About instance.name look up order

[Kev Dwyer]

Prim wrote:

> First, sorry about my poor english.
> Put these in IPython under ubuntu.
> 
-------------------------------------------------------------------------------------
> class C:
>     def __init__(self):
>         self.x = 1
>     def print(self):
>         print self.x
> c = C()
> c.x --> 1, so c.x mean a attr of c named 'x'
> c.print() --> pirnt 1, so c.print mean a method of c named 'print'
> 
-------------------------------------------------------------------------------------
> class C:
>     def __init__(self):
>         self.x = 1
>     def x(self):
>         print 'x method'
>     def y(self):
>         print 'y method'
> c = C()
> c.x --> 1
> c.x() --> TypeError: 'int' object is not callable
> c.y --> bound method C.y
> #Q1: instance.name will get the attr first, than method?
> 
-------------------------------------------------------------------------------------
> class C:
>     def x(self):
>         print 'x method'
>     def __getattr__(self, attr):
>         print 'in __getattr__ method'
>         return attr
> c = C()
> c.x --> print in __getattr__ method, then throw TypeError: 'str'
> object is not callable
> c.x() --> print in __getattr__ method, x method 2 lines
> #Q2: why c.x would get a exception?
> 
> t = c.x
> t --> print in __getattr__ method, then throw TypeError: 'str' object
> is not callable
> t() --> print x method
> t = c.x() --> print x method, t == None
> #Q3 why t=c.x() and c.x() output different?
> 
> #Q4, if when I define the class use property too, then instance.name
> look up order would be?
> 
> Thanks for your reply.

Hello,

Python always looks for attributes in the instance first, then in the class,
and then in the class's superclasses.  In your first example, by defining
"x" in C.__init__ you are creating an instance attribute named "x".  When 
the attribute c.x is requested, Python finds an attribute "x" in the 
instance and returns it; the method "x" is found in the class, but the 
attribute lookup does not proceed this far.

Try looking at C.__dict__ and c.__dict__ in the interpreter to see how the 
attributes are stored.  

See also 
http://docs.python.org/reference/datamodel.html#customizing-attribute-access

Cheers

Kev