user2667066 user2667066 - 1 year ago 121
Python Question

Is a python dict comprehension always "last wins" if there are duplicate keys

If I create a python dictionary with a dict comprehension, but there are duplicate keys, am I guaranteed that the last item will the the one that ends up in the final dictionary? It's not clear to me from looking at

new_dict = {k:v for k,v in [(1,100),(2,200),(3,300),(1,111)]}
new_dict[1] #is this guaranteed to be 111, rather than 100?

Answer Source

Last item wins. The best documentation I can find for this is in the Python 3 language reference, section 6.2.7:

A dict comprehension, in contrast to list and set comprehensions, needs two expressions separated with a colon followed by the usual “for” and “if” clauses. When the comprehension is run, the resulting key and value elements are inserted in the new dictionary in the order they are produced.

That documentation also explicitly states that the last item wins for comma-separated key-value pairs ({1: 1, 1: 2}) and for dictionary unpacking ({**{1: 1}, **{1: 2}}):

If a comma-separated sequence of key/datum pairs is given, ... you can specify the same key multiple times in the key/datum list, and the final dictionary’s value for that key will be the last one given.

A double asterisk ** denotes dictionary unpacking. Its operand must be a mapping. Each mapping item is added to the new dictionary. Later values replace values already set by earlier key/datum pairs and earlier dictionary unpackings.

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