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The documented behavior of list.index(o) is, and I suspect  
always has been (at least the first part)
"s.index(x) return smallest i such that s[i] == x" 
"Raises ValueError when x is not found in s."

If the matching were on identity, [some-literal].index(some-
literal) would raise ValueError instead of returning 0 if 
some-literal were not interned or recorded in some way so 
that both references were to the same object.  Making
.index() dependent on the (changing) details of object 
implementation and interning would make it both 
surprising, version-dependent, and pretty useless.

Since .list() was added 12 years ago, there have been two 
changes recorded in the CVS log: 1) flag multiple args as 
an error and 2) switch from list_compare() to the newer 
list_richcompare() (2 years ago)..  This latter perhaps 
fixed a previous bug.

When you write a .__eq__(s,o) method returning True, you 
are saying that you *want* the interpreter to see s==0 as 
True!  So either don't do that, or write an 'iddex' function 
that does what you want.