direct initialization of class attributes vs. declarations w/in __init__

Diez B. Roggisch deets at nospam.web.de
Sun Jun 11 18:09:31 CEST 2006


digitalorganics at gmail.com schrieb:
> What's the difference between initializing class variables within the
> class definition directly versus initializing them within the class's
> __init__ method? Is there a reason, perhaps in certain situations, to
> choose one over the other?

You are confusing class variables with instance variables. The former 
are what you can initialize inside the class-statement. However, they 
are shared amongst _all_ instances. Consider this little example:

class Foo(object):
     FOO = 1
     BAR = []

     def __init__(self, FOO):
         self.FOO = FOO
         self.BAR.append(FOO)

     def __repr__(self):
         return "FOO: %r\nBAR: %r\n" % (self.FOO, self.BAR)


f1 = Foo(1)
print f1
f2 = Foo(2)
print f2
print f1


------
meskal:~/Projects/CameraCalibrator deets$ python2.4 /tmp/test.py
FOO: 1
BAR: [1]

FOO: 2
BAR: [1, 2]

FOO: 1
BAR: [1, 2]


-----

As you can see, the list BAR is shared. And you can also see that 
_assigning_ to something like this:

self.FOO

will create an instance-variable. Even if a variable of the same name 
existed on the class before!

Which is precisely the difference between using variable initialization 
in __init__ and inside the class-statement.

BTW,

self.__class__.FOO = value

will set class-variables inside a method. Just if you wondered.

Diez



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