ÿØÿà JFIF    ÿÛ „  ( %!1!%)+//.383,7(-.+  -%%-////---/-.+/--+------/------/--0+--/-/-----.-----ÿÀ  ¥2" ÿÄ     ÿÄ J    ! 1AQ"aq2‘#BR‚¡ÁÑ3br’¢±Âð$CSƒ²á4c“%DsÓñÿÄ   ÿÄ *  !1AQa‘"2q3±ð#b¡ÿÚ   ? ¼QxJQaÍuò¸Zö Úü8,ÐÚú "SSn<rçù–´âE—^ªBÖ9À\†¸ÔÁT­ÃÛ5 ëd´³Í#Ý;Þ38œî ¶H£M:wÎ3…³…âpÔF&‚FK¸9„â4àGEõªfÿ ‘ñ(ßw­pŽF|È¥ù®häðÍѶ¹‘[ÒinÙW¶ùñY˜Q{›K"išÒ[Ú8žë\F¹@-?v"ÔU”,ìöžkÿ {I‡£šÍ?e ríV .............................................................................................................................................................................. ............................................................................. ÿØÿà JFIF    ÿÛ „  ( %!1!%)+//.383,7(-.+  -%%-////---/-.+/--+------/------/--0+--/-/-----.-----ÿÀ  ¥2" ÿÄ     ÿÄ J    ! 1AQ"aq2‘#BR‚¡ÁÑ3br’¢±Âð$CSƒ²á4c“%DsÓñÿÄ   ÿÄ *  !1AQa‘"2q3±ð#b¡ÿÚ   ? ¼QxJQaÍuò¸Zö Úü8,ÐÚú "SSn<rçù–´âE—^ªBÖ9À\†¸ÔÁT­ÃÛ5 ëd´³Í#Ý;Þ38œî ¶H£M:wÎ3…³…âpÔF&‚FK¸9„â4àGEõªfÿ ‘ñ(ßw­pŽF|È¥ù®häðÍѶ¹‘[ÒinÙW¶ùñY˜Q{›K"išÒ[Ú8žë\F¹@-?v"ÔU”,ìöžkÿ {I‡£šÍ?e ríV .............................................................................................................................................................................. .............................................................................  ^c@sdZddlmZmZmZdefdYZdefdYZdefdYZed d iZd Z e d kre nd S(sSupport 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 MetaHelpertMetaMethodWrappertEiffelMethodWrappercBseZdZdZRS(cCstj|||yt||jd|_Wntk rLd|_nXyt||jd|_Wntk rd|_nXdS(Nt_pret_post(Rt__init__tgetattrt__name__tpretAttributeErrortNonetpost(tselftfunctinst((s//usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyRDs   cOsh|jrt|j||nt|j|jf||}|jrdt|j|f||n|S(N(R tapplyRRR (R targstkwtResult((s//usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyt__call__Ss   (Rt __module__RR(((s//usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyRBs t EiffelHelpercBseZeZRS((RRRt__methodwrapper__(((s//usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyR[stEiffelMetaClasscBseZeZRS((RRRt __helper__(((s//usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyR^stEiffelcCs0dtfdY}|}|jddS(NtCcBs#eZdZdZdZRS(cSs|dS(Ni((R targ((s//usr/lib64/python2.7/Demo/metaclasses/Eiffel.pytm1fscSsdS(N((R R((s//usr/lib64/python2.7/Demo/metaclasses/Eiffel.pytm1_prehscSsdS(N((R RR((s//usr/lib64/python2.7/Demo/metaclasses/Eiffel.pytm1_postjs(RRRRR(((s//usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyRes  i (RR(Rtx((s//usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyt_testds t__main__N(( t__doc__tMetaRRRRRRRR!R(((s//usr/lib64/python2.7/Demo/metaclasses/Eiffel.pyt>s