spir wrote: > Le Fri, 12 Jun 2009 22:37:17 +1000, > Lie Ryan <lie.1...@gmail.com> s'exprima ainsi: > > Right, thank you. I continued my trials and ended with seemingly working > code, close to yours. > > # case pattern is Klass: yield String instead > if isinstance(pattern,Klass): > self = String(pattern, numMin,numMax, expression,name) > #~ self.__init__(pattern, numMin,numMax, expression,name) > return self > # else a Repetition > self = Pattern.__new__(cls,pattern, numMin,numMax, > expression,name) > return self > > I have more questions: > > 1. For the 'normal' (second) case, I read in the docs that __new__ *must* > return the new object. This should be done using the supertype's __new__. But > you don't do it and it seems to work anyway. In this case, __init__ is > (supposed to be) called automagically.
The super(Normal, cls) construct is semantically equivalent to the supertype on single inheritance cases. There is a bug in my previous code: I called "ret.__init__(arg)" manually. It is unnecessary to invoke __init__ for this since ret is an instance of super(Normal, cls) and python will do automagic __init__ call on it later after "return ret". > 2. For the special case, as you can see the __init__ line is commented out > and it works anyway. While the docs positively assert that __init__ *won't* > be called if an object of a different type is returned, it is anyway. When this statement is run: self = String(pattern, numMin,numMax, expression,name) String() is __init__ ed through normal class instantiation (the one we see everyday), therefore if you or python's automagic invoke __init__ again, __init__ will be called twice (or thrice) as a result (in short, the mechanism that invoked __init__ in this case is NOT python's automagic but String() ) > Of course, i checked that init methods are really used in both cases. > > So, how do things _actually_ work? > (Such confusions are why each time I need __new__ I have to re-learn the real > way to use it.) It seems in this case python is a little bit out-of-character. Python is being a little bit protective by doing automagic __init__ call and automagically making __new__ classmethods. Understandable though, since we can never be sure what could happen if an object's __new__ and __init__ is never called. Rule of thumb: - no need to call __init__ manually for both Normal() nor Special() >>>>> a = Normal(True) >>>>> b = Normal(False) >>>>> a.arg >> True >>>>> b.arg >> False >> generally though, avoid using __new__ >> >> >>> (also Normal's supertype has no explicite __new__, but it has an __init__) >> Haven't tested yet, but that should be no problem as python will use the >> supertype's supertype's __new__ and at the end of the method resolution >> order is object.__new__() > > That's what I thought, too. > Thanks again, > Denis > ------ > la vita e estrany > _______________________________________________ > Tutor maillist - Tutor@python.org > http://mail.python.org/mailman/listinfo/tutor > _______________________________________________ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
