"""Support Eiffel-style preconditions and postconditions.
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For example,
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class C:
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def m1(self, arg):
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require arg > 0
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return whatever
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ensure Result > arg
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can be written (clumsily, I agree) as:
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class C(Eiffel):
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def m1(self, arg):
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return whatever
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def m1_pre(self, arg):
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assert arg > 0
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def m1_post(self, Result, arg):
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assert Result > arg
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Pre- and post-conditions for a method, being implemented as methods
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themselves, are inherited independently from the method. This gives
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much of the same effect of Eiffel, where pre- and post-conditions are
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inherited when a method is overridden by a derived class. However,
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when a derived class in Python needs to extend a pre- or
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post-condition, it must manually merge the base class' pre- or
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post-condition with that defined in the derived class', for example:
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class D(C):
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def m1(self, arg):
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return arg**2
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def m1_post(self, Result, arg):
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C.m1_post(self, Result, arg)
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assert Result < 100
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This gives derived classes more freedom but also more responsibility
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than in Eiffel, where the compiler automatically takes care of this.
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In Eiffel, pre-conditions combine using contravariance, meaning a
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derived class can only make a pre-condition weaker; in Python, this is
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up to the derived class. For example, a derived class that takes away
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the requirement that arg > 0 could write:
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def m1_pre(self, arg):
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pass
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but one could equally write a derived class that makes a stronger
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requirement:
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def m1_pre(self, arg):
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require arg > 50
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It would be easy to modify the classes shown here so that pre- and
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post-conditions can be disabled (separately, on a per-class basis).
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A different design would have the pre- or post-condition testing
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functions return true for success and false for failure. This would
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make it possible to implement automatic combination of inherited
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and new pre-/post-conditions. All this is left as an exercise to the
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reader.
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"""
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from Meta import MetaClass, MetaHelper, MetaMethodWrapper
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class EiffelMethodWrapper(MetaMethodWrapper):
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def __init__(self, func, inst):
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MetaMethodWrapper.__init__(self, func, inst)
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# Note that the following causes recursive wrappers around
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# the pre-/post-condition testing methods. These are harmless
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# but inefficient; to avoid them, the lookup must be done
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# using the class.
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try:
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self.pre = getattr(inst, self.__name__ + "_pre")
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except AttributeError:
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self.pre = None
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try:
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self.post = getattr(inst, self.__name__ + "_post")
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except AttributeError:
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self.post = None
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def __call__(self, *args, **kw):
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if self.pre:
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apply(self.pre, args, kw)
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Result = apply(self.func, (self.inst,) + args, kw)
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if self.post:
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apply(self.post, (Result,) + args, kw)
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return Result
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class EiffelHelper(MetaHelper):
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__methodwrapper__ = EiffelMethodWrapper
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class EiffelMetaClass(MetaClass):
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__helper__ = EiffelHelper
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Eiffel = EiffelMetaClass('Eiffel', (), {})
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def _test():
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class C(Eiffel):
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def m1(self, arg):
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return arg+1
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def m1_pre(self, arg):
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assert arg > 0, "precondition for m1 failed"
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def m1_post(self, Result, arg):
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assert Result > arg
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x = C()
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x.m1(12)
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## x.m1(-1)
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if __name__ == '__main__':
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_test()
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