[Tutor] Object instances
john at fouhy.net
Mon Nov 7 01:43:41 CET 2005
On 07/11/05, DS <ds-python-tutor at sidorof.com> wrote:
> So, I can see the error of my ways. I can also see that this behavior
> gives me an opportunity to globally change a value in all of the object
> instances, if I ever had to do something like that. I just don't have a
> clue as to why objects were designed this way.
> Can anyone point me in the right direction?
The difference is between class objects and class instances.
When you say "class Foo: ...", you are creating a class object. When
you call a class object, you create an instance of that class.
Let's have a look:
>>> class Foo(object):
... testList = 
... print 'Foo class object being created now!'
... def __init__(self):
... self.testList2 = 
... print 'Foo instance being initialised now!'
Foo class object being created now!
When I hit <return>, the python interpreted executed the class
definition. This is why it printed out "Foo class object being
created now!". This is also when it executed "testList = ".
testList is an attribute of Foo:
On the other hand, the code in Foo.__init__ did not execute. That
code will only execute when we create an instance of Foo. So, Foo has
no testList2 attribute, because "self.testList2 = " only happens in
Traceback (most recent call last):
File "<stdin>", line 1, in ?
AttributeError: type object 'Foo' has no attribute 'testList2'
We can create an instance in the usual way, by calling the class object:
>>> f = Foo()
Foo instance being initialised now!
Now, we can get at testList2.
We can also get at testList in the same way --- but testList belongs
to the class, not the instance.
>>> f.testList is f.__class__.testList is Foo.testList
I guess the same thing happens with methods --- that there is only one
copy of each method --- but you don't notice, becuase when you call a
method on an instance, it gets passed the instance as the first
Hope this helps!
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