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?

• S.G. Harmonia: Possible duplicate of How to merge two Python dictionaries in a single expression?
• AChampion: If you don't care if this done in a single line (why would you) then d = dict(b); d.update(c)
• Cory Madden: @AChampion Why did you cast b to a dict?
• AChampion: @CoryMadden It's not really casting, it is creating a copy of b, so the update() doesn't change b.
• NPE: Please could you make sure that the code in your question is syntactically valid. (Presumably, data, station_id etc are strings?)

Answer 1

For Py>3.5:

It's easy. Just enter:

d = {**b, **c}

• Root: I only know how to use Py3. :p
• cs95: @Root You should know that there are solutions that work on python3 as well as python2.
• AChampion: I would note that in your answer - also note this in only Py>3.5.
• Root: Good advice. I will add it.
• Robφ: @root, have you run this with the sample data? Does this give the answer OP expects?

Answer 2

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)

• kia: 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??
• Robφ: 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.