Pick keys/values from dict

pick.py

def pick(d, filter_tree):
    '''Returns a new dictionary by picking keys/values from d per filter_tree.
    Works recursively: values in filter_tree can be either True or dictionaries
    which are their own filters.

    >>> d = {'a': 1, 'b': 2, 'c': {'foo': -1, 'bar': -2}, 'd': {'r': 'red', 'b': 'blue'}}
    >>> pick(d, {'a': True, 'b': True})
    {'a': 1, 'b': 2}
    >>> pick(d, {'a': True, 'b': True, 'd': True})
    {'a': 1, 'b': 2, 'd': {'r': 'red', 'b': 'blue'}}
    >>> pick(d, {'a': True, 'b': True, 'd': {'r': True}})
    {'a': 1, 'b': 2, 'd': {'r': 'red'}}
    '''
    r = {}
    for k, filter_branch in filter_tree.items():
        if isinstance(filter_branch, dict):
            r[k] = pick(d[k], filter_branch)
        elif filter_branch is True:
            r[k] = d[k]
    return r