requiresthat 2025.7.2.0

Support for requirements-assisted development

Decorate an instance method with pre- and/or postconditions that must be fulfilled

Example usage

from requiresthat import (
    requires,
    RequirementNotFulfilledError,
    APRIORI,
    POSTMORTEM,
    BEFOREANDAFTER,
)
class C:

    def __init__(self, data=None):
        self.data = data

    @requires(that='self.data is not None')
    @requires(that='self.data == "spam"', when=APRIORI)
    @requires(that='True is not False')
    @requires(that='self.data != "spam"', when=POSTMORTEM)
    @requires(that='len(self.data) >= 3', when=BEFOREANDAFTER)
    def method(self):
        self.data = 'ham'

X = C(data='spam')
X.method()

See the tests for more.

The that can be almost any valid Python statement which can be evaluated and whose veracity can be asserted, and the result thereof will decide whether or not the method fires/will be considered a success. Then details should be observed here.

The parameter when decides if the condition is a-priori, post-mortem, or before-and-after. The default is a-priori, meaning a precondition. Note that before-and-after does not mean during; you cannot mandate an invariant this way!

RequirementNotFulfilledError is the exception you have to deal with in case a condition is not met. NoCallableConstructError gets raised should you apply the decorator to a construct that is not callable. Both of these derive from the base class RequirementError.

Installation

The project is on PyPI, so simply run

python -m pip install requiresthat

If you want to cook it on your own, do this:

git clone https://gitlab.com/bedhanger/mwe.git
cd mwe/python/requiresthat
python -m venv --system-site-packages venv
source venv/bin/activate
python -m build
python -m pip install .
deactivate

It is recommended to carry out the build and install steps in a venv. This is shown in an exemplary manner in the snipped above that deals with building from scratch.

Running the tests

Build the package from scratch like shown above, but do not leave the venv (i.e., don’t call deactivate). Then

python -m pytest --verbose