‘too many values to unpack’, iterating over a dict. key=>string, value=>list

Python 3

Use items().

for field, possible_values in fields.items():
    print(field, possible_values)

Python 2

Use iteritems().

for field, possible_values in fields.iteritems():
    print field, possible_values

See this answer for more information on iterating through dictionaries, such as using items(), across Python versions.

For reference, iteritems() was removed in Python 3.