Binary Data Semantics

April 24, 2026 · View on GitHub

openapi-schema-validator

About

openapi-schema-validator is a Python library that validates schemas against:

OpenAPI Schema Specification v3.0 <https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.0.md#schemaObject>__ which is an extended subset of the JSON Schema Specification Wright Draft 00 <http://json-schema.org/>__.
OpenAPI Schema Specification v3.1 <https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.1.0.md#schemaObject>__ which is an extended superset of the JSON Schema Specification Draft 2020-12 <http://json-schema.org/>__.
OpenAPI Schema Specification v3.2 <https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.2.0.md#schemaObject>__ using the published OAS 3.2 JSON Schema dialect resources.

Documentation #############

Check documentation to see more details about the features. All documentation is in the "docs" directory and online at openapi-schema-validator.readthedocs.io <https://openapi-schema-validator.readthedocs.io>__

Installation ############

Recommended way (via pip):

.. code-block:: console

pip install openapi-schema-validator

Alternatively you can download the code and install from the repository:

.. code-block:: console

pip install "git+https://github.com/python-openapi/openapi-schema-validator.git"

Usage

validate call signature is:

.. code-block:: python

validate( instance, schema, cls=OAS32Validator, allow_remote_references=False, check_schema=True, enforce_properties_required=False, **kwargs, )

The first argument is always the value you want to validate. The second argument is always the OpenAPI schema object. The cls keyword argument is optional and defaults to OAS32Validator. Use cls when you need a specific validator version/behavior.

.. code-block:: python

from openapi_schema_validator import OAS30Validator from openapi_schema_validator import OAS31Validator from openapi_schema_validator import validate

OpenAPI 3.0 behavior

validate(instance, schema, cls=OAS30Validator)

OpenAPI 3.1 behavior

validate(instance, schema, cls=OAS31Validator)

OpenAPI 3.2 behavior (default)

validate(instance, schema)

Common forwarded keyword arguments include registry (reference context) and format_checker (format validation behavior). By default, validate uses a local-only empty registry to avoid implicit remote $ref retrieval. To resolve external references, pass an explicit registry. Set allow_remote_references=True only if you explicitly accept jsonschema's default remote retrieval behavior.

check_schema defaults to True and validates the schema before validating an instance. For trusted pre-validated schemas in hot paths, set check_schema=False to skip schema checking.

When enforce_properties_required=True is passed, all properties declared in the schema's properties object are strictly required to be present in the instance (except those marked as writeOnly or readOnly where appropriate), regardless of the schema's required array. This is useful for response or contract testing to ensure no documented fields are missing.

The validate helper keeps an internal compiled-validator cache. You can control cache size using the OPENAPI_SCHEMA_VALIDATOR_COMPILED_VALIDATOR_CACHE_MAX_SIZE environment variable (default: 128). The value is loaded once at first use and reused for the lifetime of the process.

To validate an OpenAPI schema:

.. code-block:: python

from openapi_schema_validator import validate

A sample schema

schema = { "type": "object", "required": [ "name" ], "properties": { "name": { "type": "string" }, "age": { "type": ["integer", "null"], "format": "int32", "minimum": 0, }, "birth-date": { "type": "string", "format": "date", }, "address": { "type": 'array', "prefixItems": [ { "type": "number" }, { "type": "string" }, { "enum": ["Street", "Avenue", "Boulevard"] }, { "enum": ["NW", "NE", "SW", "SE"] } ], "items": False, } }, "additionalProperties": False, }

If no exception is raised by validate(), the instance is valid.

validate({"name": "John", "age": 23, "address": [1600, "Pennsylvania", "Avenue"]}, schema)

validate({"name": "John", "city": "London"}, schema)

Expected failure output:

.. code-block:: text

Traceback (most recent call last): ... ValidationError: Additional properties are not allowed ('city' was unexpected)

By default, the latest OpenAPI schema syntax is expected.

Default dialect resolution

The OpenAPI 3.1 and 3.2 base dialect URIs are registered for jsonschema.validators.validator_for resolution. Schemas declaring "$schema" as either "https://spec.openapis.org/oas/3.1/dialect/base" or "https://spec.openapis.org/oas/3.2/dialect/2025-09-17" resolve directly to OAS31Validator and OAS32Validator without unresolved-metaschema fallback warnings.

.. code-block:: python

from jsonschema.validators import validator_for

from openapi_schema_validator import OAS31Validator from openapi_schema_validator import OAS32Validator

schema = { " $schema": "https://spec.openapis.org/oas/3.1/dialect/base", "type": "object", } schema32 = { "$ schema": "https://spec.openapis.org/oas/3.2/dialect/2025-09-17", "type": "object", }

assert validator_for(schema) is OAS31Validator assert validator_for(schema32) is OAS32Validator

Binary Data Semantics

The handling of binary-like payloads differs between OpenAPI versions.

OpenAPI 3.0

OpenAPI 3.0 keeps historical format: binary / format: byte usage on type: string.

OAS30Validator (default - compatibility behavior)

type: string accepts str
type: string, format: binary accepts Python bytes and strings
useful when validating Python-native runtime data

OAS30StrictValidator

type: string accepts str only
type: string, format: binary uses strict format validation
use when you want strict, spec-oriented behavior for 3.0 schemas

OpenAPI 3.1+

OpenAPI 3.1+ follows JSON Schema semantics for string typing in this library.

type: string accepts str only (not bytes)
format: binary and format: byte are not treated as built-in formats
for base64-in-JSON, model with contentEncoding: base64 (optionally contentMediaType)
for raw binary payloads, model via media type (for example application/octet-stream) rather than schema string formats

Regex Behavior

By default, pattern handling follows host Python regex behavior. For ECMAScript-oriented regex validation and matching (via regress), install the optional extra:

.. code-block:: console

pip install "openapi-schema-validator[ecma-regex]"

For more details read about Validation <https://openapi-schema-validator.readthedocs.io/en/latest/validation.html>__.

Related projects ################

openapi-core <https://github.com/python-openapi/openapi-core>__ Python library that adds client-side and server-side support for the OpenAPI.
openapi-spec-validator <https://github.com/python-openapi/openapi-spec-validator>__ Python library that validates OpenAPI Specs against the OpenAPI 2.0 (aka Swagger) and OpenAPI 3.0 specification