Equality between frozensets

Question

Example:

>>> tuple((1, 2)) == tuple((2, 1))
False
>>> frozenset((1, 2)) == frozenset((2, 1))
True

Frozen sets are immutable. I would expect that equality between immutable objects should by determined by order, but here obviously that is not the case.

How can I discard frozensets with same elements and different order, without casting to different type?

Answer 1

The short answer is you can't, since as pointed out in the comments, sets and frozensets are unordered data structures. Here are some excerpts from the docs^* to support this statement:

A set object is an unordered collection of distinct hashable objects.

There are currently two built-in set types, set and frozenset. The set type is mutable — the contents can be changed using methods like add() and remove(). Since it is mutable, it has no hash value and cannot be used as either a dictionary key or as an element of another set. The frozenset type is immutable and hashable — its contents cannot be altered after it is created; it can therefore be used as a dictionary key or as an element of another set.

^* Python 2.7.12

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For a better grasp of the equality issue, I would encourage you to run the following snippet using the Online Python Tutor:

tup_1 = tuple((1, 2))
tup_2 = tuple((2, 1))
fs_1 = frozenset((1, 2))
fs_2 = frozenset((2, 1))

This is an extremely handy tool that renders a graphical representation of the objects in memory while the code is executed step by step. I'm attaching a screenshot: