validate.rst

Schema Validation

.. currentmodule:: jsonschema

Tip

Most of the documentation for this package assumes you're familiar with the fundamentals of writing JSON schemas themselves, and focuses on how this library helps you validate with them in Python.

If you aren't already comfortable with writing schemas and need an introduction which teaches about JSON Schema the specification, you may find :ujs:`Understanding JSON Schema </>` to be a good read!

The Basics

The simplest way to validate an instance under a given schema is to use the validate <jsonschema.validators.validate> function.

.. autofunction:: validate
    :noindex:

The Validator Protocol

jsonschema defines a protocol <typing.Protocol> that all validator classes adhere to.

Hint

If you are unfamiliar with protocols, either as a general notion or as specifically implemented by typing.Protocol, you can think of them as a set of attributes and methods that all objects satisfying the protocol have.

Here, in the context of jsonschema, the Validator.iter_errors method can be called on jsonschema.validators.Draft202012Validator, or jsonschema.validators.Draft7Validator, or indeed any validator class, as all of them have it, along with all of the other methods described below.

.. autoclass:: jsonschema.protocols.Validator
    :noindex:
    :members:

All of the versioned validators <versioned-validators> that are included with jsonschema adhere to the protocol, and any extensions of these validators <jsonschema.validators.extend> will as well. For more information on creating <jsonschema.validators.create> or extending <jsonschema.validators.extend> validators see creating-validators.

Type Checking

To handle JSON Schema's :kw:`type` keyword, a Validator uses an associated TypeChecker. The type checker provides an immutable mapping between names of types and functions that can test if an instance is of that type. The defaults are suitable for most users - each of the versioned validators <versioned-validators> that are included with jsonschema have a TypeChecker that can correctly handle their respective versions.

.. seealso:: `validating-types`

    For an example of providing a custom type check.

.. autoclass:: TypeChecker
    :members:
    :noindex:

.. autoexception:: jsonschema.exceptions.UndefinedTypeCheck
    :noindex:

    Raised when trying to remove a type check that is not known to this
    TypeChecker, or when calling `jsonschema.TypeChecker.is_type`
    directly.

Validating With Additional Types

Occasionally it can be useful to provide additional or alternate types when validating JSON Schema's :kw:`type` keyword.

jsonschema tries to strike a balance between performance in the common case and generality. For instance, JSON Schema defines a number type, which can be validated with a schema such as {"type" : "number"}. By default, this will accept instances of Python numbers.Number. This includes in particular ints and floats, along with decimal.Decimal objects, complex numbers etc. For integer and object, however, rather than checking for numbers.Integral and collections.abc.Mapping, jsonschema simply checks for int and dict, since the more general instance checks can introduce significant slowdown, especially given how common validating these types are.

If you do want the generality, or just want to add a few specific additional types as being acceptable for a validator object, then you should update an existing jsonschema.TypeChecker or create a new one. You may then create a new Validator via jsonschema.validators.extend.

.. testcode::

    from jsonschema import validators

    class MyInteger:
        pass

    def is_my_int(checker, instance):
        return (
            Draft202012Validator.TYPE_CHECKER.is_type(instance, "number") or
            isinstance(instance, MyInteger)
        )

    type_checker = Draft202012Validator.TYPE_CHECKER.redefine(
        "number", is_my_int,
    )

    CustomValidator = validators.extend(
        Draft202012Validator,
        type_checker=type_checker,
    )
    validator = CustomValidator(schema={"type" : "number"})

.. autoexception:: jsonschema.exceptions.UnknownType
    :noindex:

Versioned Validators

jsonschema ships with validator classes for various versions of the JSON Schema specification. For details on the methods and attributes that each validator class provides see the Validator protocol, which each included validator class implements.

Each of the below cover a specific release of the JSON Schema specification.

.. autoclass:: Draft202012Validator
    :noindex:

.. autoclass:: Draft201909Validator
    :noindex:

.. autoclass:: Draft7Validator
    :noindex:

.. autoclass:: Draft6Validator
    :noindex:

.. autoclass:: Draft4Validator
    :noindex:

.. autoclass:: Draft3Validator
    :noindex:

For example, if you wanted to validate a schema you created against the Draft 2020-12 meta-schema, you could use:

.. testcode::

    from jsonschema import Draft202012Validator

    schema = {
        "$schema": Draft202012Validator.META_SCHEMA["$id"],

        "type": "object",
        "properties": {
            "name": {"type": "string"},
            "email": {"type": "string"},
        },
        "required": ["email"]
    }
    Draft202012Validator.check_schema(schema)

Validating Formats

JSON Schema defines the :kw:`format` keyword which can be used to check if primitive types (strings, numbers, booleans) conform to well-defined formats. By default, as per the specification, no validation is enforced. Optionally however, validation can be enabled by hooking a format-checking object <jsonschema.FormatChecker> into a Validator.

>>> validate("127.0.0.1", {"format" : "ipv4"})
>>> validate(
...     instance="-12",
...     schema={"format" : "ipv4"},
...     format_checker=Draft202012Validator.FORMAT_CHECKER,
... )
Traceback (most recent call last):
    ...
ValidationError: "-12" is not a "ipv4"

Some formats require additional dependencies to be installed.

The easiest way to ensure you have what is needed is to install jsonschema using the format or format-nongpl extras.

For example:

$ pip install jsonschema[format]

Or if you want to avoid GPL dependencies, a second extra is available:

$ pip install jsonschema[format-nongpl]

At the moment, it supports all the available checkers except for iri and iri-reference.

Warning

It is your own responsibility ultimately to ensure you are license-compliant, so you should be double checking your own dependencies if you rely on this extra.

The more specific list of formats along with any additional dependencies they have is shown below.

Warning

If a dependency is not installed when using a checker that requires it, validation will succeed without throwing an error, as also specified by the specification.

Checker	Notes
color	requires webcolors
date
date-time	requires rfc3339-validator
duration	requires isoduration
email
hostname	requires fqdn
idn-hostname	requires idna
ipv4
ipv6	OS must have socket.inet_pton function
iri	requires rfc3987
iri-reference	requires rfc3987
json-pointer	requires jsonpointer
regex
relative-json-pointer	requires jsonpointer
time	requires rfc3339-validator
uri	requires rfc3987 or rfc3986-validator
uri-reference	requires rfc3987 or rfc3986-validator
uri-template	requires uri-template

The supported mechanism for ensuring these dependencies are present is again as shown above, not by directly installing the packages.

.. autoclass:: FormatChecker
    :members:
    :noindex:
    :exclude-members: cls_checks

    .. attribute:: checkers

        A mapping of currently known formats to tuple of functions that validate them and errors that should be caught.
        New checkers can be added and removed either per-instance or globally for all checkers using the `FormatChecker.checks` decorator.

    .. classmethod:: cls_checks(format, raises=())

        Register a decorated function as *globally* validating a new format.

        Any instance created after this function is called will pick up the supplied checker.

        :argument str format: the format that the decorated function will check
        :argument Exception raises: the exception(s) raised
            by the decorated function when an invalid instance is
            found. The exception object will be accessible as the
            `jsonschema.exceptions.ValidationError.cause` attribute
            of the resulting validation error.

        .. deprecated:: v4.14.0

            Use `FormatChecker.checks` on an instance instead.

.. autoexception:: jsonschema.exceptions.FormatError
    :noindex:
    :members:

Format-Specific Notes

regex

The JSON Schema specification recommends (but does not require) that implementations use ECMA 262 regular expressions.

Given that there is no current library in Python capable of supporting the ECMA 262 dialect, the regex format will instead validate Python regular expressions, which are the ones used by this implementation for other keywords like :kw:`pattern` or :kw:`patternProperties`.

email

Since in most cases "validating" an email address is an attempt instead to confirm that mail sent to it will deliver to a recipient, and that that recipient is the correct one the email is intended for, and since many valid email addresses are in many places incorrectly rejected, and many invalid email addresses are in many places incorrectly accepted, the email format keyword only provides a sanity check, not full RFC 5322 validation.

The same applies to the idn-email format.

If you indeed want a particular well-specified set of emails to be considered valid, you can use FormatChecker.checks to provide your specific definition.