114 lines
3.2 KiB
Python
114 lines
3.2 KiB
Python
"""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 whatever**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()
|