nicolaiarocci/cerberus — pull request #56 by RussellLuo (2014-08-20 14:22:57+00:00)
Title: add `validator` rule for customized function

`validator` rule may be very useful for user to customize validators by defining functions with the following prototype:

def validate_fieldname(field, value, error):
pass

for usage, see `test_validator_rule` in tests.py.