API Documentation

Validator Class

class cerberus.Validator(*args, **kwargs)

Validator class. Normalizes and/or validates any mapping against a validation-schema which is provided as an argument at class instantiation or upon calling the validate(), validated() or normalized() method. An instance itself is callable and executes a validation.

All instantiation parameters are optional.

There are the introspective properties types, validators, coercers, default_setters, rules, normalization_rules and validation_rules.

The attributes reflecting the available rules are assembled considering constraints that are defined in the docstrings of rules’ methods and is effectively used as validation schema for schema.

  • schema (any mapping) – See schema. Defaults to None.
  • ignore_none_values (bool) – See ignore_none_values. Defaults to False.
  • allow_unknown (bool or any mapping) – See allow_unknown. Defaults to False.
  • require_all (bool) – See require_all. Defaults to False.
  • purge_unknown (bool) – See purge_unknown. Defaults to to False.
  • purge_readonly (bool) – Removes all fields that are defined as readonly in the normalization phase.
  • error_handler (class or instance based on BaseErrorHandler or tuple) – The error handler that formats the result of errors. When given as two-value tuple with an error-handler class and a dictionary, the latter is passed to the initialization of the error handler. Default: BasicErrorHandler.

Drops rules from the queue of the rules that still need to be evaluated for the currently processed field. If no arguments are given, the whole queue is emptied.


Creates and adds one or multiple errors.


Accepts different argument’s signatures.

1. Bulk addition of errors:

The errors will be added to _errors.

2. Custom error:

  • the invalid field’s name
  • the error message

A custom error containing the message will be created and added to _errors. There will however be fewer information contained in the error (no reference to the violated rule and its constraint).

3. Defined error:

  • the invalid field’s name
  • the error-reference, see cerberus.errors
  • arbitrary, supplemental information about the error

A ValidationError instance will be created and added to _errors.

_get_child_validator(document_crumb=None, schema_crumb=None, **kwargs)
Creates a new instance of Validator-(sub-)class. All initial
parameters of the parent are passed to the initialization, unless a parameter is given as an explicit keyword-parameter.
  • document_crumb (tuple or hashable) – Extends the document_path of the child-validator.
  • schema_crumb (tuple or hashable) – Extends the schema_path of the child-validator.
  • kwargs (dict) – Overriding keyword-arguments for initialization.

an instance of self.__class__

Searches for a field as defined by path. This method is used by the
dependency evaluation logic.
Parameters:path (str) – Path elements are separated by a .. A leading ^ indicates that the path relates to the document root, otherwise it relates to the currently evaluated document, which is possibly a subdocument. The sequence ^^ at the start will be interpreted as a literal ^.
Returns:Either the found field name and its value or None for both.
Return type:A two-value tuple.

If True unknown fields that are not defined in the schema will be ignored. If a mapping with a validation schema is given, any undefined field will be validated against its rules. Also see Allowing the Unknown. Type: bool or any mapping

classmethod clear_caches()

Purge the cache of known valid schemas.


The errors of the last processing formatted by the handler that is bound to error_handler.


Whether to not process None-values in a document or not. Type: bool


True for child-validators obtained with _get_child_validator(). Type: bool

normalized(document, schema=None, always_return_document=False)

Returns the document normalized according to the specified rules of a schema.

  • document (any mapping) – The document to normalize.
  • schema (any mapping) – The validation schema. Defaults to None. If not provided here, the schema must have been provided at class instantiation.
  • always_return_document (bool) – Return the document, even if an error occurred. Defaults to: False.

A normalized copy of the provided mapping or None if an error occurred during normalization.


If True, unknown fields will be deleted from the document unless a validation is called with disabled normalization. Also see Purging Unknown Fields. Type: bool


If True known fields that are defined in the schema will be required. Type: bool


The allow_unknown attribute of the first level ancestor of a child validator.


The document attribute of the first level ancestor of a child validator.


The require_all attribute of the first level ancestor of a child validator.


The schema attribute of the first level ancestor of a child validator.


The registry that holds referenced rules sets. Type: Registry


The validation schema of a validator. When a schema is passed to a method, it replaces this attribute. Type: any mapping or None


The registry that holds referenced schemas. Type: Registry

validate(document, schema=None, update=False, normalize=True)

Normalizes and validates a mapping against a validation-schema of defined rules.

  • document (any mapping) – The document to normalize.
  • schema (any mapping) – The validation schema. Defaults to None. If not provided here, the schema must have been provided at class instantiation.
  • update (bool) – If True, required fields won’t be checked.
  • normalize (bool) – If True, normalize the document before validation.

True if validation succeeds, otherwise False. Check the errors() property for a list of processing errors.

Return type:


validated(*args, **kwargs)

Wrapper around validate() that returns the normalized and validated document or None if validation failed.

Rules Set & Schema Registry

class cerberus.schema.Registry(definitions={})

A registry to store and retrieve schemas and parts of it by a name that can be used in validation schemas.

Parameters:definitions (any mapping) – Optional, initial definitions.
add(name, definition)

Register a definition to the registry. Existing definitions are replaced silently.

  • name (str) – The name which can be used as reference in a validation schema.
  • definition (any mapping) – The definition.

Returns a dict with all registered definitions mapped to their name.


Purge all definitions in the registry.


Add several definitions at once. Existing definitions are replaced silently.

Parameters:definitions (a mapping or an iterable with two-value tuple s) – The names and definitions.
get(name, default=None)

Retrieve a definition from the registry.

  • name (str) – The reference that points to the definition.
  • default – Return value if the reference isn’t registered.

Unregister definitions from the registry.

Parameters:names – The names of the definitions that are to be unregistered.

Type Definitions

class cerberus.TypeDefinition(name, included_types, excluded_types)

Error Handlers

class cerberus.errors.BaseErrorHandler(*args, **kwargs)

Base class for all error handlers. Subclasses are identified as error-handlers with an instance-test.


Returns errors in a handler-specific format.

Parameters:errors (iterable of ValidationError instances or a Validator instance) – An object containing the errors.
__init__(*args, **kwargs)

Optionally initialize a new instance.


Be a superhero and implement an iterator over errors.


list of weak references to the object (if defined)


Add an error to the errors’ container object of a handler.

Parameters:error (ValidationError) – The error to add.
Optionally emits an error in the handler’s format to a stream.
Or light a LED, or even shut down a power plant.
Parameters:error (ValidationError) – The error to emit.

Gets called when a validation ends.

Parameters:validator (Validator) – The calling validator.

Adds all errors to the handler’s container object.

Parameters:errors (iterable of ValidationError instances) – The errors to add.

Gets called when a validation starts.

Parameters:validator (Validator) – The calling validator.
class cerberus.errors.BasicErrorHandler(tree=None)

Models cerberus’ legacy. Returns a dict. When mangled through str a pretty-formatted representation of that tree is returned.

Python Error Representations

class cerberus.errors.ErrorDefinition(code, rule)
class cerberus.errors.ValidationError(document_path, schema_path, code, rule, constraint, value, info)

A simple class to store and query basic error information.


A list that contains the individual errors of a bulk validation error.


Dictionary with errors of an *of-rule mapped to the index of the definition it occurred in. Returns None if not applicable.


Field of the contextual mapping, possibly None.


True for errors of bulk validations.


True for validation errors against different schemas with *of-rules.


True for normalization errors.

Error Codes

Its code attribute uniquely identifies an ErrorDefinition that is used a concrete error’s code. Some codes are actually reserved to mark a shared property of different errors. These are useful as bitmasks while processing errors. This is the list of the reserved codes:

0110 0000 0x60 96 An error that occurred during normalization.
1000 0000 0x80 128 An error that contains child errors.
1001 0000 0x90 144 An error that was emitted by one of the *of-rules.

None of these bits in the upper nibble must be used to enumerate error definitions, but only to mark one with the associated property.


Users are advised to set bit 8 for self-defined errors. So the code 0001 0000 0001 / 0x101 would the first in a domain-specific set of error definitions.

This is a list of all error defintions that are shipped with the errors module:

Code (dec.) Code (hex.) Name Rule
0 0x0 CUSTOM None
2 0x2 REQUIRED_FIELD required
4 0x4 DEPENDENCIES_FIELD dependencies
6 0x6 EXCLUDES_FIELD excludes
34 0x22 EMPTY_NOT_ALLOWED empty
35 0x23 NOT_NULLABLE nullable
36 0x24 BAD_TYPE type
37 0x25 BAD_TYPE_FOR_SCHEMA schema
38 0x26 ITEMS_LENGTH items
39 0x27 MIN_LENGTH minlength
40 0x28 MAX_LENGTH maxlength
65 0x41 REGEX_MISMATCH regex
66 0x42 MIN_VALUE min
67 0x43 MAX_VALUE max
68 0x44 UNALLOWED_VALUE allowed
69 0x45 UNALLOWED_VALUES allowed
70 0x46 FORBIDDEN_VALUE forbidden
71 0x47 FORBIDDEN_VALUES forbidden
72 0x48 MISSING_MEMBERS contains
97 0x61 COERCION_FAILED coerce
98 0x62 RENAMING_FAILED rename_handler
99 0x63 READONLY_FIELD readonly
100 0x64 SETTING_DEFAULT_FAILED default_setter
128 0x80 ERROR_GROUP None
129 0x81 MAPPING_SCHEMA schema
130 0x82 SEQUENCE_SCHEMA schema
131 0x83 KEYSRULES keysrules
131 0x83 KEYSCHEMA keysrules
132 0x84 VALUESRULES valuesrules
132 0x84 VALUESCHEMA valuesrules
143 0x8f BAD_ITEMS items
144 0x90 LOGICAL None
145 0x91 NONEOF noneof
146 0x92 ONEOF oneof
147 0x93 ANYOF anyof
148 0x94 ALLOF allof

Error Containers

class cerberus.errors.ErrorList

A list for ValidationError instances that can be queried with the in keyword for a particular ErrorDefinition.

class cerberus.errors.ErrorTree(errors=[])

Base class for DocumentErrorTree and SchemaErrorTree.


Add an error to the tree.


Returns all errors for a particular path.

Parameters:pathtuple of hashable s.
Return type:ErrorList

Returns a node for a path.

Parameters:path – Tuple of hashable s.
Return type:ErrorTreeNode or None
class cerberus.errors.DocumentErrorTree(errors=[])

Implements a dict-like class to query errors by indexes following the structure of a validated document.

class cerberus.errors.SchemaErrorTree(errors=[])

Implements a dict-like class to query errors by indexes following the structure of the used schema.


exception cerberus.SchemaError

Raised when the validation schema is missing, has the wrong format or contains errors.

exception cerberus.DocumentError

Raised when the target document is missing or has the wrong format


class cerberus.utils.TypeDefinition(name, included_types, excluded_types)

Alias for field number 2


Alias for field number 1


Alias for field number 0


Be aware that this treats any sequence type with the equal members as equal. As it is used to identify equality of schemas, this can be considered okay as definitions are semantically equal regardless the container type.

class cerberus.utils.readonly_classproperty
cerberus.utils.validator_factory(name, bases=None, namespace={})
Dynamically create a Validator subclass.
Docstrings of mixin-classes will be added to the resulting class’ one if __doc__ is not in namespace.
  • name (str) – The name of the new class.
  • bases (tuple of or a single class) – Class(es) with additional and overriding attributes.
  • namespace (dict) – Attributes for the new class.

The created class.

Schema Validation Schema

Against this schema validation schemas given to a vanilla Validator will be validated:

{'allof': {'logical': 'allof', 'type': 'list'},
'allow_unknown': {'oneof': [{'type': 'boolean'},
                            {'check_with': 'bulk_schema',
                             'type': ['dict', 'string']}]},
'allowed': {'type': 'container'},
'anyof': {'logical': 'anyof', 'type': 'list'},
'check_with': {'oneof': [{'type': 'callable'},
                         {'schema': {'oneof': [{'type': 'callable'},
                                               {'allowed': (),
                                                'type': 'string'}]},
                          'type': 'list'},
                         {'allowed': (), 'type': 'string'}]},
'coerce': {'oneof': [{'type': 'callable'},
                     {'schema': {'oneof': [{'type': 'callable'},
                                           {'allowed': (),
                                            'type': 'string'}]},
                      'type': 'list'},
                     {'allowed': (), 'type': 'string'}]},
'contains': {'empty': False},
'default': {'nullable': True},
'default_setter': {'oneof': [{'type': 'callable'},
                             {'allowed': (), 'type': 'string'}]},
'dependencies': {'check_with': 'dependencies',
                 'type': ('dict', 'hashable', 'list')},
'empty': {'type': 'boolean'},
'excludes': {'schema': {'type': 'hashable'},
             'type': ('hashable', 'list')},
'forbidden': {'type': 'list'},
'items': {'check_with': 'items', 'type': 'list'},
'keysrules': {'check_with': 'bulk_schema',
              'forbidden': ['rename', 'rename_handler'],
              'type': ['dict', 'string']},
'max': {'nullable': False},
'maxlength': {'type': 'integer'},
'meta': {},
'min': {'nullable': False},
'minlength': {'type': 'integer'},
'noneof': {'logical': 'noneof', 'type': 'list'},
'nullable': {'type': 'boolean'},
'oneof': {'logical': 'oneof', 'type': 'list'},
'purge_unknown': {'type': 'boolean'},
'readonly': {'type': 'boolean'},
'regex': {'type': 'string'},
'rename': {'type': 'hashable'},
'rename_handler': {'oneof': [{'type': 'callable'},
                             {'schema': {'oneof': [{'type': 'callable'},
                                                   {'allowed': (),
                                                    'type': 'string'}]},
                              'type': 'list'},
                             {'allowed': (), 'type': 'string'}]},
'require_all': {'type': 'boolean'},
'required': {'type': 'boolean'},
'schema': {'anyof': [{'check_with': 'schema'},
                     {'check_with': 'bulk_schema'}],
           'type': ['dict', 'string']},
'type': {'check_with': 'type', 'type': ['string', 'list']},
'valuesrules': {'check_with': 'bulk_schema',
                'forbidden': ['rename', 'rename_handler'],
                'type': ['dict', 'string']}}