update json property value using for loop in python?

The straight forward way:

for obj in json:
    if obj['id'] == 2:
        obj['name'] = 'something'

Objects are mutable, so you're directly mutating the object here. This is the simplest way. The typical Javascript equivalent would be:

json.forEach(obj => {
    if (obj.id == 2) {
        obj.name = 'something';
    }
});

The slightly more condensed version:

for obj in (o for o in json if o['id'] == 2):
    obj['name'] = 'something'

This inlines a generator expression which pre-filters the objects to loop over. Alternatively:

for obj in filter(lambda o: o['id'] == 2, json):
    obj['name'] = 'something'

Somewhat equivalent to:

json.filter(o => o.id == 2).forEach(obj => obj.name = 'something')

The even more condensed version:

json = [{**obj, 'name': 'something' if obj['id'] == 2 else obj['name']} for obj in json]

This is a list comprehension which builds a new object and new list, somewhat equivalent to:

json = json.map(o => ({...obj, name: obj.id == 2 ? 'something' : obj.name}))

You decide what you find most readable…

Since your json is a list of dicts, you can loop through the dicts while checking your required condition:

json = [{
    "id": 1, "name": "a"
}, {
    "id": 2, "name": "b"
}]

for obj in json:
  if obj["id"] == 2:
    obj["name"] = 'something'

Result:

[{'id': 1, 'name': 'a'}, {'id': 2, 'name': 'something'}]

(That's a list of dictionaries, not a JSON string.)

The most concise (you can argue about readability) way I can think of is:

>>> [{**d, 'name': 'something' if d['id'] == 2 else d['name']} for d in json]
[{'id': 1, 'name': 'a'}, {'id': 2, 'name': 'something'}]

This builds a new list of dicts (not an inplace operation), so you would have to reassign the name json.

edit: fixed error from misreading the question.

The syntax in your question is very closed, here are two working versions.

Option 1 use enumerate

If you prefer to access json by index, enumerate is what you are looking for:

for i, x in enumerate(json):
    if x['id'] == 2:
        json[i]['name'] = 'something'

print(json) # what you want

Option 2 directly modify x

From your example, you actually don't need the index i. You can directly modify x to achieve this. The reason is that when iterating a list through for x in, x is a reference to the original item in the list, not a copy, so any modification to x will be reflected in the original list.

for x in json:
    if x['id'] == 2:
        x['name'] = 'something'

print(json) # what you want

Yes, Python has map and friends, see https://docs.python.org/3/library/functions.html .

As others stated, it's more pythonic to just use for loops. But if you want to use (and abuse) functional programming style, you can do it as:

any(map(lambda item: (item['id'] == 2) and item.update(name='something'), json))

The same with some comments:

any( # map will return an iterator, any is there to force the evaluation on each element
    map(
         lambda item:          # function to be applied
             (item['id'] == 2) # our condition
             and               # 'and' is lazy evaluated, if 'id' != 2
                               # we don't evaluate the following line
             item.update(      # update item with another dict
                 name = 'something' # use the fact that keyword arguments are given as a dict
             ), # the return value will always be false
                # forcing 'any' to look further
         json                  # the list we apply the map to
       )
   )

Python have all these functions like map, reduce, and filter. Please refer https://www.learnpython.org/en/Map,_Filter,_Reduce

You also can use the list comprehension feature which is quite useful and handy. Here I have written the list_comprehension for getting the list of specified condition which is id is 2 here.

output_list = [i.get("name") for i in json if i.get("id") == 2]

expected output

['b']