Any built-in ishashable method ?

Terry Reedy tjreedy at
Fri Jan 18 12:25:51 CET 2013

On 1/18/2013 5:36 AM, Jean-Michel Pichavant wrote:
> Hello people,
> Is there any built-in way to know if an object is a valid dictionary key ?

For the instances of a class to be properly useable as set members or 
dict keys, __eq__ must return bool, __hash__ must return int, the __eq__ 
and __hash__ methods must define equivalence relations on the instances 
of the class, and the hash relation should be a coarsening of the eq 
relation. By default, for objects()s, equality is identity and hash is 
id, so these condition are met.

If the instances of a class can compare equal to instances of another 
class then apply the , then apply the above to the union of the classes 
if they are used together in one set or dict.

An example of when the union condition was broken. We had 0.0 == 0 and 
== decimal(0) but 0.0 != decimal(0). The breaking of transitivity of == 
has nasty effects when mixing floats, ints, and decimals in one 
set/class. (This is fixed now.) For one thing, set((0, 0.0, decimal(0))) 
had one member while set((0.0, 0, decimal(0))) had two ;-)

> I was trying to know if any custom class can be used as a dict key.

Yes, unless the __eq__ and __hash__ methods have been over-written in 
the inheritance chain in a way that violates the conditions, or if the 
class instances are mixed with other class instances that joint violate 
the conditions.

 > Yet I'm a little bit concerned, because last time I used invalid
 > objects as keys, I got a bug that was really difficult to spot.

Yeh, see the example above ;-).

Terry Jan Reedy

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