Merging two dictionaries in Python?

Question

I want to merge this dictionary:

b = {data:[{station_id: 7000,
     name: "Ft. York / Capreol Crt."
     },
     {station_id: 7001,
      name: "Lower Jarvis St / The Esplanade"}
     ]}

and this one :

c = {data:[{station_id: 7000,
     num_bikes_available: 18,
     },
     {station_id: 7001,
      num_bikes_available: 4,
      }
    ]}

and get one dictionary like this:

d = {data:[{station_id: 7000,
 name: "Ft. York / Capreol Crt.",
 num_bikes_available: 18
 },
{station_id: 7001,
 name: "Lower Jarvis St / The Esplanade",                         
 num_bikes_available: 4}
]}

How can I do that?

Possible duplicate of [How to merge two Python dictionaries in a single expression?](https://stackoverflow.com/questions/38987/how-to-merge-two-python-dictionaries-in-a-single-expression) — S.G. Harmonia, Jul 20 '17 at 01:29
If you don't care if this done in a single line (why would you) then `d = dict(b); d.update(c)` — AChampion, Jul 20 '17 at 01:29
@CoryMadden It's not really casting, it is creating a copy of `b`, so the `update()` doesn't change `b`. — AChampion, Jul 20 '17 at 01:31
nevermind. I reread it, I wasn't computing that `d` was a new variable — Cory Madden, Jul 20 '17 at 01:32
Please could you make sure that the code in your question is syntactically valid. (Presumably, `data`, `station_id` etc are strings?) — NPE, Jul 20 '17 at 01:54

Root · Answer 1 · 2017-07-20T01:40:57.027

2

For Py>3.5:

It's easy. Just enter:

d = {**b, **c}

edited Jul 20 '17 at 01:40

answered Jul 20 '17 at 01:28

Root

97
1
12

I only know how to use Py3. :p – Root Jul 20 '17 at 01:34
@Root You should know that there are solutions that work on python3 as well as python2. – cs95 Jul 20 '17 at 01:35
I would note that in your answer - also note this in only Py>3.5. – AChampion Jul 20 '17 at 01:36
Good advice. I will add it. – Root Jul 20 '17 at 01:40
@root, have you run this with the sample data? Does this give the answer OP expects? – Robᵩ Jul 20 '17 at 02:03
@Robᵩ The question didn't look like this when I answered it. It was just two simple dicts that need to be merged. – Root Jul 20 '17 at 02:15
Ah, that makes much more sense. – Robᵩ Jul 20 '17 at 03:04

Robᵩ · Accepted Answer · 2017-07-20T16:53:22.647

The key to this problem is picking the right data structure. Instead of b['data'] being a list, it should be a dict indexed by the merge key. The following code first converts b and c into dicts indexed by station_id, then merges those dictionaries.

Try this:

from pprint import pprint

b = {'data': [{'station_id': 7000,
     'name': "Ft. York / Capreol Crt."
     },
     {'station_id': 7001,
      'name': "Lower Jarvis St / The Esplanade"},
     {'station_id':7002,'num_bikes_available':10},
     ]}

c = {'data': [{'station_id': 7000,
     'num_bikes_available': 18,
     },
     {'station_id': 7001,
      'num_bikes_available': 4,
      }
    ]}

# First, convert B and C to a more useful format:

b1 = {item['station_id']: item for item in b['data']}
c1 = {item['station_id']: item for item in c['data']}

# Now construct D by merging the individual values
d = {'data': []}
for station_id, b_item in sorted(b1.items()):
    z = b_item.copy()
    z.update(c1.get(station_id, {}))
    d['data'].append(z)

pprint(d)

But what if for example in b there was also {'station_id':7002,'num_bikes_available':10} which in c there isn't. what about then?? — kia, Jul 20 '17 at 13:57
Anytime you need an item from a dictionary, but it might not be present, use `dict.get()`. I have modified the `z.update(...)` line in response to your question. — Robᵩ, Jul 20 '17 at 16:53

Merging two dictionaries in Python?

2 Answers2