python equality precedence

Question

class L(object):
    def __eq__(self, other):
        print 'invoked L.__eq__'
        return False

class R(object):
    def __eq__(self, other):
        print 'invoked R.__eq__'
        return False

left = L()
right = R()

With this code, left side gets the first shot at comparison, as documented in the data model:

>>> left == right
invoked L.__eq__
False

But if we make a slight modification on line 6 (everything else the same):

class R(L):

Now the right side gets to have the first shot at comparison.

>>> left == right
invoked R.__eq__
False

Why is that? Where is it documented, and what's the reason for the design decision?

Answer 1

This is documented under the numeric operations, further down the page, with an explanation for why it works that way:

Note: If the right operand’s type is a subclass of the left operand’s type and that subclass provides the reflected method for the operation, this method will be called before the left operand’s non-reflected method. This behavior allows subclasses to override their ancestors’ operations.

The Python 3 documentation additionally mentions it in the section you were looking at:

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. Virtual subclassing is not considered.