Question

I want to use a class instance as a dictionary key, like:

classinstance = class()
dictionary[classinstance] = 'hello world'

Python seems to be not able to handle classes as dictionary key, or am I wrong?
In addition, I could use a Tuple-list like [(classinstance, helloworld),…] instead of a dictionary, but that looks very unprofessional.
Do you have any clue for fixing that issue?

Answer 1

Your instances need to be hashable. The python glossary tells us:

An object is hashable if it has a hash value which never changes during its lifetime (it needs a __hash__() method), and can be compared to other objects (it needs an __eq__() or __cmp__() method). Hashable objects which compare equal must have the same hash value.

Hashability makes an object usable as a dictionary key and a set member, because these data structures use the hash value internally.

All of Python’s immutable built-in objects are hashable, while no mutable containers (such as lists or dictionaries) are. Objects which are instances of user-defined classes are hashable by default; they all compare unequal, and their hash value is their id().