Fast JSON schema for Python


pip install fastjsonschema

Support only for Python 3.3 and higher.


fastjsonschema implements validation of JSON documents by JSON schema. The library implements JSON schema drafts 04, 06 and 07. The main purpose is to have a really fast implementation. See some numbers:

  • Probably most popular jsonschema can take up to 5 seconds for valid inputs and 1.2 seconds for invalid inputs.
  • Second most popular json-spec is even worse with up to 7.2 and 1.7 seconds.
  • Last validictory, now deprecated, is much better with 370 or 23 milliseconds, but it does not follow all standards and it can be still slow for some purposes.

With this library you can gain big improvements as fastjsonschema takes only about 25 milliseconds for valid inputs and 2 milliseconds for invalid ones. Pretty amazing, right? :-)

Technically it works by generating the most stupid code on the fly which is fast but is hard to write by hand. The best efficiency is achieved when compiled once and used many times, of course. It works similarly like regular expressions. But you can also generate the code to the file which is even slightly faster.

You can do the performance on your computer or server with an included script:

$ make performance
fast_compiled        valid      ==> 0.030474655970465392
fast_compiled        invalid    ==> 0.0017561429995112121
fast_file            valid      ==> 0.028758891974575818
fast_file            invalid    ==> 0.0017655809642747045
fast_not_compiled    valid      ==> 4.597834145999514
fast_not_compiled    invalid    ==> 1.139162228035275
jsonschema           valid      ==> 5.014410221017897
jsonschema           invalid    ==> 1.1362981660058722
jsonspec             valid      ==> 8.1144932230236
jsonspec             invalid    ==> 2.0143173419637606
validictory          valid      ==> 0.4084212710149586
validictory          invalid    ==> 0.026061681972350925

This library follows and implements JSON schema draft-04, draft-06, and draft-07. Sometimes it’s not perfectly clear so I recommend also check out this understanding JSON schema.

Note that there are some differences compared to JSON schema standard:

  • Regular expressions are full Python ones, not only what JSON schema allows. It’s easier to allow everything and also it’s faster to compile without limits. So keep in mind that when you will use a more advanced regular expression, it may not work with other library or in other languages.
  • Because Python matches new line for a dollar in regular expressions (a$ matches a and a\\n), instead of $ is used \Z and all dollars in your regular expression are changed to \\Z as well. When you want to use dollar as regular character, you have to escape it (\$).
  • JSON schema says you can use keyword default for providing default values. This implementation uses that and always returns transformed input data.


exception fastjsonschema.JsonSchemaException(message, value=None, name=None, definition=None, rule=None)[source]

Exception raised by validation function. Available properties:

  • message containing human-readable information what is wrong (e.g.[index] must be smaller than or equal to 42),
  • invalid value (e.g. 60),
  • name of a path in the data structure (e.g. data.propery[index]),
  • path as an array in the data structure (e.g. ['data', 'propery', 'index']),
  • the whole definition which the value has to fulfil (e.g. {'type': 'number', 'maximum': 42}),
  • rule which the value is breaking (e.g. maximum)
  • and rule_definition (e.g. 42).

Changed in version 2.14.0: Added all extra properties.

exception fastjsonschema.JsonSchemaDefinitionException(message, value=None, name=None, definition=None, rule=None)[source]

Exception raised by generator of validation function.

fastjsonschema.validate(definition, data, handlers={}, formats={})[source]

Validation function for lazy programmers or for use cases, when you need to call validation only once, so you do not have to compile it first. Use it only when you do not care about performance (even thought it will be still faster than alternative implementations).

import fastjsonschema

fastjsonschema.validate({'type': 'string'}, 'hello')
# same as: compile({'type': 'string'})('hello')

Preferred is to use compile function.

fastjsonschema.compile(definition, handlers={}, formats={})[source]

Generates validation function for validating JSON schema passed in definition. Example:

import fastjsonschema

validate = fastjsonschema.compile({'type': 'string'})

This implementation support keyword default:

validate = fastjsonschema.compile({
    'type': 'object',
    'properties': {
        'a': {'type': 'number', 'default': 42},

data = validate({})
assert data == {'a': 42}

Supported implementations are draft-04, draft-06 and draft-07. Which version should be used is determined by $draft in your definition. When not specified, the latest implementation is used (draft-07).

validate = fastjsonschema.compile({
    '$schema': '',
    'type': 'number',

You can pass mapping from URI to function that should be used to retrieve remote schemes used in your definition in parameter handlers.

Also, you can pass mapping for custom formats. Key is the name of your formatter and value can be regular expression which will be compiled or callback returning bool (or you can raise your own exception).

validate = fastjsonschema.compile(definition, formats={
    'foo': r'foo|bar',
    'bar': lambda value: value in ('foo', 'bar'),

Exception JsonSchemaDefinitionException is raised when generating the code fails (bad definition).

Exception JsonSchemaException is raised from generated funtion when validation fails (data do not follow the definition).

fastjsonschema.compile_to_code(definition, handlers={}, formats={})[source]

Generates validation code for validating JSON schema passed in definition. Example:

import fastjsonschema

code = fastjsonschema.compile_to_code({'type': 'string'})
with open('', 'w') as f:

You can also use it as a script:

echo "{'type': 'string'}" | python3 -m fastjsonschema >
python3 -m fastjsonschema "{'type': 'string'}" >

Exception JsonSchemaDefinitionException is raised when generating the code fails (bad definition).