Upgrading to Cerberus 1.0

Major Additions

Error Handling

The inspection on and representation of errors is thoroughly overhauled and allows a more detailed and flexible handling. Make sure you have look on Errors & Error Handling.

Also, errors (as provided by the default BasicErrorHandler) values are lists containing error messages, and possibly a dict as last item containing nested errors. Previously, they were strings if single errors per field occured; lists otherwise.


Validator class


In the past you could override the schema validation by setting transparent_schema_rules to True. Now all rules whose implementing method’s docstring contain a schema to validate the arguments for that rule in the validation schema, are validated. To omit the schema validation for a particular rule, just omit that definition, but consider it a bad practice. The Validator-attribute and -initialization-argument transparent_schema_rules are removed without replacement.


The method validate_update has been removed from Validator. Instead use validate() with the keyword-argument update set to True.


items (for mappings)

The usage of the items-rule is restricted to sequences. If you still had schemas that used that rule to validate mappings, just rename these instances to schema (docs).

keyschema & valueschema

To reflect the common terms in the Pythoniverse [1], the rule for validating all values of a mapping was renamed from keyschema to valueschema (docs). Furthermore a rule was implemented to validate all keys, introduced as propertyschema, now renamed to keyschema (docs). This means code using prior versions of cerberus would not break, but bring up wrong results!

To update your code you may adapt cerberus’ iteration:

  1. Rename keyschema to valueschema in your schemas. (0.9)
  2. Rename propertyschema to keyschema in your schemas. (1.0)
Note that propertyschema will not be handled as an alias like
keyschema was in the 0.9-branch.

Custom validators

Data types

Since the type-rule allowed multiple arguments cerberus’ type validation code was somewhat cumbersome as it had to deal with the circumstance that each type checking method would file an error though another one may not - and thus positively validate the constraint as a whole. The refactoring of the error handling allows cerberus’ type validation to be much more lightweight and to formulate the corresponding methods in a simpler way.

Previously such a method would test what a value is not and submit an error. Now a method tests what a value is to be expected and returns True in that case.

This is the most critical part of updating your code, but still easy when your head is clear. Of course your code is well tested. It’s essentially these three steps. Search, Replace and Regex may come at your service.

  1. Remove the second method’s argument (probably named field).
  2. Invert the logic of the conditional clauses where is tested what a value is not / has not.
  3. Replace calls to self._error below such clauses with return True.

A method doesn’t need to return False or any value when expected criteria are not met.

Here’s the change from the documentation example.


def _validate_type_objectid(self, field, value):
    if not re.match('[a-f0-9]{24}', value):
        self._error(field, errors.BAD_TYPE)


def _validate_type_objectid(self, value):
    if re.match('[a-f0-9]{24}', value):
        return True
[1]compare dictionary