A Python library for design by contract (DbC) and checking values, exceptions, and side-effects. In a nutshell, deal empowers you to write bug-free code. By adding a few decorators to your code, you get for free tests, static analysis, formal verification, and much more. Read intro to get started.

Features

• Classic DbC: precondition, postcondition, invariant.
• Tracking exceptions and side-effects.
• Property-based testing.
• Static checker.
• Integration with pytest, flake8, sphinx, and hypothesis.
• Type annotations support.
• External validators support.
• Contracts for importing modules.
• Can be enabled or disabled on production.
• Colorless: annotate only what you want. Hence, easy integration into an existing project.
• Colorful: syntax highlighting for every piece of code in every command.
• Memory leaks detection: deal makes sure that pure functions don’t leave unexpected objects in the memory.
• DRY: test discovery, error messages generation.
• Partial execution: linter executes contracts to statically check possible values.
• Formal verification: prove that your code works for all input (or find out when it doesn’t).
• Zero-dependency runtime: there are some dependencies for analysis tools, but nothing of it is required on the production.
• Fast: each code change is benchmarked and profiled.
• Reliable: the library has 100% test coverage, partially verified, and runs on production by multiple companies since 2018.

Deal in 30 seconds

# the result is always non-negative
@deal.post(lambda result: result >= 0)
# the function has no side-effects
@deal.pure
def count(items: List[str], item: str) -> int:
    return items.count(item)

# generate test function
test_count = deal.cases(count)

Now we can:

• Run python3 -m deal lint or flake8 to statically check errors.
• Run python3 -m deal test or pytest to generate and run tests.
• Just use the function in the project and check errors in runtime.

Read more in the documentation.

Installation

python3 -m pip install --user 'deal[all]'

Contributing

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 README.md.
• Don’t have time to code? No worries! Just tell your friends and subscribers about the project. More users -> more contributors -> more cool features.

To run tests locally, all you need is task. Run task all to run all code formatters, linters, and tests.

Thank you ❤️