Demo/metaclasses/Eiffel.py

"""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()