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Define the shape of your data with simple python data structures. Use those data descriptions to validate your application.

Project description

Schemagic / Schemagic.web
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Remove the Guesswork from Data Processing

Schemagic is a rather utilitarian re-imagining of the wonderful and powerful clojure library `Schema <>`_!
Schemagic.web is what programmers do when they hate web programming, but want to make their programs accessible to the web.


It's a wheel on Pypi, and it's 2 and 3 compatible.
To install Schemagic, simply:

.. code-block:: bash

$ pip install schemagic

What is schemagic?

One of the difficulties with large scale, multi-team python efforts is the overhead of understanding the kind of data
(e.g., list of strings, nested map from long to string to double) that a function or a webservice expects and returns.
Python lacks static typing and, moreover, static typing is insufficient to capture and validate custom business types,
which ultimately is what holds back teams from rapidly iterating on each others work.[1]

To you, the programmer, schemagic is all about three things:

* data **description** using the simplest python data structures and an easily extensible syntax
* data **communication** between teams, enhancing documentation, giving feedback when something went wrong.
* data **validation** based on descriptions of data that have been documented and communicated.
Comments describing the shape of data are insufficient in real world applications.
Unless the documentation is backed up by programmatic verification, the documentation gets initially ignored,
and ultimately falls behind the actual program behavior.

In other words, **schemagic is all about data**.

Getting Acquainted with Schemagic

Lets build a schema and start using it.

.. code-block:: python

>>> import schemagic
>>> list_of_ints = [int]
>>> schemagic.validate_against_schema(list_of_ints, [1, 2, 3])
[1, 2, 3]
>>> schemagic.validate_against_schema(list_of_ints, ["hello", "my friends"])
Traceback (most recent call last):
ValueError: invalid literal for int() with base 10: 'hello'

The error you see here (customizeable) is the error you get when you try to call:

.. code-block:: python

>>> int("hello")
Traceback (most recent call last):
ValueError: invalid literal for int() with base 10: 'hello'

And it occurred because list_of_ints specified that the function to check every member of the list against was int()

Basic Schemagic Usage

Schema checking is quite flexible, and all checks are done recursively. Lets go through some more examples:

**Map Template**:
*if you only provide a schema with one (callable) key and one value*

.. code-block:: python

>>> string_to_int_map = {str:int}
>>> schemagic.validate_against_schema(string_to_int_map, {"hello": 5, "friends": 6})
{'friends': 6, 'hello': 5}

**Map with Specific Keys**
*if you provide a schema with strings as keys*

.. code-block:: python

>>> friend_record = {"name":str, "age": int}
>>> schemagic.validate_against_schema(friend_record, {"name": "Tyler", "age": 400})
{'name': 'Tyler', 'age': 400}

**Sequence Template**:
*if you provide a sequence containing only one item as a schema*

.. code-block:: python

>>> list_of_ints = [int]
>>> schemagic.validate_against_schema(list_of_ints, [1, 2, 3, 4])
[1, 2, 3, 4]

**Strict Sequence**:
*if you provide a sequence with multiple items as a schema*

.. code-block:: python

>>> list_with_3_items_int_str_and_intstrmap = [int, str, {int: str}]
>>> schemagic.validate_against_schema(list_with_3_items_int_str_and_intstrmap, [1, "hello", {5: "friends", 12: "and", 90: "world"}])
[1, "hello", {5: "friends", 12: "and", 90: "world"}]

**Validation Function**:
*if you provide a function as a schema*

.. code-block:: python

>>> def null(data):
... if data is not None:
... raise TypeError("expected Nonetype, got {0}".format(data))
>>> schemagic.validate_against_schema(null, None)
>>> schemagic.validate_against_schema(null, "hello!")
Traceback (most recent call last):
TypeError: expected Nonetype, got hello

**Compose Schema Definitions Recursively Ad Nauseam**:
*this is where the real value lies*

.. code-block:: python

>>> def enum(*possible_values):
... def _validator(data):
... if not data in possible_values:
... raise ValueError()
... return data
... return _validator
>>> event = {
... "event_type": enum("PRODUCTION", "DEVELOPMENT"),
... "event_name": str
>>> dispatch_request = {
... "events": [event],
... "requested_by": str
>>> schemagic.validate_against_schema(dispatch_request,
... {"events": [{"event_type": "DEVELOPMENT",
... "event_name": "demo_business_process"},
... {"event_type": "DEVELOPMENT",
... "event_name": "demo_other_business_process"}],
... "requested_by": "Tyler Tolton"})
{"events": [{"event_type": "DEVELOPMENT", "event_name": "demo_business_process"}, {"event_type": "DEVELOPMENT", "event_name": "demo_other_business_process"}], "requested_by": "Tyler Tolton"}

Schemagic.validator Usage

**Use the Schemagic.validator for increased message clarity and control**:

.. code-block:: python

>>> list_of_ints_validator = schemagic.validator([int], "Business Type: list of integers")
>>> list_of_ints_validator([1, "not an int", 3])
Traceback (most recent call last):
ValueError: Bad value provided for Business Type: list of integers. - error: ValueError: invalid literal for int() with base 10: 'not an int' schema: [<type 'int'>] value: [1, 'not an int', 3]

**Supply predicate to prevent/enable validation conditionally**:

.. code-block:: python

>>> __env__ = None
>>> WHEN_IN_DEV_ENV = lambda: __env__ == "DEV"
>>> validate_in_dev = partial(schemagic.validator, validation_predicate=WHEN_IN_DEV_ENV)
>>> list_of_ints_validator = validate_in_dev([int], "integer list")
>>> __env__ = "DEV"
>>> list_of_ints_validator([1, "not an int", 3])
Traceback (most recent call last):
ValueError: Bad value provided for integer list. - error: ValueError: invalid literal for int() with base 10: 'not an int' schema: [<type 'int'>] value: [1, 'not an int', 3]
>>> __env__ = "PROD"
>>> list_of_ints_validator([1, "not an int", 3])
[1, "not an int", 3]

**Coerce data as it is validated**:
*note: validate_against_schema will do this automatically. see docs on validator.*

.. code-block:: python

>>> validate_and_coerce = partial(schemagic.validator, coerce_data=True)
>>> list_of_ints_validator_and_coercer = validate_and_coerce([int], "integer list")
>>> list_of_ints_validator_only = schemagic.validator([int], "integer_list")
>>> list_of_ints_validator_only(["1", "2", "3"])
["1", "2", "3"]
>>> # Note that the if you pass an integer string to int() it returns an integer.
>>> # this makes it s dual purpose validator and coercer.
>>> list_of_ints_validator_and_coercer(["1", "2", "3"])
[1, 2, 3]


Schemagic.web is where rubber meets the road in practical usage. It provides an easy way to communicate between
services, between developers, and between development teams in an agile environment. The webservice business world was
the furnace in which schemagic was forged. Get ready to outsource yourself.

To demo the schemagic.web workflow, lets assume the roles of the first people in the world to discover a way
to (gasp) compute the fibonacci sequence in python.

*note: this code is all pulled from Peter Norvig's excellent* `Design of Computer Programs <>`_ *Udacity class.*

.. code-block:: python

def memo(fn):
_cache = {}
def _f(*args):
return _cache[args]
except KeyError:
_cache[args] = result = fn(*args)
return result
except TypeError:
return fn(*args)
_f.cache = _cache
return _f

def fib(n):
if n == 0 or n == 1:
return 1
return fib(n - 1) + fib(n - 2)

>>> fib(30)

Brilliant! Well, now we'll of course want to share this discovery with the world in the form of a microservice, so that
others need not know the inner workings of this complex and dangerous algorithm.

Lets walk through how we might set up this webservice in flask:

.. code-block:: python

from flask import Flask, json
from fibonacci import fib # assuming we implemented the function in

app = Flask(__name__)

def web_fib_endpoint(index):
index = int(index)
except ValueError:
return Response(
response="Argument to /fibonacci/ must be an integer"
return Response(

if __name__ == '__main__':

While this pattern is certainly serviceable, it is rather heavyweight to simply expose a function to the web.
Additionally, the code doesn't lend itself well to easily documenting its input and output.
Lets see an adapted version of this code using schemagic.web utilities.

.. code-block:: python

from import Flask
from fibonacci import fib # assuming we implemented the function in
from schemagic.web import service_registry

app = Flask(__name__)
register_fibonnacci_services = service_registry(app)

input_schema={"n" : int},

if __name__ == '__main__':

There, now we simply *describe* our service with data.
What is the service endpoint, what is the input, what is the output,
and what is the implementation that delivers the contract defined herein.

Important notes:

#. The webservices all uniformally use POST requests to transmit data. The data supplied to the endpoints comes from the payload of the request.
#. Regarding the above example, there are alternate ways of describing the input to fib(). We could have said "input_schema=int", which would imply that the POST request payload should be an int, unwrapped.
the notation used in the example requires the POST request to provide its data via keyword.

How to Contribute
#. This codebase uses the popular `git flow <>`_ model for version control
#. Fork `the repository`_ and make a branch off of develop, (ideally using the naming convention feature/your-feature)
#. When you've finished your feature, make a pull request back into develop.
#. Once you've made your pull request, email `the maintainer`_ and let me know!
#. Finally, if you ever have any questions about how or what to contribute, feel free to send an email!

.. _`the repository`:
.. _`the maintainer`:


This project autogenerates it's documentation using sphinx and hosts it using readthedocs. It can be viewed `here <>`_

.. [1] Please note: this description is adapted from the excellently phrased introduction to the `prismatic/schema <>`_ clojure library this project was based on

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