Best way to check Python multi-level dictionary

Question

Currently I am using the following method, assume dictionary

data[a][b][c]

I use:

if "a" in data and "b" in data["a"] and "c" in data["a"]["b"]:
  ...

Are there any better way?

score 8 · Answer 1 · answered Dec 26 '13 at 15:26

8

You can wrap it in a try/except block

try:
    x = data[a][b][c]
    ...do stuff in your "if" clause
except KeyError:
    ...do stuff in your "else" clause

answered Dec 26 '13 at 15:26

tdelaney

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score 7 · Answer 2 · answered Dec 26 '13 at 15:15

7

I would typically use the pattern

foo = data.get("a",{}).get("b",{}).get("c",False)
if foo:
...

This requires that the nesting is always the same depth and that data["a"]["b"]["c"] is not "falsy".

answered Dec 26 '13 at 15:15

colcarroll

3,632
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score 3 · Answer 3 · answered Dec 31 '20 at 04:55

3

If you need to do this multi-level lookup very often, this helper method can be useful:

def multi_get(dict_obj, *attrs, default=None):
    result = dict_obj
    for attr in attrs:
        if attr not in result:
            return default
        result = result[attr]
    return result

To use it, just call it like:

x = multi_get(data, 'a', 'b', 'c', default=False)

answered Dec 31 '20 at 04:55

Martín De la Fuente

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Maybe change the if to the following one. Then the function can be used to iterate through json-like data structures that include lists as well: if (attr in result) or (isinstance(result, list) and isinstance(attr, int) and (attr < len(result))): It checks if the current iteration of result is a list and the attribute an int and whether the list has enough elements. If yes the element can be accessed using the squre brackets syntax as well. – Pinolpier Jul 12 '22 at 08:38

score 1 · Answer 4 · edited May 23 '17 at 10:29

There's two solutions:

Return a default value with defaultdict

You can use a defaultdict, and intstantiate an inner level with another defaultdict. Then, if the key is missing, it'll return an empty dict without throwing errors:

from collections import defaultdict

>>> d = defaultdict(lambda: defaultdict(dict))
>>> d['a']['b']
{}

If you need more levels, you just have to chain many lambda.

Partial reference: Multiple levels of 'collection.defaultdict' in Python

Handle the missing key with a try/except

Another way is to surronding the dict access with try/except block and manage the missing key through the KeyError exception (stop the program, return a default value, etc.)

try:
    x = data[a][b][c]
    # same code of your if statement
except KeyError:
    # stuff you do in your else

Best way to check Python multi-level dictionary

4 Answers4

Return a default value with defaultdict

Handle the missing key with a try/except