[Tutor] methods and functions

[Magnus Lyckå]

At 10:54 2003-06-13 -0700, Gerardo Arnaez wrote:
>How can an instance of a class NOT have an attribute
>that has been defined for that class.

The instance *never* has an attribute just because
its class has an attribute. They are two different
objects.

When python reaches the code:

class X:
     ...

it creates a *class* object, and calls it X.

When python reaches the code:

x1 = X()

it creates an *instance* object, which will have an
attribute x1.__class__ = X.

 >>> class X:
...     a = 'Attribute in class X'
...
 >>> type(X)
<type 'class'>

 >>> x1 = X()
 >>> type(x1)
<type 'instance'>

 >>> x1.a
'Attribute in class X'

 >>> x1.a = 'Attribute in instance x1'
 >>> x1.a
'Attribute in instance x1'

 >>> x1.__dict__
{'a': 'Attribute in instance x1'}

 >>> X.__dict__
{'a': 'Attribute in class X', '__module__': '__main__', '__doc__': None}

The same mechanism exists whether the attributes are
strings, functions, or anything else.

 >>> class X:
...     def hi(self):
...             print "Hi"
...
 >>> X.hi
<unbound method X.hi>
 >>> type(X.hi)
<type 'instance method'>
 >>> x = X()
 >>> x.hi
<bound method X.hi of <__main__.X instance at 0x01185B00>>
 >>> type(x.hi)
<type 'instance method'>
 >>> X.__dict__
{'__module__': '__main__', 'hi': <function hi at 0x01188140>, '__doc__': None}
 >>> x.__dict__
{}

You see? Here, the method "hi" is an attribute of the class X.
It's *not* an attribute of the instance object x. But it's been
bound to x...

To connect to another recent thread, we can add a method to
the actual instance object as well...

 >>> x.hi()
Hi

This is what we expect. The x object uses the method defined in
its class. Let's give it its completely private instance method!

 >>> import new
 >>> def myHello(self):
...     print "Hello there!!!"
...
 >>> x.hi = new.instancemethod(myHello, x, X)
 >>> x.__dict__
{'hi': <bound method X.myHello of <__main__.X instance at 0x01185B00>>}

As you see, it now has an attribute 'hi', which is a bound method.

 >>> X.__dict__
{'__module__': '__main__', 'hi': <function hi at 0x01188140>, '__doc__': None}

But the class X still looks just the same... So, how does x
behave now?

 >>> x.hi()
Hello there!!!

It uses the "hi" which is defined most closely of course...
But you can still do:

 >>> X.hi(x)
Hi

Confused yet? :)

As you see, the plain and simple python interpreter and the
standard functions Python has for introspection lets you explore
a lot of things...


--
Magnus Lycka (It's really Lyck&aring;), magnus@thinkware.se
Thinkware AB, Sweden, www.thinkware.se
I code Python ~ The Agile Programming Language