GenSON is a powerful, user-friendly JSON Schema generator built in Python.
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Travis CI_
GenSON is a powerful, user-friendly
JSON Schema_ generator built in Python.
.. note:: This is not the Python equivalent of the
Java Genson library. If you are coming from Java and need to create JSON objects in Python, you want
Python's builtin json library.)
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:
below_)
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
idare also not supported, so GenSON will not dereference linked nodes when building a schema.
.. code-block:: bash
$ pip install genson
The package includes a
gensonexecutable that allows you to access this functionality from the command line. For usage info, run with
--help:
.. code-block:: bash
$ genson --help
.. code-block::
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 boundaries. -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 '$schema' keyword (defaults to 'http://json-schema.org/schema#' or can be specified in a schema with the -s option). If 'NULL' is passed, the "$schema" keyword will not be included in the result.
.. note:: The
--encodingoption is only available in Python 3.
SchemaBuilderis the basic schema generator class.
SchemaBuilderinstances can be loaded up with existing schemas and objects before being serialized.
.. code-block:: python
>>> 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() {'$schema': 'http://json-schema.org/schema#', 'type': 'object', 'properties': { 'hi': {'type': ['integer', 'string']}}, 'required': ['hi']}
>>> print(builder.to_json(indent=2)) { "$schema": "http://json-schema.org/schema#", "type": "object", "properties": { "hi": { "type": [ "integer", "string" ] } }, "required": [ "hi" ] }
SchemaBuilderAPI +++++++++++++++++++++
__init__(schema_uri=None)^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
:param schema_uri: value of the
$schemakeyword. If not given, it will use the value of the first available
$schemakeyword on an added schema or else the default:
'http://json-schema.org/schema#'. A value of
Falseor
Nonewill direct GenSON to leave out the
"$schema"keyword.
add_schema(schema)^^^^^^^^^^^^^^^^^^^^^^
Merge in a JSON schema. This can be a
dictor another
SchemaBuilderobject.
:param schema: a JSON Schema
.. note:: There is no schema validation. If you pass in a bad schema, you might get back a bad schema.
add_object(obj)^^^^^^^^^^^^^^^^^^^
Modify the schema to accommodate an object.
:param obj: any object or scalar that can be serialized in JSON
to_schema()^^^^^^^^^^^^^^^
Generate a schema based on previous inputs.
:rtype:
dict
to_json()^^^^^^^^^^^^^
Generate a schema and convert it directly to serialized JSON.
:rtype:
str
__eq__(other)^^^^^^^^^^^^^^^^^
Check for equality with another
SchemaBuilderobject.
:param other: another
SchemaBuilderobject. Other types are accepted, but will always return
False
SchemaBuilder object interaction ++++++++++++++++++++++++++++++++
SchemaBuilderobjects can also interact with each other:
.. code-block:: python
>>> 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 False
>>> b1.add_schema(b2) >>> b2.add_schema(b1) >>> b1 == b2 True >>> b1.to_schema() {'$schema': 'http://json-schema.org/schema#', 'type': 'object', 'properties': {'hi': {'type': ['integer', 'string']}}}
There are several cases where multiple valid schemas could be generated from the same object. GenSON makes a default choice in all these ambiguous cases, but if you want it to choose differently, you can tell it what to do using a seed schema.
Seeding Arrays ++++++++++++++
For example, suppose you have a simple array with two items:
.. code-block:: python
['one', 1]
There are always two ways for GenSON to interpret any array: List and Tuple. Lists have one schema for every item, whereas Tuples have a different schema for every array position. This is analogous to the (now deprecated)
merge_arraysoption from version 0. You can read more about JSON Schema
array validation here_.
List Validation ^^^^^^^^^^^^^^^
.. code-block:: json
{ "type": "array", "items": {"type": ["integer", "string"]} }
Tuple Validation ^^^^^^^^^^^^^^^^
.. code-block:: json
{ "type": "array", "items": [{"type": "integer"}, {"type": "string"}] }
By default, GenSON always interprets arrays using list validation, but you can tell it to use tuple validation by seeding it with a schema.
.. code-block:: python
>>> from genson import SchemaBuilder>>> builder = SchemaBuilder() >>> builder.add_object(['one', 1]) >>> builder.to_schema() {'$schema': 'http://json-schema.org/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() {'$schema': 'http://json-schema.org/schema#', 'type': 'array', 'items': [{'type': 'string'}, {'type': 'integer'}]}
Note that in this case, the seed schema is actually invalid. You can't have an empty array as the value for an
itemskeyword. But GenSON is a generator, not a validator, so you can fudge a little. GenSON will modify the generated schema so that it is valid, provided that there aren't invalid keywords beyond the ones it knows about.
Seeding patternProperties +++++++++++++++++++++++++
Support for patternProperties_ is new in version 1; however, since GenSON's default behavior is to only use
properties, this powerful keyword can only be utilized with seed schemas. You will need to supply an
objectschema with a
patternPropertiesobject whose keys are RegEx strings. Again, you can fudge here and set the values to null instead of creating valid subschemas.
.. code-block:: python
>>> from genson import SchemaBuilder>>> builder = SchemaBuilder() >>> builder.add_schema({'type': 'object', 'patternProperties': {r'^\d+$': None}}) >>> builder.add_object({'1': 1, '2': 2, '3': 3}) >>> builder.to_schema() {'$schema': 'http://json-schema.org/schema#', 'type': 'object', 'patternProperties': {'^\d+$': {'type': 'integer'}}}
There are a few gotchas you should be aware of here:
Python flavor of RegEx_.
propertiesto
patternPropertiesif a property already exists that matches one of your patterns, the normal property will be updated, not the pattern property.
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.
SchemaBuilder
You can extend the
SchemaBuilderclass to add in your own logic (e.g. recording
minimumand
maximumfor a number). In order to do this, you need to:
SchemaStrategyclass.
SchemaBuildersubclass that includes your custom
SchemaStrategyclass(es).
SchemaBuilderjust like you would the stock
SchemaBuilder.
SchemaStrategyClasses ++++++++++++++++++++++++++
GenSON uses the Strategy Pattern to parse, update, and serialize different kinds of schemas that behave in different ways. There are several
SchemaStrategyclasses 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
SchemaStrategyclass, though
SchemaStrategyand
TypedSchemaStrategyare the most useful base classes. You should call
superand 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
SchemaStrategysubclasses maintain the public API though, so you can extend any of them in this way.
SchemaStrategyAPI ++++++++++++++++++++++
[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
KEYWORDSinto the new tuple.
[class method]
match_schema(cls, schema)^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Return
trueif this strategy should be used to handle the passed-in schema.
:param schema: a JSON Schema in
dictform :rtype:
bool
[class method]
match_object(cls, obj)^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Return
trueif this strategy should be used to handle the passed-in object.
:param obj: any object or scalar that can be serialized in JSON :rtype:
bool
__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
dictform
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
to_schema(self)^^^^^^^^^^^^^^^^^^^
Override this method to customize how a schema object is constructed from the inputs. It is suggested that you invoke
superas the basis for the return value, but it is not required.
:rtype:
dict
.. note:: There is no schema validation. If you return a bad schema from this method,
SchemaBuilderwill output a bad schema.
__eq__(self, other)^^^^^^^^^^^^^^^^^^^^^^^
When checking for
SchemaBuilderequality, 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.
:rtype:
bool
TypedSchemaStrategyAPI +++++++++++++++++++++++++++
This is an abstract schema strategy for making simple schemas that only deal with the
typekeyword, but you can extend it to add more functionality. Subclasses must define the following two class constants, but you get the entire
SchemaStrategyinterface for free.
[class constant]
JS_TYPE^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This will be the value of the
typekeyword 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
SchemaStrategytypes, you'll need to create a
SchemaBuilderclass that uses them, since the default
SchemaBuilderonly incorporates the default strategies. To do this, extend the
SchemaBuilderclass 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_STRATEGIEShave 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.
.. note:: This example is written in Python 3.3+. Custom strategies also work in Python 2.7, but you need different syntax (
superarguments & no splatting
KEYWORDS).
.. code-block:: python
from genson import SchemaBuilder from genson.schema.strategies import Numberclass MinNumber(Number): # add 'minimum' to list of keywords KEYWORDS = (*Number.KEYWORDS, 'minimum')
# create a new instance variable def __init__(self, node_class): super().__init__(node_class) self.min = None # capture 'minimum's from schemas def add_schema(self, schema): super().add_schema(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): super().add_object(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.
.. code-block:: python
>>> builder = MinNumberSchemaBuilder() >>> builder.add_object(5) >>> builder.add_object(7) >>> builder.to_schema() {'$schema': 'http://json-schema.org/schema#', 'type': 'integer', 'minimum': 5} >>> builder.add_object(-2) >>> builder.to_schema() {'$schema': 'http://json-schema.org/schema#', 'type': 'integer', 'minimum': -2} >>> builder.add_schema({'$schema': 'http://json-schema.org/schema#', 'type': 'integer', 'minimum': -7}) >>> builder.to_schema() {'$schema': 'http://json-schema.org/schema#', 'type': 'integer', 'minimum': -7}
Note that the exclusive builder is much more particular.
.. code-block:: python
>>> builder = MinNumberSchemaBuilder() >>> picky_builder = ExclusiveMinNumberSchemaBuilder() >>> picky_builder.add_object(5) >>> picky_builder.to_schema() {'$schema': 'http://json-schema.org/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
When contributing, please follow these steps:
Flake8_.
tox_ to make sure the test suite passes.
Tests +++++
Tests are written in
unittestand are run using
tox_ and
nose_. Tox will run all tests with coverage against each supported Python version that is installed on your machine.
.. code-block:: bash
$ tox
You should always run tox before submitting a PR, but it takes some time, so when developing, it may be faster just to run
nosetestsin your own environment. e.g.
.. code-block:: bash
$ nosetests $ nosetests --with-coverate --cover-package=genson $ nosetests test.test_gen_single:TestBasicTypes.test_number
Integration +++++++++++
When you submit a PR,
Travis CI_ performs the following steps:
If any of these steps fail, your PR cannot be merged until it is fixed.
Potential Future Features +++++++++++++++++++++++++
The following are extra features under consideration.
logical support for more keywords:
enum
minimum/
maximum
minLength/
maxLength
minItems/
maxItems
minProperties/
maxProperties
additionalItems
additionalProperties
format&
pattern
$ref&
id
.. _JSON Schema: http://json-schema.org/ .. _Java Genson library: https://owlike.github.io/genson/ .. _
Python's builtin json library: https://docs.python.org/library/json.html .. _below: #typeless-schemas .. _array validation here: https://spacetelescope.github.io/understanding-json-schema/reference/array.html#items .. _patternProperties: https://spacetelescope.github.io/understanding-json-schema/reference/object.html#pattern-properties .. _Python flavor of RegEx: https://docs.python.org/3.6/library/re.html .. _the code: https://github.com/wolverdude/GenSON/tree/master/genson/schema/strategies .. _minimum number: https://json-schema.org/understanding-json-schema/reference/numeric.html#range .. _Flake8: https://pypi.python.org/pypi/flake8 .. _tox: https://pypi.python.org/pypi/tox .. _Travis CI: https://travis-ci.com/github/wolverdude/GenSON