

print("a: {}".format(a)) # output "a: <__main__.A object at 0x7f8fda95f860>"
print("b: {}".format(b)) # output "b: <__main__.B object at 0x7f8fda8bcfd0>"
print("c: {}".format(c)) # output "c: <__main__.C object at 0x7f8fda8bcef0>"

a == a # case 1

a == b # case 2.1
b == a # case 2.2

a == c # case 3.1
c == a # case 3.2

In case 1, I expect a.__eq__ to be called twice and this is also what I get:

Calling A(<__main__.A object at 0x7f8fda95f860>).__eq__
Calling A(<__main__.A object at 0x7f8fda95f860>).__eq__

However, in cases 2.1 and 2.2, b.__eq__ is always executed first, no matter on which side of the comparison it stands:

Calling B(<__main__.B object at 0x7f8fda8bcfd0>).__eq__ # case 2.1
Calling B(<__main__.B object at 0x7f8fda8bcfd0>).__eq__ # case 2.2

In cases 3.1 and 3.2 then, the left-hand side is again evaluated first, as I expected:

Calling A(<__main__.A object at 0x7f8fda95f860>).__eq__ # case 3.1
Calling C(<__main__.C object at 0x7f8fda8bcef0>).__eq__ # case 3.1
Calling C(<__main__.C object at 0x7f8fda8bcef0>).__eq__ # case 3.2

It seems that, if the compared objects are related to each other, __eq__ of the object of the child class is always evaluated first. Is there a deeper reasoning behind this behavior? If so, is this documented somewhere? PEP 207 doesn't mention this case, as far as I can see. Or am I maybe missing something obvious here?

Answer 1

From the official documentation for __eq__ :

If the operands are of different types, and right operand’s type is a direct or indirect subclass of the left operand’s type, the reflected method of the right operand has priority, otherwise the left operand’s method has priority.