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"""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()
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