python: union keys from multiple dictionary?

Question

I have 5 dictionaries and I want a union of their keys.

alldict =  [dict1, dict2, dict3, dict4, dict5]

I tried

allkey = reduce(lambda x, y: set(x.keys()).union(y.keys()), alldict)

but it gave me an error

AttributeError: 'set' object has no attribute 'keys'

Am I doing it wrong ? I using normal forloop but I wonder why the above code didn't work.

Answer 1

I think @chuck already answered the question why it doesn't work, but a simpler way to do this would be to remember that the union method can take multiple arguments:

allkey = set().union(*alldict)

does what you want without any loops or lambdas.

Answer 2

Your solution works for the first two elements in the list, but then dict1 and dict2 got reduced into a set and that set is put into your lambda as the x. So now x does not have the method keys() anymore.

The solution is to make x be a set from the very beginning by initializing the reduction with an empty set (which happens to be the neutral element of the union).

Try it with an initializer:

allkey = reduce(lambda x, y: x.union(y.keys()), alldict, set())

An alternative without any lambdas would be:

allkey = reduce(set.union, map(set, map(dict.keys, alldict)))