symbian-qemu-0.9.1-12/python-2.6.1/Demo/metaclasses/Eiffel.py
author Gareth Stockwell <gareth.stockwell@accenture.com>
Wed, 22 Sep 2010 15:40:40 +0100
branchgraphics-phase-3
changeset 111 345f1c88c950
parent 1 2fb8b9db1c86
permissions -rw-r--r--
Fixes to syborg-graphicswrapper.vcproj These changes allow syborg-graphicswrapper to link against the hostthreadadapter and khronosapiwrapper libraries built by the graphics.simulator component. The .vcproj file uses relative paths, which requires that the following three packages are laid out as follows: os/ graphics adapt/ graphics.simulator qemu

"""Support Eiffel-style preconditions and postconditions.

For example,

class C:
    def m1(self, arg):
        require arg > 0
        return whatever
        ensure Result > arg

can be written (clumsily, I agree) as:

class C(Eiffel):
    def m1(self, arg):
        return whatever
    def m1_pre(self, arg):
        assert arg > 0
    def m1_post(self, Result, arg):
        assert Result > arg

Pre- and post-conditions for a method, being implemented as methods
themselves, are inherited independently from the method.  This gives
much of the same effect of Eiffel, where pre- and post-conditions are
inherited when a method is overridden by a derived class.  However,
when a derived class in Python needs to extend a pre- or
post-condition, it must manually merge the base class' pre- or
post-condition with that defined in the derived class', for example:

class D(C):
    def m1(self, arg):
        return arg**2
    def m1_post(self, Result, arg):
        C.m1_post(self, Result, arg)
        assert Result < 100

This gives derived classes more freedom but also more responsibility
than in Eiffel, where the compiler automatically takes care of this.

In Eiffel, pre-conditions combine using contravariance, meaning a
derived class can only make a pre-condition weaker; in Python, this is
up to the derived class.  For example, a derived class that takes away
the requirement that arg > 0 could write:

    def m1_pre(self, arg):
        pass

but one could equally write a derived class that makes a stronger
requirement:

    def m1_pre(self, arg):
        require arg > 50

It would be easy to modify the classes shown here so that pre- and
post-conditions can be disabled (separately, on a per-class basis).

A different design would have the pre- or post-condition testing
functions return true for success and false for failure.  This would
make it possible to implement automatic combination of inherited
and new pre-/post-conditions.  All this is left as an exercise to the
reader.

"""

from Meta import MetaClass, MetaHelper, MetaMethodWrapper

class EiffelMethodWrapper(MetaMethodWrapper):

    def __init__(self, func, inst):
        MetaMethodWrapper.__init__(self, func, inst)
        # Note that the following causes recursive wrappers around
        # the pre-/post-condition testing methods.  These are harmless
        # but inefficient; to avoid them, the lookup must be done
        # using the class.
        try:
            self.pre = getattr(inst, self.__name__ + "_pre")
        except AttributeError:
            self.pre = None
        try:
            self.post = getattr(inst, self.__name__ + "_post")
        except AttributeError:
            self.post = None

    def __call__(self, *args, **kw):
        if self.pre:
            apply(self.pre, args, kw)
        Result = apply(self.func, (self.inst,) + args, kw)
        if self.post:
            apply(self.post, (Result,) + args, kw)
        return Result

class EiffelHelper(MetaHelper):
    __methodwrapper__ = EiffelMethodWrapper

class EiffelMetaClass(MetaClass):
    __helper__ = EiffelHelper

Eiffel = EiffelMetaClass('Eiffel', (), {})


def _test():
    class C(Eiffel):
        def m1(self, arg):
            return arg+1
        def m1_pre(self, arg):
            assert arg > 0, "precondition for m1 failed"
        def m1_post(self, Result, arg):
            assert Result > arg
    x = C()
    x.m1(12)
##    x.m1(-1)

if __name__ == '__main__':
    _test()