covenant - Code contracts for Python 3¶
covenant is a Python 3 library for enforcing code contracts.
Preconditions¶
Preconditions are checked before a function is called.
Preconditions are applied to a function via the @pre() decorator. The decorator takes a single argument: a callable with the same number of parameters as the function being decorated:
from covenant import pre
@pre(lambda x: x < 10)
def some_function(x):
...
If the decorated function may be called with keyword arguments then the names of the precondition’s arguments should match those of the function. When decorating a class method, the precondition should include the self argument:
@pre(lambda self, x: x < self.max)
def some_method(self, x):
...
If the value returned by the precondition evalues to False or it raises an Exception then the precondition has been violated and a PreconditionViolationError will be raised.
Postconditions¶
Postconditions are checked after a function is called.
Postconditions are applied to a function via the @post() decorator. The decorator takes a single argument: a callable whose first argument is the function’s return value and the remaining arguments match those of the function being decorated:
from covenant import post
@post(lambda r, x: r < x)
def some_function(x):
return x - 20
If the value returned by the postcondition evalues to False or it raises an Exception then the precondition has been violated and a PostconditionViolationError will be raised.
