[Tutor] Is there a test for hashability?

Richard D. Moores rdmoores at gmail.com
Thu Sep 1 16:32:01 CEST 2011

The glossary defines "hashable" as:

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__() 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().

I'm trying to write a general test for hashability. How can I test if
an object has both a  __hash__() method and an __eq__() method?


Dick Moores

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