Making a python user-defined class sortable, hashable

python class sorting hash magic-methods
Matt Fenwick picture Matt Fenwick · Aug 22, 2011 · Viewed 44.7k times · Source

What methods need to be overridden/implemented when making user-defined classes sortable and/or hashable in python?

What are the gotchas to watch out for?

I type dir({}) into my interpreter to get a list of methods on built-in dicts. Of those, I assume I need to some implement some subset of

['__cmp__', '__eq__', '__ge__', '__gt__', '__hash__', '__le__', '__lt__', '__ne__']

Is there a difference in which methods must be implemented for Python3 as opposed to Python2?

Answer

agf picture agf · Aug 22, 2011

I almost posted this as a comment to the other answers but it's really an answer in and of itself.

To make your items sortable, they only need to implement __lt__. That's the only method used by the built in sort.

The other comparisons or functools.total_ordering are only needed if you actually want to use the comparison operators with your class.

To make your items hashable, you implement __hash__ as others noted. You should also implement __eq__ in a compatible way -- items that are equivalent should hash the same.

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