Iterate over a list inside a nested dictionary

Question

Lets say I have a dictionary like this:

myDict = {
    1: {
        "a": "something",
        "b": [0, 1, 2],
        "c": ["a", "b", "c"]
    },
    2: {
        "a": "somethingElse",
        "b": [3, 4, 5],
        "c": ["d", "e", "f"]
    },
    3: {
        "a": "another",
        "b": [6, 7, 8],
        "c": ["g", "h", "i"]
    }
}

And this is my code:

for id, obj in myDict.items():
    for key, val in obj.items():
        if key is "b":
            for item in val:
                # apply some function to item

Is there a better way to iterate over a list inside a nested dict? Or is there a pythonic way to do this?

`if key is "b"`, but what about `"c"`? They are lists too. You don't want them? — gil, Feb 12 '16 at 04:35
@gill I don't need "c". I only need values from "b" because I need to apply some function for each value inside the list. — Mico, Feb 12 '16 at 04:39

David Zemens · Answer 1 · 2016-02-12T04:43:51.987

2

You absolutely do not need to iterate the list to print it (unless this is a functional requirement for the code you are writing).

Very simply, you could do this:

for id, obj in myDict.items():
    if "b" in obj:
        print obj["b"]

To map the list object, represented by obj['b'] to another function, you can use the map function:

map(foo, obj["b"])

edited Feb 12 '16 at 04:43

answered Feb 12 '16 at 04:36

David Zemens

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I don't want to just print it. I need to apply some function for each value inside the list. – Mico Feb 12 '16 at 04:37
OK, then in my answer you could do `map(foo, obj['b'])` instead of `print`. – David Zemens Feb 12 '16 at 04:39
@Mico, see my response for a version that will update the list if that's what you want – Jared Goguen Feb 12 '16 at 04:47

Jared Goguen · Accepted Answer · 2019-04-26T21:28:06.260

If you're dictionary is always two levels deep, I don't see anything wrong with your approach. In your implementation, I would use key == "b" rather than key is "b". Using is will test for identity (e.g. id(a) == id(b)), while == will test for equality (e.g. a.__eq__(b)). This functions the same way when I test it in IDLE, but it's not a good habit to get into. There's more info on it here: How is the 'is' keyword implemented in Python?

If you want to deal with varying level dictionaries, you could use something like:

def test_dict_for_key(dictionary, key, function):
    for test_key, value in dictionary.items():
        if key == test_key:
            dictionary[key] = type(value)(map(function, value))
        if isinstance(value, dict):
            test_dict_for_key(value, key, function)

An example usage might be something like:

myDict = {
    1: {
        "a": "something",
        "b": [0, 1, 2],
        "c": ["a", "b", "c"]
    },
    2: {
        "a": "somethingElse",
        "b": [3, 4, 5],
        "c": ["d", "e", "f"]
    },
    3: {
        "a": "another",
        "b": [6, 7, 8],
        "c": ["g", "h", "i"]
    }
}

# adds 1 to every entry in each b
test_dict_for_key(myDict, "b", lambda x: x + 1)

# prints [1, 2, 3]
print(myDict[1]["b"])

Pretty much answers all of my questions, Thanks! – Mico Feb 12 '16 at 04:49 — Mico, Feb 12 '16 at 04:49

score 1 · Answer 3 · answered Feb 12 '16 at 04:43

I'm a fan of generator expressions.

inner_lists = (inner_dict['b'] for inner_dict in myDict.values())
# if 'b' is not guaranteed to exist,
# replace inner_dict['b'] with inner_dict.get('b', [])
items = (item for ls in inner_lists for item in ls)

Now you can either use a foo loop

for item in items:
    # apply function

or map

transformed_items = map(func, items)

score 0 · Answer 4 · answered Feb 12 '16 at 04:35

A couple fixes could be made.

Don't use is when comparing two strings (if key is "b":)
Simply say print(item) instead of using .format(), since you only have one variable that you're printing, with no additional string formatting

Revised code:

for id, obj in myDict.items():
    for key, val in obj.items():
        if key == "b":
            for item in val:
                print(item)

score 0 · Answer 5 · answered Feb 12 '16 at 04:37

0

If you are sure that you will have a b key in every case, you can simply do:

for id, obj in myDict.items():
    for item in obj["b"]:
        print item

answered Feb 12 '16 at 04:37

Selcuk

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Iterate over a list inside a nested dictionary

5 Answers5