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metaclasses
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Editing: Eiffel.pyo
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� ��^c @ s� d Z d d l m Z m Z m Z d e f d � � YZ d e f d � � YZ d e f d � � YZ e d d i � Z d � Z e d k r� e � n d S( s� 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. i����( t MetaClasst MetaHelpert MetaMethodWrappert EiffelMethodWrapperc B s e Z d � Z d � Z RS( c C s� t j | | | � y t | | j d � | _ Wn t k rL d | _ n Xy t | | j d � | _ Wn t k r� d | _ n Xd S( Nt _pret _post( R t __init__t getattrt __name__t pret AttributeErrort Nonet post( t selft funct inst( ( s/ /usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyR D s c O sh | j r t | j | | � n t | j | j f | | � } | j rd t | j | f | | � n | S( N( R t applyR R R ( R t argst kwt Result( ( s/ /usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyt __call__S s ( R t __module__R R ( ( ( s/ /usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyR B s t EiffelHelperc B s e Z e Z RS( ( R R R t __methodwrapper__( ( ( s/ /usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyR [ s t EiffelMetaClassc B s e Z e Z RS( ( R R R t __helper__( ( ( s/ /usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyR ^ s t Eiffelc C s0 d t f d � � Y} | � } | j d � d S( Nt Cc B s# e Z d � Z d � Z d � Z RS( c S s | d S( Ni ( ( R t arg( ( s/ /usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyt m1f s c S s d S( N( ( R R ( ( s/ /usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyt m1_preh s c S s d S( N( ( R R R ( ( s/ /usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyt m1_postj s ( R R R R R ( ( ( s/ /usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyR e s i ( R R ( R t x( ( s/ /usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyt _testd s t __main__N( ( t __doc__t MetaR R R R R R R R! R ( ( ( s/ /usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyt <module>> s
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