Replacing a specific item of a dict in a list of dicts?

Question

Say I have the following list of dicts:

dicts = [
    {'name': "Tom", 'age': 20, 'height': 1.8},
    {'name': "Isa", 'age': 31, 'height': 1.5},
    ... ]

I'd like to replace the age of a given person with a given value.

def replace_age(person, age):
    dicts[?]['age'] = age

replace_age("Tom", 45)

Assuming that name is unique, what's the most elegant way to go about this?

In a golden world: dicts[name=person]['age'] = age

Not a duplicate of Find the index of a dict within a list, by matching the dict's value: I'm looking to change the value, not get the index. And Tom is a pretty common name.

You first have to identify the proper dict in your list if you want to update it, that's the same as finding it's index. Also, your last remark 'Tom is a pretty common name' makes me wonder what you would want to do if there are many Toms in your list. That makes your question unclear... — Thierry Lathuille, Aug 20 '19 at 13:28
@snazzybouche it's the critical part of your question. changing the value for a particular dictionary key is trivial and part of the Python language. Finding the right dictionary is covered by the duplicated question. I like code golf as much as the next person but it's not going to help you any more than the dupe will (though I admit I am always fond of the crazy one-liners people come up with) — , Aug 20 '19 at 13:35
@ThierryLathuille For the purposes of this question I have asserted that `name` is unique — snazzybouche, Aug 20 '19 at 13:36

hiro protagonist · Accepted Answer · 2019-08-20T13:06:08.740

3

this is a variant:

def replace_age(person, age):
    try:
        dct = next(item for item in dicts if item["name"] == person)
    except StopIteration:
        # person not found
        # here you could print a message or raise an error...
        return
    dct["age"] = age

this will only work if the names are unique. if they are not only the first occurrence will be replaced.

edited Aug 20 '19 at 13:06

answered Aug 20 '19 at 13:00

hiro protagonist

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the dictionary is changed in-place. for these functions it is conventional to return None. i have no idea what the OP needs if the person is not found in the dict... – hiro protagonist Aug 20 '19 at 13:04
1

I agree with hiro, it's better to pass None as second argument in next function. – Arsal Aug 20 '19 at 13:07

Pitto · Answer 2 · 2019-08-26T12:44:25.327

Here's my version

dictionaries = [
    {'name': "Tom", 'age': 20, 'height': 1.8},
    {'name': "Isa", 'age': 31, 'height': 1.5}
    ]

def change_dict_person_age(dictionaries, person, age):
    for dictionary in dictionaries:
        if dictionary['name'] == person:
            dictionary['age'] = age
            # Uncomment the following line if you want to stop at the 1st
            # match. Leave it as is if you want to modify all occurrences.
            #break

change_dict_person_age(dictionaries, "Tom", 40)
print(dictionaries)
#[{'name': 'Tom', 'age': 40, 'height': 1.8}, {'name': 'Isa', 'age': 31, 'height': 1.5}]

I also wrote a more generic version for broader user:

dictionaries = [
    {'name': "Tom", 'age': 20, 'height': 1.8},
    {'name': "Isa", 'age': 31, 'height': 1.5}
    ]

def change_dict(dictionaries, key_to_check, value_to_match, key_to_change, value_to_change):
    for dictionary in dictionaries:
        if dictionary[key_to_check] == value_to_match:
            dictionary[key_to_change] = value_to_change
            # Uncomment the following line if you want to stop at the 1st
            # match. Leave it as is if you want to modify all occurrences.
            #break

change_dict(dictionaries, "name", "Tom", "age", 50)
print(dictionaries)
#[{'name': 'Tom', 'age': 50, 'height': 1.8}, {'name': 'Isa', 'age': 31, 'height': 1.5}]

Good point @snazzybouche but then how could I have more fine grained control about which is the value to check and which is the one to change? Just positional? — Pitto, Aug 20 '19 at 13:20
Mmm, good counterpoint. I don't believe kwargs are ordered so you're right, that wouldn't work at all. — snazzybouche, Aug 20 '19 at 13:23

score 1 · Answer 3 · answered Aug 20 '19 at 13:14

since the name is unique you can change your data structure where you keep your data to achieve your task efficiently:

efficient_dict = {e['name']: {'age' : e.get('age'), 'height': e.get('height')} for e in dicts}

def replace_age(person, age):
    if person in efficient_dict:
        efficient_dict[person]['age'] = age

Replacing a specific item of a dict in a list of dicts?

3 Answers3