.. image:: https://circleci.com/gh/AndreaCensi/contracts.svg?style=svg
:target: https://circleci.com/gh/AndreaCensi/contracts
PyContracts is a Python package that allows to declare constraints on function parameters and
return values. It supports a basic type system, variables binding, arithmetic constraints, and
has several specialized contracts (notably for Numpy arrays).
As a quick intro, please see this presentation about PyContracts_.
.. _this presentation about PyContracts: http://censi.mit.edu/pub/research/201410-pycontracts/201410-pycontracts.pdf
.. image:: http://censi.mit.edu/pub/research/201410-pycontracts/201410-pycontracts.border.png
:height: 100px
:target: http://censi.mit.edu/pub/research/201410-pycontracts/201410-pycontracts.pdf
:alt: A presentation about PyContracts
.. container:: brief_summary
A brief summary follows. See the full documentation at: <http://andreacensi.github.com/contracts/>
Why: The purpose of PyContracts is not to turn Python into a statically-typed language
(albeit you can be as strict as you wish), but, rather, to avoid the time-consuming and
obfuscating checking of various preconditions. In fact, more than the type constraints, I found
useful the ability to impose value and size constraints. For example, "I need a list of at least
3 positive numbers" can be expressed as list[>=3](number, >0)). If you find that
PyContracts is overkill for you, you might want to try a simpler alternative, such as
typecheck_. If you find that PyContracts is not enough for you, you probably want to be
using Haskell_ instead of Python.
Specifying contracts: Contracts can be specified in three ways:
-
Using the
@contractdecorator: ::@contract(a='int,>0', b='list[N],N>0', returns='list[N]')
def my_function(a, b):
... -
Using annotations (for Python 3): ::
@contract
def my_function(a : 'int,>0', b : 'list[N],N>0') -> 'list[N]':
# Requires b to be a nonempty list, and the return
# value to have the same length.
... -
Using docstrings, with the
:type:and:rtype:tags: ::@contract
def my_function(a, b):
""" Function description.
:type a: int,>0
:type b: list[N],N>0
:rtype: list[N]
"""
...
..
In any case, PyContracts will include the spec in the __doc__ attribute.
Deployment: In production, all checks can be disabled using the function contracts.disable_all(), so the performance hit is 0.
Extensions: You can extend PyContracts with new contracts types: ::
new_contract('valid_name', lambda s: isinstance(s, str) and len(s)>0)
@contract(names='dict(int: (valid_name, int))')
def process_accounting(records):
...
Any Python type is a contract: ::
@contract(a=int, # simple contract
b='int,>0' # more complicated
)
def f(a, b):
...
Enforcing interfaces: ContractsMeta is a metaclass,
like ABCMeta, which propagates contracts to the subclasses: ::
from contracts import contract, ContractsMeta, with_metaclass
class Base(with_metaclass(ContractsMeta, object)):
@abstractmethod
@contract(probability='float,>=0,<=1')
def sample(self, probability):
pass
class Derived(Base):
# The contract above is automatically enforced,
# without this class having to know about PyContracts at all!
def sample(self, probability):
....
Numpy: There is special support for Numpy: ::
@contract(image='array[HxWx3](uint8),H>10,W>10')
def recolor(image):
...
Status: The syntax is stable and it won't be changed. PyContracts is very well tested on Python 2.x.
Status on Python 3.x: We reached feature parity! Everything works on Python 3 now.
Contributors:
Chris Beaumont_ (Harvard-Smithsonian Center for Astrophysics):$varsyntax; kwargs/args for extensions.Brett Graham_ (Rowland Institute at Harvard University):attr(name:type)syntax for checking types of attributes.William Furr_: bug reports and performance improvementsKarol Kuczmarski_ (Google Zurich): implementation of "string" and "unicode" contractsMaarten Derickx_ (Leiden U.): documentation fixesCalen Pennington_ (EdX): disabling checks inside check() function.Adam Palay_ (EdX): implementation of environment variable enabling/disabling override.Ryan Heimbuch_: bug reports- Bernhard Biskup: bug reports
asharp_: bug fixesDennis Kempin_ (Google mothership): Sphinx-style constraints specsAndy Hayden_: Python 3 support, more efficient Numpy checksJonathan Sharpe_: contracts for file-like objects, not operator
(Please let me know if I forgot anybody.)
.. _Jonathan Sharpe: http://jonathansharpe.me.uk/
.. _Chris Beaumont: http://chrisbeaumont.org/
.. _asharp: https://github.com/asharp
.. _Maarten Derickx: http://mderickx.nl/
.. _Ryan Heimbuch: https://github.com/ryanheimbuch-wf
.. _Calen Pennington: https://github.com/cpennington
.. _Adam Palay: https://github.com/adampalay
.. _William Furr: http://www.ccs.neu.edu/home/furrwf/
.. _Karol Kuczmarski: http://xion.org.pl/
.. _Brett Graham: https://github.com/braingram
.. _Dennis Kempin: https://github.com/denniskempin
.. _Andy Hayden: http://careers.stackoverflow.com/hayd
.. _typecheck: http://oakwinter.com/code/typecheck/
.. _Haskell: http://www.haskell.org/