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:
@contract decorator: ::
@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
: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
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: ::
'int,>0' # more complicated ) def f(a, b): ...b=
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)): 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.
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 improvements
Karol Kuczmarski_ (Google Zurich): implementation of "string" and "unicode" contracts
Maarten Derickx_ (Leiden U.): documentation fixes
Calen Pennington_ (EdX): disabling checks inside check() function.
Adam Palay_ (EdX): implementation of environment variable enabling/disabling override.
Ryan Heimbuch_: bug reports
asharp_: bug fixes
Dennis Kempin_ (Google mothership): Sphinx-style constraints specs
Andy Hayden_: Python 3 support, more efficient Numpy checks
Jonathan 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/
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