Current File : //proc/thread-self/root/lib64/python2.7/Demo/metaclasses/Eiffel.pyc
�
��^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 k s t d � � d S( Ni s precondition for m1 failed( t AssertionError( R
R ( ( s/ /usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyt m1_preh s c S s | | k s t � d S( N( R ( 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
Mini Shell