Finding a key recursively in a dictionary

I'm trying to write a very simple function to recursively search through a possibly nested (in the most extreme cases ten levels deep) Python dictionary and return the first value it finds from the given key.

I cannot understand why my code doesn't work for nested dictionaries.

def _finditem(obj, key):
    if key in obj: return obj[key]
    for k, v in obj.items():
        if isinstance(v,dict):
            _finditem(v, key)

print _finditem({"B":{"A":2}},"A")

It returns None.

It does work, however, for _finditem({"B":1,"A":2},"A"), returning 2.

I'm sure it's a simple mistake but I cannot find it. I feel like there already might be something for this in the standard library or collections, but I can't find that either.

Solution 1:

when you recurse, you need to return the result of _finditem

def _finditem(obj, key):
    if key in obj: return obj[key]
    for k, v in obj.items():
        if isinstance(v,dict):
            return _finditem(v, key)  #added return statement

To fix the actual algorithm, you need to realize that _finditem returns None if it didn't find anything, so you need to check that explicitly to prevent an early return:

def _finditem(obj, key):
    if key in obj: return obj[key]
    for k, v in obj.items():
        if isinstance(v,dict):
            item = _finditem(v, key)
            if item is not None:
                return item

Of course, that will fail if you have None values in any of your dictionaries. In that case, you could set up a sentinel object() for this function and return that in the case that you don't find anything -- Then you can check against the sentinel to know if you found something or not.

Solution 2:

Here's a function that searches a dictionary that contains both nested dictionaries and lists. It creates a list of the values of the results.

def get_recursively(search_dict, field):
    """
    Takes a dict with nested lists and dicts,
    and searches all dicts for a key of the field
    provided.
    """
    fields_found = []

    for key, value in search_dict.iteritems():

        if key == field:
            fields_found.append(value)

        elif isinstance(value, dict):
            results = get_recursively(value, field)
            for result in results:
                fields_found.append(result)

        elif isinstance(value, list):
            for item in value:
                if isinstance(item, dict):
                    more_results = get_recursively(item, field)
                    for another_result in more_results:
                        fields_found.append(another_result)

    return fields_found