Orri Ganel wrote: > Hello all, > I was just messing around with a suggestion w chun (wescpy at gmail > dot com) gave (to make a class that handles time, ie: > >> >> import myTime >> >> c = myTime.myTime(10,30) >> >> print c > 10:30 >> >> d = myTime.myTime(8,45) >> >> print c + d > 19:15 > > etc), and I was trying to make it like the str class in that, by > default, when you str() a string, it returns the same object. I tried > to do this with the following (irrelevant code removed), but it didn't > work, despite original indications to the contrary, and I'm not sure I > understand why: > > >>> class stime: > def __init__(self, minutes, seconds=None): > if seconds is None: > if minutes.__class__ is stime: > print minutes, type(minutes), minutes.__class__, id(minutes) > self = minutes > print self, type(self), self.__class__, id(self) > def __repr__(self): > return str(self) > def __str__(self): > return "%02d:%02d" % (self.minutes, self.seconds)
Assigning to self in __init__() just changes the value of the local variable, it has no effect outside the method; inside a method, self is just a method parameter, nothing more. There is some magic that gives self its value for the call. When __init__() is called the new instance has already been created, you are just initializing its state, not creating the object. You can do what you want by defining stime.__new__(). __new__() is responsible for actually creating a new object. You still have to check for an stime instance in __init__() because __init__() will be called even when __new__() returns an existing instance. Here is a simplified example: >>> class stime(object): ... def __new__(cls, value): ... if isinstance(value, cls): ... return value ... return object.__new__(cls, value) ... def __init__(self, value): ... print 'stime.__init__', value ... if isinstance(value, stime): ... return ... self.value = value ... >>> a=stime(3) stime.__init__ 3 >>> a <__main__.stime object at 0x00A32E90> >>> b=stime(a) stime.__init__ <__main__.stime object at 0x00A32E90> >>> b <__main__.stime object at 0x00A32E90> >>> a.value 3 >>> b.value 3 >>> a is b True Kent > > >>> a = stime(10,3) > >>> b = stime(a) > 10:03 <type 'instance'> __main__.stime 12858832 > 10:03 <type 'instance'> __main__.stime 12858832 > >>> # so, it appears to work, because it prints the same id both times. > however, > # . . . > >>> id(a) > 12858832 > >>> id(b) > 12858792 > >>> # they're not the same anymore! and that's not all: > >>> a > 10:03 > >>> b > > Traceback (most recent call last): > File "<pyshell#163>", line 1, in -toplevel- > b > File "C:/Documents and Settings/Cookie/Desktop/stime.py", line 20, in > __repr__ > return str(self) > File "C:/Documents and Settings/Cookie/Desktop/stime.py", line 22, in > __str__ > return "%02d:%02d" % (self.minutes, self.seconds) > AttributeError: stime instance has no attribute 'minutes' > > So, not only do the two variables not refer to the same object after > exiting __init__, but b is now a broken stime instance. Anyone know > what's going on? > > Thanks in advance, > Orri > > -- > Email: singingxduck AT gmail DOT com > AIM: singingxduck > Programming Python for the fun of it. > > > ------------------------------------------------------------------------ > > _______________________________________________ > Tutor maillist - Tutor@python.org > http://mail.python.org/mailman/listinfo/tutor _______________________________________________ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
