requiresthat 2025.6.18.1

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()

The that can be almost any valid Python statement which can be evaluated for its veracity, and whose result will decide whether or not the method fires/will be considered a success.

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.