author | martin.trojer@nokia.com |
Fri, 31 Jul 2009 15:01:17 +0100 | |
changeset 1 | 2fb8b9db1c86 |
permissions | -rw-r--r-- |
1
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1 |
"""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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|
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can be written (clumsily, I agree) as: |
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|
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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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|
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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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|
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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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|
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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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|
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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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|
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def m1_pre(self, arg): |
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pass |
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|
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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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|
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def m1_pre(self, arg): |
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require arg > 50 |
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|
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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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|
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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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""" |
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|
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from Meta import MetaClass, MetaHelper, MetaMethodWrapper |
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|
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class EiffelMethodWrapper(MetaMethodWrapper): |
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|
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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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|
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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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|
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class EiffelHelper(MetaHelper): |
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__methodwrapper__ = EiffelMethodWrapper |
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|
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class EiffelMetaClass(MetaClass): |
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__helper__ = EiffelHelper |
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|
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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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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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## x.m1(-1) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
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111 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
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if __name__ == '__main__': |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
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113 |
_test() |