requiresthat 2025.6.22.1

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.