Union with nested list/multiple for loops

Question

I have a dictionary like so:

myDict = {'items':
            [{'names': [{'longName1', 'shortName1'},
                      {'shortName2', 'longName2'}]},
            {'names': [{'longName3', 'shortName3'},
                      {'shortName4', 'longName4'}]}]}

Attempting to get the keys (i.e. shortName) in a set Pythonically. I have the following statement, but it's complaining that i isn't defined. What am I doing wrong?

shortNames = set().union(*(j.values() for j in i["names"] for i in myDict["items"]))

Expected result:

set(['shortName1', 'shortName2', 'shortName3', 'shortName4'])

score 2 · Answer 1 · answered Nov 19 '19 at 12:41

2

You are accessing i["names"] before i is defined by i in myDict["items"].

answered Nov 19 '19 at 12:41

dacx

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Ah...so I need to swap the statements. – ILostMySpoon Nov 19 '19 at 12:43

score 1 · Accepted Answer · answered Nov 19 '19 at 13:20

1

You have to swap the for loops:

set().union(*[j for i in myDict["items"] for j in i["names"]])

answered Nov 19 '19 at 13:20

Mykola Zotko

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Sayandip Dutta · Answer 3 · 2019-11-19T13:11:32.340

See, it is not advisable to chain list comprehensions just for the sake of brevity. Since you are asking, one way would be:

>>> from itertools import chain
>>> {j for j in (chain.from_iterable(sum([i['names'] for i in myDict['items']],[]))) if j.startswith('short')}

{'shortName1', 'shortName2', 'shortName3', 'shortName4'}

Your method could not work:

As dacx mentioned.
j.values() would not give you shortNames as those are inside sets not dicts, and sets do not have .values() method.

OR:

>>> {j for j in set().union(*(set().union(*i['names']) for i in myDict['items'])) if j.startswith('short')}
{'shortName1', 'shortName2', 'shortName3', 'shortName4'}

Union with nested list/multiple for loops

3 Answers3