Extending Cerberus#

Though you can use functions in conjunction with the coerce and the check_with rules, you can easily extend the Validator class with custom rules, types, check_with handlers, coercers and default_setters. While the function-based style is more suitable for special and one-off uses, a custom class leverages these possibilities:

  • custom rules can be defined with constrains in a schema

  • extending the available type s

  • use additional contextual data

  • schemas are serializable

The references in schemas to these custom methods can use space characters instead of underscores, e.g. {'foo': {'check_with': 'is odd'}} is an alias for {'foo': {'check_with': 'is_odd'}}.

Custom Rules#

Suppose that in our use case some values can only be expressed as odd integers, therefore we decide to add support for a new is_odd rule to our validation schema:

schema = {'amount': {'is odd': True, 'type': 'integer'}}

This is how we would go to implement that:

from cerberus import Validator

class MyValidator(Validator):
    def _validate_is_odd(self, constraint, field, value):
        """ Test the oddity of a value.

        The rule's arguments are validated against this schema:
        {'type': 'boolean'}
        if constraint is True and not bool(value & 1):
            self._error(field, "Must be an odd number")

By subclassing Cerberus Validator class and adding the custom _validate_<rulename> method, we just enhanced Cerberus to suit our needs. The custom rule is_odd is now available in our schema and, what really matters, we can use it to validate all odd values:

>>> v = MyValidator(schema)
>>> v.validate({'amount': 10})
>>> v.errors
{'amount': ['Must be an odd number']}
>>> v.validate({'amount': 9})

As schemas themselves are validated, you can provide constraints as literal Python expression in the docstring of the rule’s implementing method to validate the arguments given in a schema for that rule. Either the docstring contains solely the literal or the literal is placed at the bottom of the docstring preceded by The rule's arguments are validated against this schema: See the source of the contributed rules for more examples.

Custom Data Types#

Cerberus supports and validates several standard data types (see type). When building a custom validator you can add and validate your own data types.

Additional types can be added on the fly by assigning a TypeDefinition to the designated type name in types_mapping:

from decimal import Decimal

decimal_type = cerberus.TypeDefinition('decimal', (Decimal,), ())

Validator.types_mapping['decimal'] = decimal_type


As the types_mapping property is a mutable type, any change to its items on an instance will affect its class.

They can also be defined for subclasses of Validator:

from decimal import Decimal

decimal_type = cerberus.TypeDefinition('decimal', (Decimal,), ())

class MyValidator(Validator):
    types_mapping = Validator.types_mapping.copy()
    types_mapping['decimal'] = decimal_type

New in version 0.0.2.

Changed in version 1.0: The type validation logic changed, see Upgrading to Cerberus 1.0.

Changed in version 1.2: Added the types_mapping property and marked methods for testing types as deprecated.

Methods that can be referenced by the check_with rule#

If a validation test doesn’t depend on a specified constraint from a schema or needs to be more complex than a rule should be, it’s possible to rather define it as value checker than as a rule. There are two ways to use the check_with rule.

One is by extending Validator with a method prefixed with _check_with_. This allows to access the whole context of the validator instance including arbitrary configuration values and state. To reference such method using the check_with rule, simply pass the unprefixed method name as a string constraint.

For example, one can define an oddity validator method as follows:

class MyValidator(Validator):
    def _check_with_oddity(self, field, value):
        if not value & 1:
            self._error(field, "Must be an odd number")

Usage would look something like:

schema = {'amount': {'type': 'integer', 'check_with': 'oddity'}}

The second option to use the rule is to define a standalone function and pass it as the constraint. This brings with it the benefit of not having to extend Validator. To read more about this implementation and see examples check out the rule’s documentation.

Custom Coercers#

You can also define custom methods that return a coerce d value or point to a method as rename_handler. The method name must be prefixed with _normalize_coerce_.

class MyNormalizer(Validator):
    def __init__(self, multiplier, *args, **kwargs):
        super(MyNormalizer, self).__init__(*args, **kwargs)
        self.multiplier = multiplier

    def _normalize_coerce_multiply(self, value):
        return value * self.multiplier
>>> schema = {'foo': {'coerce': 'multiply'}}
>>> document = {'foo': 2}
>>> MyNormalizer(multiplier=2).normalized(document, schema)
{'foo': 4}

Custom Default Setters#

Similar to custom rename handlers, it is also possible to create custom default setters.

from datetime import datetime

class MyNormalizer(Validator):
    def _normalize_default_setter_utcnow(self, document):
        return datetime.utcnow()
>>> schema = {'creation_date': {'type': 'datetime', 'default_setter': 'utcnow'}}
>>> MyNormalizer().normalized({}, schema)
{'creation_date': datetime.datetime(...)}


It may be a bad idea to overwrite particular contributed rules.

Attaching Configuration Data And Instantiating Custom Validators#

It’s possible to pass arbitrary configuration values when instantiating a Validator or a subclass as keyword arguments (whose names are not used by Cerberus). These can be used in all of the handlers described in this document that have access to the instance. Cerberus ensures that this data is available in all child instances that may get spawned during processing. When you implement an __init__ method on a customized validator, you must ensure that all positional and keyword arguments are also passed to the parent class’ initialization method. Here’s an example pattern:

class MyValidator(Validator):
    def __init__(self, *args, **kwargs):
        # assign a configuration value to an instance property
        # for convenience
        self.additional_context = kwargs.get('additional_context')
        # pass all data to the base classes
        super(MyValidator, self).__init__(*args, **kwargs)

    # alternatively a dynamic property can be defined, rendering
    # the __init__ method unnecessary in this example case
    def additional_context(self):
        return self._config.get('additional_context', 'bar')

    # an optional property setter if you deal with state
    def additional_context(self, value):
        self._config["additional_context"] = value

    def _check_with_foo(self, field, value):


It is neither recommended to access the _config property in other situations than outlined in the sketch above nor to to change its contents during the processing of a document. Both cases are not tested and are unlikely to get officially supported.

New in version 0.9.

There’s a function validator_factory() to get a Validator mutant with concatenated docstrings.

New in version 1.0.

Relevant Validator-attributes#

There are some attributes of a Validator that you should be aware of when writing custom Validators.


A validator accesses the document property when fetching fields for validation. It also allows validation of a field to happen in context of the rest of the document.

New in version 0.7.1.


Alike, the schema property holds the used schema.


This attribute is not the same object that was passed as schema to the validator at some point. Also, its content may differ, though it still represents the initial constraints. It offers the same interface like a dict.


There are three signatures that are accepted to submit errors to the Validator’s error stash. If necessary the given information will be parsed into a new instance of ValidationError.

Full disclosure#

In order to be able to gain complete insight into the context of an error at a later point, you need to call _error() with two mandatory arguments:

For custom rules you need to define an error as ErrorDefinition with a unique id and the causing rule that is violated. See errors for a list of the contributed error definitions. Keep in mind that bit 7 marks a group error, bit 5 marks an error raised by a validation against different sets of rules.

Optionally you can submit further arguments as information. Error handlers that are targeted for humans will use these as positional arguments when formatting a message with str.format(). Serializing handlers will keep these values in a list.

New in version 1.0.

Simple custom errors#

A simpler form is to call _error() with the field and a string as message. However the resulting error will contain no information about the violated constraint. This is supposed to maintain backward compatibility, but can also be used when an in-depth error handling isn’t needed.

Multiple errors#

When using child-validators, it is a convenience to submit all their errors ; which is a list of ValidationError instances.

New in version 1.0.


If you need another instance of your Validator-subclass, the _get_child_validator()-method returns another instance that is initiated with the same arguments as self was. You can specify overriding keyword-arguments. As the properties document_path and schema_path (see below) are inherited by the child validator, you can extend these by passing a single value or values-tuple with the keywords document_crumb and schema_crumb. Study the source code for example usages.

New in version 0.9.

Changed in version 1.0: Added document_crumb and schema_crumb as optional keyword- arguments.

Validator.root_document, .root_schema, .root_allow_unknown & .root_require_all#

A child-validator - as used when validating a schema - can access the first generation validator’s document and schema that are being processed as well as the constraints for unknown fields via its root_document, root_schema, root_allow_unknown and root_require_all properties.

New in version 1.0.

Changed in version 1.3: Added root_require_all

Validator.document_path & Validator.schema_path#

These properties maintain the path of keys within the document respectively the schema that was traversed by possible parent-validators. Both will be used as base path when an error is submitted.

New in version 1.0.


The last single error that was submitted is accessible through the recent_error-attribute.

New in version 1.0.

Validator.mandatory_validations, Validator.priority_validations & Validator._remaining_rules#

You can use these class properties and instance instance property if you want to adjust the validation logic for each field validation. mandatory_validations is a tuple that contains rules that will be validated for each field, regardless if the rule is defined for a field in a schema or not. priority_validations is a tuple of ordered rules that will be validated before any other. _remaining_rules is a list that is populated under consideration of these and keeps track of the rules that are next in line to be evaluated. Thus it can be manipulated by rule handlers to change the remaining validation for the current field. Preferably you would call _drop_remaining_rules() to remove particular rules or all at once.

New in version 1.0.

Changed in version 1.2: Added _remaining_rules for extended leverage.