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GenSON is a powerful, user-friendly JSON Schema generator.

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GenSON is a powerful, user-friendly JSON Schema generator built in Python.

GenSON’s core function is to take JSON objects and generate schemas that describe them, but it is unique in its ability to merge schemas. It was originally built to describe the common structure of a large number of JSON objects, and it uses its merging ability to generate a single schema from any number of JSON objects and/or schemas.

GenSON’s schema builder follows these three rules:

  1. Every object it is given must validate under the generated schema.

  2. Any object that is valid under any schema it is given must also validate under the generated schema. (there is one glaring exception to this, detailed below)

  3. The generated schema should be as strict as possible given the first 2 rules.

JSON Schema Implementation

GenSON is compatible with JSON Schema Draft 6 and above.

It is important to note that GenSON uses only a subset of JSON Schema’s capabilities. This is mainly because it doesn’t know the specifics of your data model, and it tries to avoid guessing them. Its purpose is to generate the basic structure so that you can skip the boilerplate and focus on the details of the schema.

Currently, GenSON only deals with these keywords:

  • "$schema"

  • "type"

  • "items"

  • "properties"

  • "patternProperties"

  • "required"

  • "anyOf"

You should be aware that this limited vocabulary could cause GenSON to violate rules 1 and 2. If you feed it schemas with advanced keywords, it will just blindly pass them on to the final schema. Note that "$ref" and id are also not supported, so GenSON will not dereference linked nodes when building a schema.


$ pip install genson

CLI Tool

The package includes a genson executable that allows you to access this functionality from the command line. For usage info, run with --help:

$ genson --help
usage: genson [-h] [--version] [-d DELIM] [-e ENCODING] [-i SPACES]
              [-s SCHEMA] [-$ SCHEMA_URI]

Generate one, unified JSON Schema from one or more JSON objects and/or JSON
Schemas. Compatible with JSON-Schema Draft 4 and above.

positional arguments:
  object                Files containing JSON objects (defaults to stdin if no
                        arguments are passed).

optional arguments:
  -h, --help            Show this help message and exit.
  --version             Show version number and exit.
  -d DELIM, --delimiter DELIM
                        Set a delimiter. Use this option if the input files
                        contain multiple JSON objects/schemas. You can pass
                        any string. A few cases ('newline', 'tab', 'space')
                        will get converted to a whitespace character. If this
                        option is omitted, the parser will try to auto-detect
  -e ENCODING, --encoding ENCODING
                        Use ENCODING instead of the default system encoding
                        when reading files. ENCODING must be a valid codec
                        name or alias.
  -i SPACES, --indent SPACES
                        Pretty-print the output, indenting SPACES spaces.
  -s SCHEMA, --schema SCHEMA
                        File containing a JSON Schema (can be specified
                        multiple times to merge schemas).
  -$ SCHEMA_URI, --schema-uri SCHEMA_URI
                        The value of the '$schema' keyword (defaults to
                        '' or can be specified
                        in a schema with the -s option). If 'NULL' is passed,
                        the "$schema" keyword will not be included in the

GenSON Python API

SchemaBuilder is the basic schema generator class. SchemaBuilder instances can be loaded up with existing schemas and objects before being serialized.

>>> from genson import SchemaBuilder

>>> builder = SchemaBuilder()
>>> builder.add_schema({"type": "object", "properties": {}})
>>> builder.add_object({"hi": "there"})
>>> builder.add_object({"hi": 5})

>>> builder.to_schema()
{'$schema': '',
 'type': 'object',
 'properties': {
    'hi': {'type': ['integer', 'string']}},
    'required': ['hi']}

>>> print(builder.to_json(indent=2))
  "$schema": "",
  "type": "object",
  "properties": {
    "hi": {
      "type": [
  "required": [

SchemaBuilder API

param schema_uri:

value of the $schema keyword. If not given, it will use the value of the first available $schema keyword on an added schema or else the default: ''. A value of False or None will direct GenSON to leave out the "$schema" keyword.


Merge in a JSON schema. This can be a dict or another SchemaBuilder object.

param schema:

a JSON Schema


Modify the schema to accommodate an object.

param obj:

any object or scalar that can be serialized in JSON


Generate a schema based on previous inputs.




Generate a schema and convert it directly to serialized JSON.




Check for equality with another SchemaBuilder object.

param other:

another SchemaBuilder object. Other types are accepted, but will always return False

SchemaBuilder object interaction

SchemaBuilder objects can also interact with each other:

  • You can pass one schema directly to another to merge them.

  • You can compare schema equality directly.

>>> from genson import SchemaBuilder

>>> b1 = SchemaBuilder()
>>> b1.add_schema({"type": "object", "properties": {
...   "hi": {"type": "string"}}})
>>> b2 = SchemaBuilder()
>>> b2.add_schema({"type": "object", "properties": {
...   "hi": {"type": "integer"}}})
>>> b1 == b2

>>> b1.add_schema(b2)
>>> b2.add_schema(b1)
>>> b1 == b2
>>> b1.to_schema()
{'$schema': '',
 'type': 'object',
 'properties': {'hi': {'type': ['integer', 'string']}}}

Seed Schemas

There are several cases where multiple valid schemas could be generated from the same object. GenSON makes a default choice in all these ambiguous cases, but if you want it to choose differently, you can tell it what to do using a seed schema.

Seeding Arrays

For example, suppose you have a simple array with two items:

['one', 1]

There are always two ways for GenSON to interpret any array: List and Tuple. Lists have one schema for every item, whereas Tuples have a different schema for every array position. This is analogous to the (now deprecated) merge_arrays option from version 0. You can read more about JSON Schema array validation here.

List Validation
  "type": "array",
  "items": {"type": ["integer", "string"]}
Tuple Validation
  "type": "array",
  "items": [{"type": "integer"}, {"type": "string"}]

By default, GenSON always interprets arrays using list validation, but you can tell it to use tuple validation by seeding it with a schema.

>>> from genson import SchemaBuilder

>>> builder = SchemaBuilder()
>>> builder.add_object(['one', 1])
>>> builder.to_schema()
{'$schema': '',
 'type': 'array',
 'items': {'type': ['integer', 'string']}}

>>> builder = SchemaBuilder()
>>> seed_schema = {'type': 'array', 'items': []}
>>> builder.add_schema(seed_schema)
>>> builder.add_object(['one', 1])
>>> builder.to_schema()
{'$schema': '',
 'type': 'array',
 'items': [{'type': 'string'}, {'type': 'integer'}]}

Note that in this case, the seed schema is actually invalid. You can’t have an empty array as the value for an items keyword. But GenSON is a generator, not a validator, so you can fudge a little. GenSON will modify the generated schema so that it is valid, provided that there aren’t invalid keywords beyond the ones it knows about.

Seeding patternProperties

Support for patternProperties is new in version 1; however, since GenSON’s default behavior is to only use properties, this powerful keyword can only be utilized with seed schemas. You will need to supply an object schema with a patternProperties object whose keys are RegEx strings. Again, you can fudge here and set the values to null instead of creating valid subschemas.

>>> from genson import SchemaBuilder

>>> builder = SchemaBuilder()
>>> builder.add_schema({'type': 'object', 'patternProperties': {r'^\d+$': None}})
>>> builder.add_object({'1': 1, '2': 2, '3': 3})
>>> builder.to_schema()
{'$schema': '', 'type': 'object', 'patternProperties':  {'^\\d+$': {'type': 'integer'}}}

There are a few gotchas you should be aware of here:

  • GenSON is written in Python, so it uses the Python flavor of RegEx.

  • GenSON still prefers properties to patternProperties if a property already exists that matches one of your patterns, the normal property will be updated, not the pattern property.

  • If a key matches multiple patterns, there is no guarantee of which one will be updated.

  • The patternProperties docs themselves have some more useful pointers that can save you time.

Typeless Schemas

In version 0, GenSON did not accept a schema without a type, but in order to be flexible in the support of seed schemas, support was added for version 1. However, GenSON violates rule #2 in its handling of typeless schemas. Any object will validate under an empty schema, but GenSON incorporates typeless schemas into the first-available typed schema, and since typed schemas are stricter than typless ones, objects that would validate under an added schema will not validate under the result.

Customizing SchemaBuilder

You can extend the SchemaBuilder class to add in your own logic (e.g. recording minimum and maximum for a number). In order to do this, you need to:

  1. Create a custom SchemaStrategy class.

  2. Create a SchemaBuilder subclass that includes your custom SchemaStrategy class(es).

  3. Use your custom SchemaBuilder just like you would the stock SchemaBuilder.

SchemaStrategy Classes

GenSON uses the Strategy Pattern to parse, update, and serialize different kinds of schemas that behave in different ways. There are several SchemaStrategy classes that roughly correspond to different schema types. GenSON maps each node in an object or schema to an instance of one of these classes. Each instance stores the current schema state and updates or returns it when required.

You can modify the specific ways these classes work by extending them. You can inherit from any existing SchemaStrategy class, though SchemaStrategy and TypedSchemaStrategy are the most useful base classes. You should call super and pass along all arguments when overriding any instance methods.

The documentation below explains the public API and what you need to extend and override at a high level. Feel free to explore the code to see more, but know that the public API is documented here, and anything else you depend on could be subject to change. All SchemaStrategy subclasses maintain the public API though, so you can extend any of them in this way.

SchemaStrategy API

[class constant] KEYWORDS

This should be a tuple listing all of the JSON-schema keywords that this strategy knows how to handle. Any keywords encountered in added schemas will be be naively passed on to the generated schema unless they are in this list (or you override that behavior in to_schema).

When adding keywords to a new SchemaStrategy, it’s best to splat the parent class’s KEYWORDS into the new tuple.

[class method] match_schema(cls, schema)

Return true if this strategy should be used to handle the passed-in schema.

param schema:

a JSON Schema in dict form



[class method] match_object(cls, obj)

Return true if this strategy should be used to handle the passed-in object.

param obj:

any object or scalar that can be serialized in JSON



__init__(self, node_class)

Override this method if you need to initialize an instance variable.

param node_class:

This param is not part of the public API. Pass it along to super.

add_schema(self, schema)

Override this to modify how a schema is parsed and stored.

param schema:

a JSON Schema in dict form

add_object(self, obj)

Override this to change the way a schemas are inferred from objects.

param obj:

any object or scalar that can be serialized in JSON


Override this method to customize how a schema object is constructed from the inputs. It is suggested that you invoke super as the basis for the return value, but it is not required.



__eq__(self, other)

When checking for SchemaBuilder equality, strategies are matched using __eq__. The default implementation uses a simple __dict__ equality check.

Override this method if you need to override that behavior. This may be useful if you add instance variables that aren’t relevant to whether two SchemaStrategies are considered equal.



TypedSchemaStrategy API

This is an abstract schema strategy for making simple schemas that only deal with the type keyword, but you can extend it to add more functionality. Subclasses must define the following two class constants, but you get the entire SchemaStrategy interface for free.

[class constant] JS_TYPE

This will be the value of the type keyword in the generated schema. It is also used to match any added schemas.

[class constant] PYTHON_TYPE

This is a Python type or tuple of types that will be matched against an added object using isinstance.

Extending SchemaBuilder

Once you have extended SchemaStrategy types, you’ll need to create a SchemaBuilder class that uses them, since the default SchemaBuilder only incorporates the default strategies. To do this, extend the SchemaBuilder class and define one of these two constants inside it:

[class constant] EXTRA_STRATEGIES

This is the standard (and suggested) way to add strategies. Set it to a tuple of all your new strategies, and they will be added to the existing list of strategies to check. This preserves all the existing functionality.

Note that order matters. GenSON checks the list in order, so the first strategy has priority over the second and so on. All EXTRA_STRATEGIES have priority over the default strategies.

[class constant] STRATEGIES

This clobbers the existing list of strategies and completely replaces it. Set it to a tuple just like for EXTRA_STRATEGIES, but note that if any object or schema gets added that your exhaustive list of strategies doesn’t know how to handle, you’ll get an error. You should avoid doing this unless you’re extending most or all existing strategies in some way.

Example: MinNumber

Here’s some example code creating a number strategy that tracks the minimum number seen and includes it in the output schema.

from genson import SchemaBuilder
from genson.schema.strategies import Number

class MinNumber(Number):
    # add 'minimum' to list of keywords
    KEYWORDS = (*Number.KEYWORDS, 'minimum')

    # create a new instance variable
    def __init__(self, node_class):
        self.min = None

    # capture 'minimum's from schemas
    def add_schema(self, schema):
        if self.min is None:
            self.min = schema.get('minimum')
        elif 'minimum' in schema:
            self.min = min(self.min, schema['minimum'])

    # adjust minimum based on the data
    def add_object(self, obj):
        self.min = obj if self.min is None else min(self.min, obj)

    # include 'minimum' in the output
    def to_schema(self):
        schema = super().to_schema()
        schema['minimum'] = self.min
        return schema

# new SchemaBuilder class that uses the MinNumber strategy in addition
# to the existing strategies. Both MinNumber and Number are active, but
# MinNumber has priority, so it effectively replaces Number.
class MinNumberSchemaBuilder(SchemaBuilder):
    """ all number nodes include minimum """
    EXTRA_STRATEGIES = (MinNumber,)

# this class *ONLY* has the MinNumber strategy. Any object that is not
# a number will cause an error.
class ExclusiveMinNumberSchemaBuilder(SchemaBuilder):
    """ all number nodes include minimum, and only handles number """
    STRATEGIES = (MinNumber,)

Now that we have the MinNumberSchemaBuilder class, let’s see how it works.

>>> builder = MinNumberSchemaBuilder()
>>> builder.add_object(5)
>>> builder.add_object(7)
>>> builder.to_schema()
{'$schema': '', 'type': 'integer', 'minimum': 5}
>>> builder.add_object(-2)
>>> builder.to_schema()
{'$schema': '', 'type': 'integer', 'minimum': -2}
>>> builder.add_schema({'$schema': '', 'type': 'integer', 'minimum': -7})
>>> builder.to_schema()
{'$schema': '', 'type': 'integer', 'minimum': -7}

Note that the exclusive builder is much more particular.

>>> builder = MinNumberSchemaBuilder()
>>> picky_builder = ExclusiveMinNumberSchemaBuilder()
>>> picky_builder.add_object(5)
>>> picky_builder.to_schema()
{'$schema': '', 'type': 'integer', 'minimum': 5}
>>> builder.add_object(None) # this is fine
>>> picky_builder.add_object(None) # this fails
genson.schema.node.SchemaGenerationError: Could not find matching schema type for object: None


GenSON has been tested and verified under the following Python versions:

  • Python 2.7.13

  • Python 3.3.5

  • Python 3.4.5

  • Python 3.5.4

  • Python 3.6.2

  • Python 3.7.3

  • Python 3.8.2


When contributing, please follow these steps:

  1. Clone the repo and make your changes.

  2. Make sure your code has test cases written against it.

  3. Make sure all the tests pass.

  4. Lint your code with Flake8.

  5. Ensure the docs are accurate.

  6. Add your name to the list of contributers.

  7. Submit a Pull Request.


Tests are written in unittest. You can run them all easily with the included executable bin/

$ bin/

You can also invoke individual test suites:

$ bin/ --test-suite test.test_gen_single

Potential Future Features

The following are extra features under consideration.

  • recognize every validation keyword and ignore any that don’t apply

  • option to set error level

  • custom serializer plugins

  • logical support for more keywords:

    • enum

    • minimum/maximum

    • minLength/maxLength

    • minItems/maxItems

    • minProperties/maxProperties

    • additionalItems

    • additionalProperties

    • format & pattern

    • $ref & id



  • add __version__ attr to module & --version option to CLI tool

  • add --encoding option to CLI tool that overrides default file encoding (fixes #47)


  • expose SchemaStrategy.__eq__() for extension

  • add support for Python 3.8

  • update Trove classifiers

  • Bugfix: SchemaBuilder.__eq__() wasn’t matching the $schema keyword correctly

  • Bugfix: only activate empty required option when required is actualy empty


  • SchemaStrategies are now extendable, enabling custom SchemaBuilder classes.

  • optimize __eq__ logic


  • add support for Python 3.7

  • drop support for Python 3.3

  • drop support for JSON-Schema Draft 4 (because it doesn’t allow empty required arrays)

  • Bugfix: preserve empty required arrays (fixes #25)

  • Bugfix: handle nested anyOf keywords (fixes #35)


  • add support for long integers in Python 2.7

  • update test-skipping decorator to use standard version requirement strings


  • Bugfix: seeding an object schema with a "required" keyword caused an error

  • Docs: fix mislabeled method


This version was a total overhaul. The main change was to split Schema into three separate classes, making it simpler to add more complicated functionality by having different generator types to handle the different schema types.

  1. SchemaNode to manage the tree structure

  2. SchemaGenerator for the schema generation logic

  3. SchemaBuilder to manage the public API

Interface Changes

  • SchemaBuilder is the new Schema

  • to_dict() is now called to_schema()

To make the transition easier, there is still a Schema class that wraps SchemaBuilder with a backwards-compatibility layer, but you will trigger a PendingDeprecationWarning if you use it.

Seed Schemas

The merge_arrays option has been removed in favor of seed schemas. You can now seed specific nodes as list or tuple instead of setting a global option for every node in the schema tree.

You can also now seed object nodes with patternProperties, which was a highly requested feature.

Other Changes

  • include "$schema" keyword

  • accept schemas without "type" keyword

  • use "anyOf" keyword to help combine schemas

  • add SchemaGenerationError for better error handling

  • empty "properties" and "items" are not included in generated schemas

  • genson executable

    • new --schema-uri option

    • auto-detect object boundaries by default


  • Docs: add installation instructions


  • Docs: Python 3.6 is now explicitly tested and listed as compatible.


  • Bugfix: add_schema failed when adding list-style array schemas

  • Bugfix: typo in readme


  • Bugfix: Options were not propagated down to subschemas.

  • Bugfix: Empty arrays resulted in invalid schemas because it still included an items property.

  • Bugfix: items was being set to a list even when merge_arrays was set to True. This resulted in overly permissive schemas because items are matched optionally by default.

  • Improvement: Positional Array Matching - In order to be more consistent with the way JSON Schema works, the alternate to merge_arrays is no longer never to merge list items, but instead to merge them based on their position in the list.

  • Improvement: Schema Incompatibility Warning - A schema incompatibility used to cause a fatal error with a nondescript warning. The message has been improved and it has been reduced to a warning.

0.1.0 (2014-11-29)

  • Initial release


GenSON is written and maintained by Jon Wolverton.


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