Edit the values in a list of dictionaries?

my_dicts = [ 
    { 'key1' : 'value1',
      'key2' : 'value2' },

    { 'key1' : 'value1',  
      'key2' : 'value2' },

    { 'key1' : 'value1',  
      'key2' : 'value2' }]

What would be the most efficient way to replace all instances of 'value2' with 'value3' ?

Solution 1:

I did not do any timings, but you probably can't get much better than

for d in my_dicts:
    d.update((k, "value3") for k, v in d.iteritems() if v == "value2")

Update for Python3

for d in my_dicts:
    d.update((k, "value3") for k, v in d.items() if v == "value2")

Solution 2:

for x in my_dicts:
    for y in x:
        if x.get(y) == 'value2':
            x.update({y: "value3"})

Solution 3:

Here's a very general answer designed to handle multiple occurrences of multiple values in large dictionaries. Handling simpler more specific cases and/or with small dictionaries -- like your example -- could be done significantly faster.

from collections import defaultdict

my_dicts = [
    { 'key1' : 'value1',
      'key2' : 'value2' },

    { 'key1' : 'value1',
      'key2' : 'value2',
      'key3' : 'value2'  }, # dup added for testing

    { 'key1' : 'value1',
      'key2' : 'value2' }]

def reverse(dct):
    """ Create dictionary mapping each value to list of one or more keys """
    ret = defaultdict(list)
    for key,val in dct.iteritems():
        ret[val].append(key)
    return ret

def replace_values(dicts, replacments):
    """ Replace values in each dict in dicts """
    for dct in dicts:
        revdict = reverse(dct)
        for oldval,newval in replacments.iteritems():
            for key in revdict.get(oldval, []):
                dct[key] = newval

replace_values(my_dicts, {'value2':'value3'})
print my_dicts
# [{'key2': 'value3', 'key1': 'value1'},
#  {'key3': 'value3', 'key2': 'value3', 'key1': 'value1'},
#  {'key2': 'value3', 'key1': 'value1'}]

Solution 4:

Python3.7 dict comprehension (will return a deep copy though):

update = {'key2':'value3'}
new_dicts = [{**d,**update} for d in my_dicts]