Ezio Melotti added the comment:
This doesn't modify f, it replaces it with a new frozenset:
>>> f = frozenset({1, 2})
>>> f
frozenset({1, 2})
>>> id(f)
3071990668
>>> f -= frozenset({1})
>>> f
frozenset({2})
>>> id(f)
3066719340
Notice how the two ids are different.
In other words,
f -= frozenset({1})
is equivalent to
f = f - frozenset({1})
You get an error with += because
f = f + frozenset({1})
is not a valid operation for (frozen)sets.
----------
nosy: +ezio.melotti
resolution: -> not a bug
stage: -> resolved
status: open -> closed
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue22498>
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