python - Elegant way to remove fields from nested dictionaries

Question

I had to remove some fields from a dictionary, the keys for those fields are on a list. So I wrote this function:

def delete_keys_from_dict(dict_del, lst_keys):
    """
    Delete the keys present in lst_keys from the dictionary.
    Loops recursively over nested dictionaries.
    """
    dict_foo = dict_del.copy()  #Used as iterator to avoid the 'DictionaryHasChanged' error
    for field in dict_foo.keys():
        if field in lst_keys:
            del dict_del[field]
        if type(dict_foo[field]) == dict:
            delete_keys_from_dict(dict_del[field], lst_keys)
    return dict_del

This code works, but it's not very elegant and I'm sure that there is a better solution.

Answer 1

First of, I think your code is working and not inelegant. There's no immediate reason not to use the code you presented.

There are a few things that could be better though:

Comparing the type

Your code contains the line:

if type(dict_foo[field]) == dict:

That can be definitely improved. Generally (see also PEP8) you should use isinstance instead of comparing types:

if isinstance(dict_foo[field], dict)

However that will also return True if dict_foo[field] is a subclass of dict. If you don't want that, you could also use is instead of ==. That will be marginally (and probably unnoticeable) faster.

If you also want to allow arbitary dict-like objects you could go a step further and test if it's a collections.abc.MutableMapping. That will be True for dict and dict subclasses and for all mutable mappings that explicitly implement that interface without subclassing dict, for example UserDict:

>>> from collections import MutableMapping
>>> # from UserDict import UserDict # Python 2.x
>>> from collections import UserDict  # Python 3.x - 3.6
>>> # from collections.abc import MutableMapping # Python 3.7+
>>> isinstance(UserDict(), MutableMapping)
True
>>> isinstance(UserDict(), dict)
False

Inplace modification and return value

Typically functions either modify a data structure inplace or return a new (modified) data structure. Just to mention a few examples: list.append, dict.clear, dict.update all modify the data structure inplace and return None. That makes it easier to keep track what a function does. However that's not a hard rule and there are always valid exceptions from this rule. However personally I think a function like this doesn't need to be an exception and I would simply remove the return dict_del line and let it implicitly return None, but YMMV.

Removing the keys from the dictionary

You copied the dictionary to avoid problems when you remove key-value pairs during the iteration. However, as already mentioned by another answer you could just iterate over the keys that should be removed and try to delete them:

for key in keys_to_remove:
    try:
        del dict[key]
    except KeyError:
        pass

That has the additional advantage that you don't need to nest two loops (which could be slower, especially if the number of keys that need to be removed is very long).

If you don't like empty except clauses you can also use: contextlib.suppress (requires Python 3.4+):

from contextlib import suppress

for key in keys_to_remove:
    with suppress(KeyError):
        del dict[key] 

Variable names

There are a few variables I would rename because they are just not descriptive or even misleading:

• delete_keys_from_dict should probably mention the subdict-handling, maybe delete_keys_from_dict_recursive.

• dict_del sounds like a deleted dict. I tend to prefer names like dictionary or dct because the function name already describes what is done to the dictionary.

• lst_keys, same there. I'd probably use just keys there. If you want to be more specific something like keys_sequence would make more sense because it accepts any sequence (you just have to be able to iterate over it multiple times), not just lists.

• dict_foo, just no...

• field isn't really appropriate either, it's a key.

Putting it all together:

As I said before I personally would modify the dictionary in-place and not return the dictionary again. Because of that I present two solutions, one that modifies it in-place but doesn't return anything and one that creates a new dictionary with the keys removed.

The version that modifies in-place (very much like Ned Batchelders solution):

from collections import MutableMapping
from contextlib import suppress

def delete_keys_from_dict(dictionary, keys):
    for key in keys:
        with suppress(KeyError):
            del dictionary[key]
    for value in dictionary.values():
        if isinstance(value, MutableMapping):
            delete_keys_from_dict(value, keys)

And the solution that returns a new object:

from collections import MutableMapping

def delete_keys_from_dict(dictionary, keys):
    keys_set = set(keys)  # Just an optimization for the "if key in keys" lookup.

    modified_dict = {}
    for key, value in dictionary.items():
        if key not in keys_set:
            if isinstance(value, MutableMapping):
                modified_dict[key] = delete_keys_from_dict(value, keys_set)
            else:
                modified_dict[key] = value  # or copy.deepcopy(value) if a copy is desired for non-dicts.
    return modified_dict

However it only makes copies of the dictionaries, the other values are not returned as copy, you could easily wrap these in copy.deepcopy (I put a comment in the appropriate place of the code) if you want that.