difference between `x in list` and `list.index(x)` for instances of a new-style class

[Riccardo Murri]

Hello,

I have some code that stops when trying to find a graph in a list of
similar graphs::

  (Pydb) list
  110    try:
  111        canonical = self.base[self.base.index(graph)]
  112    except ValueError:
  113        raise ValueError, \
  114              "Cannot find canonical representative for graph `%s`." \
  115  ->                  % (repr(graph),)
  116   
  ....

The list `self.base` contains "canonical" forms of the graphs and the
`graph` object must compare equal to some item of the list, which
indeed it does::

  (Pydb) p graph == self.base[27]  
  True

  (Pydb) p graph in self.base
  True

However, I cannot directly get the index of the canonical graph (the
number "27" above was found by manual inspection)::

  (Pydb) self.base.index(graph)
  *** ValueError: list.index(x): x not in list

All graphs are instances of a `Graph` new-style class that implements
comparison operators `__eq__` and `__ne__`, but no other rich-compare
stuff.

I'm using Python 2.5::

  Python 2.5 (release25-maint, Dec  9 2006, 16:17:58) 
  [GCC 4.1.2 20061115 (prerelease) (Debian 4.1.1-20)] on linux2

So my question is: what are the implementation differences between `x
in list` and `list.index(x)` and why can one report that an item is in
the list while the other cannot find its index?  Should I add
something to the `Graph` class so that `index` works?

Thanks for any hint!


-- 
Riccardo Murri, via Galeazzo Alessi 61, 00176 Roma