Python nested dictionary lookup with default values

Question

>>> d2
{'egg': 3, 'ham': {'grill': 4, 'fry': 6, 'bake': 5}, 'spam': 2}
>>> d2.get('spamx',99)
99
>>> d2.get('ham')['fry']
6

I want to get value of fry inside of ham, if not, get value, 99 or 88 as the 2nd example. But how?

Buonarroti

Answer 1

d2.get('ham', {}).get('fry', 88)

I would probably break it down into several statements in real life.

ham = d2.get('ham', {})
fry = ham.get('fry', 88)

Eyeglass

rather d2.get('ham', {}).get('fry', 99), not ? (DRY!) – Stipulation

Making something readable != repeating yourself. Repeating yourself would be to write a function to get ham's fry, and then another function to get spam's fry, when both functions could be written as one function which takes ham/spam as a parameter. – Lexington

@Jesse, The previous version had two references to 'fry' and two references to the magic number being returned. The suggestion was a distinct improvement. – Eyeglass

I'm not seeing that version anywhere. – Lexington

@Jesse, odd. Neither am I. It took me about 3 or 4 edits to get this right, but SO seems to have collapsed it into 1, making me look far more decisive than I am. – Eyeglass

@Oddthinking, I think if the edits are clustered within a certain window of time, they are collapsed. – Lexington

Note: This breaks if d2.get('ham'... returns something that is not a dict (if d2['ham'] was 'other' say) – Eohippus

Answer 2

If you need to do this a lot, you can write a helper function

def get_nested(d, list_of_keys, default):
    for k in list_of_keys:
        if k not in d: 
            return default
        d=d[k]
    return d

print get_nested(d2,['ham','spam'],99)
print get_nested(d2,['ham','grill'],99)

Quandary

Feels like reduce. functools.reduce(lambda d,x: d[x] if x in d else default, ['ham','spam'], d2) – Spiniferous

@KennyTM, That raises a TypeError for ['bacon','spam'] – Quandary

Answer 3

For the default values of get to work correctly the first default needs to be a dictionary, so that you can chain the .get calls correctly if the first fails.

d.get('ham',{}).get('fry',88)

you could also use a try, except block

def get_ham_fry()
  try:
    return d['ham']['fry']
  except AttributeError,e:
    return 88

Db

String indexing will raise either a KeyError if no key is found, or a TypeError if the item doesn't support string indexing. I think AttributeError is only appropriate for the get calls. – Eohippus

Answer 4

Here's a solution for dealing with nested dictionaries:

def get(root, *keys):
    """
    Returns root[k_1][k_2]...[k_n] if all k_1, ..., k_n are valid keys/indices. 
    Returns None otherwise
    """
    if not keys:
        return root
    if keys[0] not in root:
        return None
    if keys[0] in root:
        return get(root[keys[0]], *keys[1:])

Usage:

>>> d = {'a': 1, 'b': {'c': 3}}
>>> get(d, 'b', 'c')
3
>>> get(d. 'key that's not in d')
None
>>> get(d)
{'a': 1, 'b': {'c': 3}}

Vale

Answer 5

another solution for the issue could be:

from 'ham' get 'fry' else return 88

result = d2['ham'].get('fry', 88)

Armenta