Python union of sets raises TypeError

Question

Consider a sequence of sets:

>>> [{n, 2*n} for n in range(5)]
[{0}, {1, 2}, {2, 4}, {3, 6}, {8, 4}]

Passing them directly into the union method yields the correct result:

>>> set().union({0}, {1, 2}, {2, 4}, {3, 6}, {8, 4})
{0, 1, 2, 3, 4, 6, 8}

But passing them as a list or generator expression results in a TypeError:

>>> set().union( [{n, 2*n} for n in range(5)] )
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'set'

>>> set().union({n, 2*n} for n in range(5))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'set'

Why does it happen and what are some solutions?

score 4 · Accepted Answer · edited Aug 16 '18 at 09:57

4

The reason for this error is that set.union() expects one or more sets (ie set.union(oneset, anotherset, andathirdone)), not a list nor generator.

The solution is to unpack your list or generator:

>>> set().union( *({n, 2*n} for n in range(5)) )
{0, 1, 2, 3, 4, 6, 8}

edited Aug 16 '18 at 09:57

bruno desthuilliers

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answered Aug 16 '18 at 07:33

U13-Forward

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score 1 · Answer 2 · edited Aug 16 '18 at 08:19

1

Here's way to union multiple sets without creating a list

s = set()

for n in range(5): 
    s = s.union({n, 2*n})

edited Aug 16 '18 at 08:19

bruno desthuilliers

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answered Aug 16 '18 at 08:08

Alistair Carscadden

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It does work indeed but that's far from being the most effective code (for range(10) it's about 50% slower than U9-Forward's solution). – bruno desthuilliers Aug 16 '18 at 08:19
@bruno fair enough, I was just adhering to the NO-LISTS rule he had made in a comment. :) – Alistair Carscadden Aug 16 '18 at 09:28

Python union of sets raises TypeError

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