python set union operation is not behaving well with named tuples

Question

I want to create a set of namedtuple in python, with the ability to add elements dynamically using the union operation.

The following code snippet creates a set of namedtuple, which is behaving nicely.

from collections import namedtuple

B = namedtuple('B', 'name x')

b1 = B('b1',90)
b2 = B('b2',92)
s = set([b1,b2])
print(s)

which prints

{B(name='b1', x=90), B(name='b2', x=92)}

Now if I create another namedtuple and add it to my set with the union operations it is not behaving as expected.

b3 = B('b3',93)
s = s.union(b3)
print(s)

The code snippet prints the following output.

{93, B(name='b1', x=90), B(name='b2', x=92), 'b3'}

The expected output should be:

{B(name='b1', x=90), B(name='b2', x=92), B(name='b3', x=93)}

Am I mis-understanding the API? Both python2 and 3 are showing the same behaviour.

Answer 1

A namedtuple instance is an iterable of items. set.union simply merges the current set with the items in the namedtuple.

However, what you want is put the namedtuple in another container/iterable, so the merge is done with the item (the namedtuple) contained in the new parent iterable:

s.union((b3,))

It becomes more obvious if you actually think of the operator equivalent:

s = s | set(b3) # set(b3) -> {93, 'b3'}

As compared to what we actually want:

s = s | {b3}

The union is performed with the outer iterable.