How can I iterate over and modify the values in a dict?

Another way

records.update((k, 0) for k,v in records.iteritems() if v is None)

Example

>>> records
{'bar': None, 'baz': 1, 'foo': None}
>>> records.update((k, 0) for k,v in records.iteritems() if v is None)
>>> records
{'bar': 0, 'baz': 1, 'foo': 0}

for k in records:
    if records[k] is None:
        records[k] = 0

Try

for key, value in records.iteritems():
    if value is None:
        records[key] = 0