OO design patterns to use for validation

Question

I am in the process of writing some validation code based on these assumptions:

• Validation code must be in an external class
  • i.e. no data class contains

Answer 1

One size does not fit all! Make it simple!

Provide validators with common methods/interface to output data, categorize warnings, filter/process warnings raised more than once. Do not create any sophisticated way of validation itself, at least not before writing a few real life validators.

Move out of the way and let the validators do what they are supposed to do:

for validator in all_validators:
    validator.validate(model)