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Programming by contract

Project description


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Deal -- python library for design by contract (DbC) programming.

That's nice assert statements in decorators style to validate function input, output, available operations and object state. Goal is make testing much easier and detect errors in your code that occasionally was missed in tests.


Available decorators

CLassic DbC:

  • deal.pre -- validate function arguments (pre-condition)
  • -- validate function return value (post-condition)
  • deal.ensure -- post-condition that accepts not only result, but also function arguments.
  • deal.inv -- validate object internal state (invariant).

Take more control:


  • deal.chain -- chain a few contracts in one.
  • deal.pure -- forbid side-effects and combine safe, silent, and offline.
  • -- forbid exceptions.


python3 -m pip install --user deal

Quick Start

import re

import attr
import deal

REX_LOGIN = re.compile(r'^[a-zA-Z][a-zA-Z0-9]+$')

class PostAlreadyLiked(Exception):

@deal.inv(lambda post: post.visits >= 0)
class Post:
    visits: int = attr.ib(default=0)
    likes: set = attr.ib(factory=set)

    @deal.pre(lambda user: REX_LOGIN.match(user), message='invalid username format')
    @deal.chain(deal.offline, deal.silent)
    def like(self, user: str) -> None:
        if user in self.likes:
            raise PostAlreadyLiked
        self.likes.add(user) result: 'visits' in result) result: 'likes' in result) result: result['likes'] > 0)
    def get_state(self):
        return dict(visits=self.visits, likes=len(self.likes))

Now, Deal controls conditions and states of the object at runtime:

  1. @deal.inv controls that visits count in post always non-negative.
  2. @deal.pre checks user name format. We assume that it should be validated somewhere before by some nice forms with user-friendly error messages. So, if we have invalid login passed here, it's definitely developer's mistake.
  3. @deal.raises says that only possible exception that can be raised is PostAlreadyLiked.
  4. @deal.chain(deal.offline, deal.silent) controls that function has no network requests and has no output in stderr or stdout. So, if we are making unexpected network requests somewhere inside, deal let us know about it.
  5. checks result format for get_state. So, all external code can be sure that fields likes and visits always represented in the result and likes always positive.

If code violates some condition, sub-exception of deal.ContractError will be raised:

p = Post()
p.visits = -1
# InvContractError:

Dive deeper on


Contributions are welcome! A few ideas what you can contribute:

  • Add new checks for the linter.
  • Improve documentation.
  • Add more tests.
  • Improve performance.
  • Found a bug? Fix it!
  • Made an article about deal? Great! Let's add it into the
  • Don't have time to code? No worries! Just tell your friends and subscribers about the project. More users -> more contributors -> more cool features.

Thank you :heart:

Project details

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