A handy function for copying properties from one dict to another with optional key mapping. Small amount of code, and well tested.

test_transfer_dict.py

from transfer_dict import transfer_dict

SOURCE = {
    'a': 'thing',
    'key': 'value',
}

def test_basic():
    dest = transfer_dict(SOURCE)
    assert dest == SOURCE
    assert dest is not SOURCE


def test_merge():
    dest = {
        'b': 'other'
    }

    d2 = transfer_dict(SOURCE, dest)
    assert d2 == {
        'a': 'thing',
        'b': 'other',
        'key': 'value',
    }
    assert d2 is dest


def test_filter():
    dest = transfer_dict(SOURCE, keys=['a'])
    assert dest == { 'a': 'thing' }


def test_mapping():
    dest = transfer_dict(SOURCE, keys=[('argh', 'a'), 'key'])
    assert dest == {
        'argh': 'thing',
        'key': 'value',
    }


def test_map_override():
    dest = {
        'argh': 'moo',
    }
    transfer_dict(SOURCE, dest, keys=[('argh', 'a'), 'key'])
    assert dest == {
        'argh': 'thing',
        'key': 'value',
    }

transfer_dict.py

def transfer_dict(src, dest=None, keys=None):
    '''
    Copies (and translates if necessary) some pairs from a source dict to a dest dict.
    
    src is the dict to copy from
    dest is the dict to copy to (or creates a new dict if not provided)
    keys is a list of keys to be copied. If not provided, it defaults to all the keys in src.
    If a key is a 2-tuple, it should contain (dest_key, src_key) and provides a mapping between the two dicts.
    '''
    if keys is None:
        keys = src.keys()
    
    result = {}
    
    for k in keys:
        if isinstance(k, tuple):
            dest_key, source_key = k
        else:
            dest_key, source_key = k, k
        result[dest_key] = src[source_key]

    if dest is None:
        return result
    else:
        dest.update(result)
        return dest