import pymongo
c = pymongo.Connection()
db = c['test']

db['test'].drop()
db['test'].ensure_index('a', unique=True)

db['test'].insert({'a': '1', 'b': 1}, safe=True)

try:
    db['test'].insert({'a': '1', 'b': 2}, safe=True)
except pymongo.errors.DuplicateKeyError:
    print 'insert gives pymongo.errors.DuplicateKeyError'

try:
    db['test'].find_and_modify(query={'a': '1', 'b': 2}, update={'$set': {'c': 1}}, upsert=True)
except pymongo.errors.DuplicateKeyError:
    print 'find_and_modify gives pymongo.errors.DuplicateKeyError'
except pymongo.errors.OperationFailure:
    print 'find_and_modify gives pymongo.errors.OperationFailure'

insert gives pymongo.errors.DuplicateKeyError
find_and_modify gives pymongo.errors.DuplicateKeyError

insert gives pymongo.errors.DuplicateKeyError
find_and_modify gives pymongo.errors.OperationFailure