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Simple data validation library

Project description

Schema validation just got Pythonic

**schema** is a library for validating Python data structures, such as those
obtained from config-files, forms, external services or command-line
parsing, converted from JSON/YAML (or something else) to Python data-types.

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Here is a quick example to get a feeling of **schema**, validating a list of
entries with personal information:

.. code:: python

>>> from schema import Schema, And, Use, Optional

>>> schema = Schema([{'name': And(str, len),
... 'age': And(Use(int), lambda n: 18 <= n <= 99),
... Optional('gender'): And(str, Use(str.lower),
... lambda s: s in ('squid', 'kid'))}])

>>> data = [{'name': 'Sue', 'age': '28', 'gender': 'Squid'},
... {'name': 'Sam', 'age': '42'},
... {'name': 'Sacha', 'age': '20', 'gender': 'KID'}]

>>> validated = schema.validate(data)

>>> assert validated == [{'name': 'Sue', 'age': 28, 'gender': 'squid'},
... {'name': 'Sam', 'age': 42},
... {'name': 'Sacha', 'age' : 20, 'gender': 'kid'}]

If data is valid, ``Schema.validate`` will return the validated data
(optionally converted with `Use` calls, see below).

If data is invalid, ``Schema`` will raise ``SchemaError`` exception.
If you just want to check that the data is valid, ``schema.is_valid(data)`` will
return ``True`` or ``False``.


Use `pip <>`_ or easy_install::

pip install schema

Alternatively, you can just drop ```` file into your project—it is

- **schema** is tested with Python 2.6, 2.7, 3.2, 3.3, 3.4, 3.5, 3.6 and PyPy.
- **schema** follows `semantic versioning <>`_.

How ``Schema`` validates data


If ``Schema(...)`` encounters a type (such as ``int``, ``str``, ``object``,
etc.), it will check if the corresponding piece of data is an instance of that type,
otherwise it will raise ``SchemaError``.

.. code:: python

>>> from schema import Schema

>>> Schema(int).validate(123)

>>> Schema(int).validate('123')
Traceback (most recent call last):
SchemaUnexpectedTypeError: '123' should be instance of 'int'

>>> Schema(object).validate('hai')


If ``Schema(...)`` encounters a callable (function, class, or object with
``__call__`` method) it will call it, and if its return value evaluates to
``True`` it will continue validating, else—it will raise ``SchemaError``.

.. code:: python

>>> import os

>>> Schema(os.path.exists).validate('./')

>>> Schema(os.path.exists).validate('./non-existent/')
Traceback (most recent call last):
SchemaError: exists('./non-existent/') should evaluate to True

>>> Schema(lambda n: n > 0).validate(123)

>>> Schema(lambda n: n > 0).validate(-12)
Traceback (most recent call last):
SchemaError: <lambda>(-12) should evaluate to True


If ``Schema(...)`` encounters an object with method ``validate`` it will run
this method on corresponding data as ``data = obj.validate(data)``. This method
may raise ``SchemaError`` exception, which will tell ``Schema`` that that piece
of data is invalid, otherwise—it will continue validating.

An example of "validatable" is ``Regex``, that tries to match a string or a
buffer with the given regular expression (itself as a string, buffer or
compiled regex ``SRE_Pattern``):

.. code:: python

>>> from schema import Regex
>>> import re

>>> Regex(r'^foo').validate('foobar')

>>> Regex(r'^[A-Z]+$', flags=re.I).validate('those-dashes-dont-match')
Traceback (most recent call last):
SchemaError: Regex('^[A-Z]+$', flags=re.IGNORECASE) does not match 'those-dashes-dont-match'

For a more general case, you can use ``Use`` for creating such objects.
``Use`` helps to use a function or type to convert a value while validating it:

.. code:: python

>>> from schema import Use

>>> Schema(Use(int)).validate('123')

>>> Schema(Use(lambda f: open(f, 'a'))).validate('LICENSE-MIT')
<open file 'LICENSE-MIT', mode 'a' at 0x...>

Dropping the details, ``Use`` is basically:

.. code:: python

class Use(object):

def __init__(self, callable_):
self._callable = callable_

def validate(self, data):
return self._callable(data)
except Exception as e:
raise SchemaError('%r raised %r' % (self._callable.__name__, e))

Sometimes you need to transform and validate part of data, but keep original data unchanged.
``Const`` helps to keep your data safe:

.. code:: python

>> from schema import Use, Const, And, Schema

>> from datetime import datetime

>> is_future = lambda date: > date

>> to_json = lambda v: {"timestamp": v}

>> Schema(And(Const(And(Use(datetime.fromtimestamp), is_future)), Use(to_json))).validate(1234567890)
{"timestamp": 1234567890}

Now you can write your own validation-aware classes and data types.

Lists, similar containers

If ``Schema(...)`` encounters an instance of ``list``, ``tuple``, ``set`` or
``frozenset``, it will validate contents of corresponding data container
against schemas listed inside that container:

.. code:: python

>>> Schema([1, 0]).validate([1, 1, 0, 1])
[1, 1, 0, 1]

>>> Schema((int, float)).validate((5, 7, 8, 'not int or float here'))
Traceback (most recent call last):
SchemaError: Or(<type 'int'>, <type 'float'>) did not validate 'not int or float here'
'not int or float here' should be instance of 'float'


If ``Schema(...)`` encounters an instance of ``dict``, it will validate data
key-value pairs:

.. code:: python

>>> d = Schema({'name': str,
... 'age': lambda n: 18 <= n <= 99}).validate({'name': 'Sue', 'age': 28})

>>> assert d == {'name': 'Sue', 'age': 28}

You can specify keys as schemas too:

.. code:: python

>>> schema = Schema({str: int, # string keys should have integer values
... int: None}) # int keys should be always None

>>> data = schema.validate({'key1': 1, 'key2': 2,
... 10: None, 20: None})

>>> schema.validate({'key1': 1,
... 10: 'not None here'})
Traceback (most recent call last):
SchemaError: Key '10' error:
None does not match 'not None here'

This is useful if you want to check certain key-values, but don't care
about others:

.. code:: python

>>> schema = Schema({'<id>': int,
... '<file>': Use(open),
... str: object}) # don't care about other str keys

>>> data = schema.validate({'<id>': 10,
... '<file>': 'README.rst',
... '--verbose': True})

You can mark a key as optional as follows:

.. code:: python

>>> from schema import Optional
>>> Schema({'name': str,
... Optional('occupation'): str}).validate({'name': 'Sam'})
{'name': 'Sam'}

``Optional`` keys can also carry a ``default``, to be used when no key in the
data matches:

.. code:: python

>>> from schema import Optional
>>> Schema({Optional('color', default='blue'): str,
... str: str}).validate({'texture': 'furry'}
... ) == {'color': 'blue', 'texture': 'furry'}

Defaults are used verbatim, not passed through any validators specified in the

default can also be a callable:

.. code:: python

>>> from schema import Schema, Optional
>>> Schema({Optional('data', default=dict): {}}).validate({}) == {'data': {}}

Also, a caveat: If you specify types, **schema** won't validate the empty dict:

.. code:: python

>>> Schema({int:int}).is_valid({})

To do that, you need ``Schema(Or({int:int}, {}))``. This is unlike what happens with
lists, where ``Schema([int]).is_valid([])`` will return True.

**schema** has classes ``And`` and ``Or`` that help validating several schemas
for the same data:

.. code:: python

>>> from schema import And, Or

>>> Schema({'age': And(int, lambda n: 0 < n < 99)}).validate({'age': 7})
{'age': 7}

>>> Schema({'password': And(str, lambda s: len(s) > 6)}).validate({'password': 'hai'})
Traceback (most recent call last):
SchemaError: Key 'password' error:
<lambda>('hai') should evaluate to True

>>> Schema(And(Or(int, float), lambda x: x > 0)).validate(3.1415)

In a dictionary, you can also combine two keys in a "one or the other" manner. To do
so, use the `Or` class as a key:

.. code:: python
>>> from schema import Or, Schema
>>> schema = Schema({
... Or("key1", "key2", only_one=True): str
... })

>>> schema.validate({"key1": "test"}) # Ok
{'key1': 'test'}

>>> schema.validate({"key1": "test", "key2": "test"}) # SchemaError
Traceback (most recent call last):
SchemaOnlyOneAllowedError: There are multiple keys present from the Or('key1', 'key2') condition

You can define hooks which are functions that are executed whenever a valid key:value is found.
The `Forbidden` class is an example of this.

You can mark a key as forbidden as follows:

.. code:: python

>>> from schema import Forbidden
>>> Schema({Forbidden('age'): object}).validate({'age': 50})
Traceback (most recent call last):
SchemaForbiddenKeyError: Forbidden key encountered: 'age' in {'age': 50}

A few things are worth noting. First, the value paired with the forbidden
key determines whether it will be rejected:

.. code:: python

>>> Schema({Forbidden('age'): str, 'age': int}).validate({'age': 50})
{'age': 50}

Note: if we hadn't supplied the 'age' key here, the call would have failed too, but with
SchemaWrongKeyError, not SchemaForbiddenKeyError.

Second, Forbidden has a higher priority than standard keys, and consequently than Optional.
This means we can do that:

.. code:: python

>>> Schema({Forbidden('age'): object, Optional(str): object}).validate({'age': 50})
Traceback (most recent call last):
SchemaForbiddenKeyError: Forbidden key encountered: 'age' in {'age': 50}

You can also define your own hooks. The following hook will call `_my_function` if `key` is encountered.

.. code:: python

from schema import Hook
def _my_function(key, scope, error):
print(key, scope, error)

Hook("key", handler=_my_function)

Here's an example where a `Deprecated` class is added to log warnings whenever a key is encountered:

.. code:: python

from schema import Hook, Schema
class Deprecated(Hook):
def __init__(self, *args, **kwargs):
kwargs["handler"] = lambda key, *args: logging.warn(f"`{key}` is deprecated. " + (self._error or ""))
super(Deprecated, self).__init__(*args, **kwargs)

Schema({Deprecated("test", "custom error message."): object}, ignore_extra_keys=True).validate({"test": "value"})
WARNING: `test` is deprecated. custom error message.

Extra Keys

The ``Schema(...)`` parameter ``ignore_extra_keys`` causes validation to ignore extra keys in a dictionary, and also to not return them after validating.

.. code:: python

>>> schema = Schema({'name': str}, ignore_extra_keys=True)
>>> schema.validate({'name': 'Sam', 'age': '42'})
{'name': 'Sam'}

If you would like any extra keys returned, use ``object: object`` as one of the key/value pairs, which will match any key and any value.
Otherwise, extra keys will raise a ``SchemaError``.

User-friendly error reporting

You can pass a keyword argument ``error`` to any of validatable classes
(such as ``Schema``, ``And``, ``Or``, ``Regex``, ``Use``) to report this error
instead of a built-in one.

.. code:: python

>>> Schema(Use(int, error='Invalid year')).validate('XVII')
Traceback (most recent call last):
SchemaError: Invalid year

You can see all errors that occurred by accessing exception's ````
for auto-generated error messages, and ``exc.errors`` for errors
which had ``error`` text passed to them.

You can exit with ``sys.exit(exc.code)`` if you want to show the messages
to the user without traceback. ``error`` messages are given precedence in that

A JSON API example

Here is a quick example: validation of
`create a gist <>`_
request from github API.

.. code:: python

>>> gist = '''{"description": "the description for this gist",
... "public": true,
... "files": {
... "file1.txt": {"content": "String file contents"},
... "other.txt": {"content": "Another file contents"}}}'''

>>> from schema import Schema, And, Use, Optional

>>> import json

>>> gist_schema = Schema(And(Use(json.loads), # first convert from JSON
... # use basestring since json returns unicode
... {Optional('description'): basestring,
... 'public': bool,
... 'files': {basestring: {'content': basestring}}}))

>>> gist = gist_schema.validate(gist)

# gist:
{u'description': u'the description for this gist',
u'files': {u'file1.txt': {u'content': u'String file contents'},
u'other.txt': {u'content': u'Another file contents'}},
u'public': True}

Using **schema** with `docopt <>`_

Assume you are using **docopt** with the following usage-pattern:

Usage: [--count=N] <path> <files>...

and you would like to validate that ``<files>`` are readable, and that
``<path>`` exists, and that ``--count`` is either integer from 0 to 5, or

Assuming **docopt** returns the following dict:

.. code:: python

>>> args = {'<files>': ['LICENSE-MIT', ''],
... '<path>': '../',
... '--count': '3'}

this is how you validate it using ``schema``:

.. code:: python

>>> from schema import Schema, And, Or, Use
>>> import os

>>> s = Schema({'<files>': [Use(open)],
... '<path>': os.path.exists,
... '--count': Or(None, And(Use(int), lambda n: 0 < n < 5))})

>>> args = s.validate(args)

>>> args['<files>']
[<open file 'LICENSE-MIT', mode 'r' at 0x...>, <open file '', mode 'r' at 0x...>]

>>> args['<path>']

>>> args['--count']

As you can see, **schema** validated data successfully, opened files and
converted ``'3'`` to ``int``.

(Beta feature) Generating JSON schema
You can also generate standard `draft-07 JSON schema <>`_ from a dict `Schema`.
This can be used to add word completion and validation directly in code editors.
Here's an example:

.. code:: python

>>> from schema import Optional, Schema
>>> import json
>>> s = Schema({"test": str,
... "nested": {Optional("other"): str}
... })
>>> json_schema = json.dumps(s.json_schema(""))

# json_schema
"properties": {
"test": {"type": "string"},
"nested": {
"properties": {
"other": {"type": "string"}
"required": [],

Please note that this is a beta feature. Some JSON schema features are not implemented. Some caveats:

- There are no object references, items of type `object` are always fully rendered
- Some JSON schema types are not implemented. In those cases, an empty dict will be rendered.
This disables all validation for the item.
- Validations other than type are not implemented. This includes features such as integers'
minimum and maximum or arrays' minItems

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