Multi tool use

Modifying list of dictionary using list comprehension

So I have the following list of dictionary

myList = [{'one':1, 'two':2,'three':3},
{'one':4, 'two':5,'three':6},
{'one':7, 'two':8,'three':9}]


This is just an example of a dictionary that I have. My question is, it possible to somehow modify say key two in all the dictionary to become twice their value, using list comprehension ?

two

I know how to use list comprehension to create new list of dictionary, but don't know how to modify them, I have come up with something like this

new_list = { <some if condiftion> for (k,v) in x.iteritems() for x in myList }


I am not sure how to specify a condition in the <some if condiftion>, also is the nested list comprehension format I am thinking of correct ?

<some if condiftion>

I want the final output as per my example like this

[ {'one':1, 'two':4,'three':3},{'one':4, 'two':10,'three':6},{'one':7, 'two':16,'three':9} ]


{

[

5 Answers
5

Use list comprehension with nested dict comprehension:

new_list = [{ k: v * 2 if k == 'two' else v for k,v in x.items()} for x in myList]
print (new_list)
[{'one': 1, 'two': 4, 'three': 3},
{'one': 4, 'two': 10, 'three': 6},
{'one': 7, 'two': 16, 'three': 9}]


myList = [ {'one':1, 'two':2,'three':3},{'one':4, 'two':5,'three':6},{'one':7, 'two':8,'three':9} ]

[ { k: 2*i[k] if k == 'two' else i[k] for k in i } for i in myList ]

[{'one': 1, 'three': 3, 'two': 4}, {'one': 4, 'three': 6, 'two': 10}, {'one': 7, 'three': 9, 'two': 16}]


A simple for loop should be sufficient. However, if you want to use a dictionary comprehension, I find defining a mapping dictionary more readable and extendable than ternary statements:

for

factor = {'two': 2}

res = [{k: v*factor.get(k, 1) for k, v in d.items()} for d in myList]

print(res)

[{'one': 1, 'two': 4, 'three': 3},
{'one': 4, 'two': 10, 'three': 6},
{'one': 7, 'two': 16, 'three': 9}]


if

else

In python 3.5+ you can use the new unpacking syntax in dict literals introduced in PEP 448. This creates a copy of each dict and then overwrites the value for the key two:

two

new_list = [{**d, 'two': d['two']*2} for d in myList]
# result:
# [{'one': 1, 'two': 4, 'three': 3},
# {'one': 4, 'two': 10, 'three': 6},
# {'one': 7, 'two': 16, 'three': 9}]


Hello did you tried this :

for d in myList:
d.update((k, v*2) for k, v in d.iteritems() if k == "two")


Thanks

d['two'] = d['two'] * 2

d.update(...)

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