Pradeep Pradeep - 1 month ago 5
Python Question

How to append key value pair to an existing list of dict from another list of dict based on matching Key in Python

I am trying to append key value pair to an existing list of dict by comparing the key column.

# Initial list of dict
lst = [{'NAME': 'TEST', 'TYPE': 'TEMP'},
{'NAME': 'TEST', 'TYPE': 'PERMANENT'},
{'NAME': 'TEST1', 'TYPE': 'TEMP'}]

# From the below list of dict, the Key (NAME only) need to compare with the initial list dict
# and for any match need to append the rest of key value pair to the initial list of dict.
compare_lst = [{'NAME': 'TEST', 'ID': 70001, 'V_KEY': 67, 'R_KEY': 1042, 'W_ID': 22},
{'NAME': 'TEST1','ID': 70005, 'V_KEY': 75, 'R_KEY': 1047, 'W_ID': 28}]

# Expected Out put
out = [{'NAME': 'TEST', 'TYPE': 'TEMP', 'ID': 70001, 'V_KEY': 67, 'R_KEY': 1042, 'W_ID': 22},
{'NAME': 'TEST', 'TYPE': 'PERMANENT', 'ID': 70001, 'V_KEY': 67, 'R_KEY': 1042, 'W_ID': 22},
{'NAME': 'TEST1', 'TYPE': 'TEMP', 'ID': 70005, 'V_KEY': 75, 'R_KEY': 1047, 'W_ID': 28}]

Answer

simple approach, no list comps, double loop but works fine:

out=[]

for l in lst:
    for c in compare_lst:
        if l['NAME']==c['NAME']:  # same "key": merge
            lc = dict(l)
            lc.update(c)  # lc is the union of both dictionaries
            out.append(lc)

print(out)

more pythonic, after having defined an helper function to return the union of 2 dictionaries:

out=[]

def dict_union(a,b):
    r = dict(a)
    r.update(b)
    return r

for l in lst:
    out.extend(dict_union(l,c) for c in compare_lst if l['NAME']==c['NAME'])

n python 3.5+, no need for union aux method, you could write more simply

out = []
for l in lst:
    out.extend({**l,**c} for c in compare_lst if l['NAME']==c['NAME'])

even better: oneliner using itertools.chain to flatten the result of the 2-level list comp

import itertools    
out=list(itertools.chain(dict_union(l,c) for l in lst for c in compare_lst if l['NAME']==c['NAME'] ))

for python 3.5

import itertools    
out=list(itertools.chain({**l,**c} for l in lst for c in compare_lst if l['NAME']==c['NAME'] ))

(the multiple methods of doing it materialize the incremental improvements I made those last minutes)

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