Can a simple a==b 'hang' in and endless loop?

Terry Hancock hancock at
Thu Jan 19 12:19:23 EST 2006

On Wed, 18 Jan 2006 17:10:09 +0100
Claudio Grondi <claudio.grondi at> wrote:
> or when the objects being members of the list redefine
> __eq__ so, that  no matter how different they are, the
> lists always compare True.

If those objects have redefined __eq__ so that they are all
equal in value to each other, then this is quite intuitive,
although not necessarily very useful -- the author of the
objects is saying that "one is as good as another".  I can't
think of any kind of object for which such a definition of
value would be useful.

But in the real world ...

Lets consider a real example. Suppose I have a variety of
"fill level" indicators (class 'fl' below).  They may
contain, let's say, a numerical value and a unit of liquid
measure (and probably an epsilon for float comparisons,

>>> a = fl(1.04, 'pint')
>>> b = fl(1.00, 'liter')
>>> a == b

>>> a = [fl(1.04,'pint')]
>>> b = [fl(1.00,'liter')]
>>> a == b

(Not tested, because I'm not going to waste time writing fl
right now!)

The point is, the author of this hypothetical 'fl' class
wants its value to be isomorphic to the amount of liquid its
representation would contain, regardless of choice of units
(and probably to within a reasonable experimental error to
take care of float precision).

The author of a class has the power to DEFINE what "value"
is in Python. And that is an extremely useful capability. If
the interpreter were to impose this definition (which is
what you seem to be asking for when you complain that
"value" has no meaning in Python), that would terribly
reduce the utility of Python.

In general, the use of "value" (that is, "value"
distinct from "identity") implies that some kind of math is
going to be done with the objects.  This is true even with
strings, in that comparison and other operators are defined
with respect to them, and depend on their literal value (not
their identity as an object in memory).

If objects NEED to only be compared by identity, then they
should leave __eq__ alone and identity will be used when
they are compared.

Terry Hancock (hancock at
Anansi Spaceworks

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