Pythonic way to copy a key/value from one dict to another

Question

I'm looking for a concise way of taking two dicts that have a common key/value, and copying a key and value into one of the dicts. Example:

d1 = [{'name': 'john', 'uid': 'ax01', 'phone': '555-555-5555'},
    {'name': 'jane', 'uid': 'ax02', 'phone': '555-555-5555'},
    {'name': 'jimmy', 'uid': 'ax03', 'phone': '555-555-5555'}]

d2 = [{'uid': 'ax01', 'orderid': '9999', 'note': 'testing this'},
      {'uid': 'ax02', 'orderid': '6666', 'note': 'testing this'},
      {'uid': 'ax03', 'orderid': '7777', 'note': 'testing this'}]

Here uid is the key that I want to use to then copy the orderid key and the value for that matching data point. In the end I'd get something like:

output = [
    {'name': 'john', 'uid': 'ax01', 'phone': '555-555-5555', 'orderid': '9999'},
    {'name': 'jane', 'uid': 'ax02', 'phone': '555-555-5555', 'orderid': '6666'},
    {'name': 'jimmy', 'uid': 'ax03', 'phone': '555-555-5555', 'orderid': '7777'}
]

Where the orderid is pulled into d1. I'm looking for the pythonic way if at all possible.

Are the list items always in corresponding order? Is `d1[i]["uid"] == d2[i]["uid"]` true for all `i`? — Messa, Feb 13 '14 at 16:45
duplicate: http://stackoverflow.com/questions/3421906/how-to-merge-lists-of-dictionaries — zhangxaochen, Feb 13 '14 at 16:48

Martijn Pieters · Accepted Answer · 2014-02-13T16:47:57.927

You can use dict() to copy one dictionary and pass in the extra key. You need to create a mapping from uid to orderid first though:

uid_to_orderid = {d['uid']: d['orderid'] for d in d2}
output = [dict(d, orderid=uid_to_orderid[d['uid']]) for d in d1]

This assumes that you want to leave the dictionaries in d1 untouched otherwise. Other assumptions made are that uid values are unique, and that all uid values in d1 are present in d2.

Demo:

>>> d1 = [{'name': 'john', 'uid': 'ax01', 'phone': '555-555-5555'},
...     {'name': 'jane', 'uid': 'ax02', 'phone': '555-555-5555'},
...     {'name': 'jimmy', 'uid': 'ax03', 'phone': '555-555-5555'}]
>>> d2 = [{'uid': 'ax01', 'orderid': '9999', 'note': 'testing this'},
...       {'uid': 'ax02', 'orderid': '6666', 'note': 'testing this'},
...       {'uid': 'ax03', 'orderid': '7777', 'note': 'testing this'}]
>>> uid_to_orderid = {d['uid']: d['orderid'] for d in d2}
>>> [dict(d, orderid=uid_to_orderid[d['uid']]) for d in d1]
[{'orderid': '9999', 'phone': '555-555-5555', 'name': 'john', 'uid': 'ax01'}, {'orderid': '6666', 'phone': '555-555-5555', 'name': 'jane', 'uid': 'ax02'}, {'orderid': '7777', 'phone': '555-555-5555', 'name': 'jimmy', 'uid': 'ax03'}]

Do you maybe mean `dict2['orderid']`? What happens if they aren't ordered? — C.B., Feb 13 '14 at 16:44
Yeah, so in this case these are two separate data sources that I need to then create an output for. The ordering can be totally anything. — bryan, Feb 13 '14 at 16:47

score 0 · Answer 2 · answered Feb 13 '14 at 17:17

Great answer @Martijn Pieters

I can't think of a clever way to protect against missing data other something brutish like this:

d1 = [{'name': 'john', 'uid': 'ax01', 'phone': '555-555-5555'},
     {'name': 'jane', 'uid': 'ax02', 'phone': '555-555-5555'},
     {'name': 'jimmy', 'uid': 'ax03', 'phone': '555-555-5555'},
     {'name': 'jack', 'uid': 'ax04', 'phone': '555-555-5555'},
    {'name': 'joan', 'uid': 'ax05', 'phone': '555-555-5555'}]
d2 = [{'uid': 'ax01', 'orderid': '9999', 'note': 'testing this'},
       {'uid': 'ax02', 'orderid': '6666', 'note': 'testing this'},
       {'uid': 'ax03', 'orderid': '6666', 'note': 'testing this'},
       {'uid': 'ax05', 'orderid-not-here': '7777', 'note': 'testing this'}]

def get_orderid(search):
    match = [x for x in d2 if x['uid']==search]
    if len(match) == 0:
        return 'None'
    else:
        return match[0].get('orderid','None')


[dict(d, orderid=get_orderid(d['uid'])) for d in d1]


[ {'name': 'john', 'orderid': '9999', 'phone': '555-555-5555', 'uid': 'ax01'},
  {'name': 'jane', 'orderid': '6666', 'phone': '555-555-5555', 'uid': 'ax02'},
  {'name': 'jimmy', 'orderid': '6666', 'phone': '555-555-5555', 'uid': 'ax03'},
  {'name': 'jack', 'orderid': 'None', 'phone': '555-555-5555', 'uid': 'ax04'},
  {'name': 'joan', 'orderid': 'None', 'phone': '555-555-5555', 'uid': 'ax05'}]

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Pythonic way to copy a key/value from one dict to another

2 Answers2