API Documentation

Validator Class

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

Parameters:

  • 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.

_drop_remaining_rules(*rules)

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.

_error(*args)

Creates and adds one or multiple errors.

Parameters:

args –

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.

_errors

The list of errors that were encountered since the last document processing was invoked. Type: ErrorList

_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.

Parameters:

  • 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.

Returns:

an instance of self.__class__

_lookup_field(path)

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.

_remaining_rules

Keeps track of the rules that are next in line to be evaluated during the validation of a field. Type: list

_valid_schemas = {}

A set of hashes derived from validation schemas that are legit for a particular Validator class.

property allow_unknown

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.

document

The document that is or was recently processed. Type: any mapping

document_error_tree

A tree representiation of encountered errors following the structure of the document. Type: DocumentErrorTree

document_path

The path within the document to the current sub-document. Type: tuple

error_handler

The error handler used to format errors and process submitted errors with _error(). Type: BaseErrorHandler

property errors

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

property ignore_none_values

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

property is_child

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

mandatory_validations = ('nullable',)

Rules that are evaluated on any field, regardless whether defined in the schema or not. Type: tuple

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

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

Parameters:

  • 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.

Returns:

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

priority_validations = ('nullable', 'readonly', 'type', 'empty')

Rules that will be processed in that order before any other. Type: tuple

property purge_unknown

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

recent_error

The last individual error that was submitted. Type: ValidationError

property require_all

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

property root_allow_unknown

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

property root_document

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

property root_require_all

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

property root_schema

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

property rules_set_registry

The registry that holds referenced rules sets. Type: Registry

property schema

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

schema_error_tree

A tree representiation of encountered errors following the structure of the schema. Type: SchemaErrorTree

schema_path

The path within the schema to the current sub-schema. Type: tuple

property schema_registry

The registry that holds referenced schemas. Type: Registry

types_mapping = {'binary': ('binary', (<class 'bytes'>, <class 'bytearray'>), ()), 'boolean': ('boolean', (<class 'bool'>,), ()), 'container': ('container', (<class 'collections.abc.Container'>,), (<class 'str'>,)), 'date': ('date', (<class 'datetime.date'>,), ()), 'datetime': ('datetime', (<class 'datetime.datetime'>,), ()), 'dict': ('dict', (<class 'collections.abc.Mapping'>,), ()), 'float': ('float', (<class 'float'>, (<class 'int'>,)), ()), 'integer': ('integer', ((<class 'int'>,),), ()), 'list': ('list', (<class 'collections.abc.Sequence'>,), (<class 'str'>,)), 'number': ('number', ((<class 'int'>,), <class 'float'>), (<class 'bool'>,)), 'set': ('set', (<class 'set'>,), ()), 'string': ('string', (<class 'str'>,), ())}

This mapping holds all available constraints for the type rule and their assigned TypeDefinition.

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

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

Parameters:

  • 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.

Returns:

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

Return type:

bool

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.

Parameters:

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

  • definition (any mapping) – The definition.

all()

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

clear()

Purge all definitions in the registry.

extend(definitions)

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.

Parameters:

  • name (str) – The reference that points to the definition.

  • default – Return value if the reference isn’t registered.

remove(*names)

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)

This class is used to define types that can be used as value in the types_mapping property. The name should be descriptive and match the key it is going to be assigned to. A value that is validated against such definition must be an instance of any of the types contained in included_types and must not match any of the types contained in 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.

__call__(errors)

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.

__iter__()

Be a superhero and implement an iterator over errors.

__weakref__

list of weak references to the object

add(error)

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

Parameters:

error (ValidationError) – The error to add.

emit(error)

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.

end(validator)

Gets called when a validation ends.

Parameters:

validator (Validator) – The calling validator.

extend(errors)

Adds all errors to the handler’s container object.

Parameters:

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

start(validator)

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)

This class is used to define possible errors. Each distinguishable error is defined by a unique error code as integer and the rule that can cause it as string. The instances’ names do not contain a common prefix as they are supposed to be referenced within the module namespace, e.g. errors.CUSTOM.

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

A simple class to store and query basic error information.

property child_errors

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

code

The error’s identifier code. Type: int

constraint

The constraint that failed.

property definitions_errors

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

document_path

The path to the field within the document that caused the error. Type: tuple

property field

Field of the contextual mapping, possibly None.

info

May hold additional information about the error. Type: tuple

property is_group_error

True for errors of bulk validations.

property is_logic_error

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

property is_normalization_error

True for normalization errors.

rule

The rule that failed. Type: string

schema_path

The path to the rule within the schema that caused the error. Type: tuple

value

The value that failed.

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.

Important

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

3

0x3

UNKNOWN_FIELD

None

4

0x4

DEPENDENCIES_FIELD

dependencies

5

0x5

DEPENDENCIES_FIELD_VALUE

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

96

0x60

NORMALIZATION

None

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(iterable=(), /)

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(error)

Add an error to the tree.

Parameters:

error – ValidationError

fetch_errors_from(path)

Returns all errors for a particular path.

Parameters:

path – tuple of hashable s.

Return type:

ErrorList

fetch_node_from(path)

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.

Exceptions

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

Utilities

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

This class is used to define types that can be used as value in the types_mapping property. The name should be descriptive and match the key it is going to be assigned to. A value that is validated against such definition must be an instance of any of the types contained in included_types and must not match any of the types contained in excluded_types.

excluded_types

Alias for field number 2

included_types

Alias for field number 1

name

Alias for field number 0

cerberus.utils.mapping_to_frozenset(mapping)

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(fget=None, fset=None, fdel=None, doc=None)
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.

Parameters:

  • 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.

Returns:

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']}}