Mailing List Archive: Unbound method

Unbound method

Jun 22, 1999, 11:31 AM

Post #1 of 3 (591 views)

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Hi everyone!!!

I'm new in Python and in Brasil we don't wave any book of Python (and
it's very expensive to import!), so I have to ask you all this boring
questions!

Why it doens't work ?

--- test.py
|
|class A:
| a = 'Greetings from Brasil!!!'
| def pa(self):
| print self.a

Python 1.5.2 (#0, Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)] on win32
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> from test import *
>>> x = A
>>> x.a
'Greetings from Brasil!!!'
>>> x.pa
<unbound method A.pa>
>>>

Henrique Faria
Desenvolvedor Multimídia
LAMI - Laboratório de Mídias Interativas Puc-PR
faria@ppgia.pucpr.br

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Unbound method [ In reply to ]

akuchlin at mems-exchange

Jun 22, 1999, 2:24 PM

Post #2 of 3 (575 views)

Permalink

faria writes:
>Python 1.5.2 (#0, Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)] on win32
>Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>>> from test import *
>>>> x = A
>>>> x.a
>'Greetings from Brasil!!!'
>>>> x.pa
><unbound method A.pa>

x is a class, not an instance. This can be made clearer with
different variable names:

>>> klass = A
>>> instance = A()
>>> klass
<class __main__.A at e21d8>
>>> instance
<__main__.A instance at f22c8>
>>> klass.a
'Greetings from Brasil!!!'
>>> instance.a
'Greetings from Brasil!!!'
>>> klass.pa
<unbound method A.pa>
>>> instance.pa
<method A.pa of A instance at f22c8>

If you're expecting x = A to create an instance of the class A, that
expection is incorrect; you have to do x = A() to create an instance.

--
A.M. Kuchling http://starship.python.net/crew/amk/
In the design of fission reactors man was not an innovator but an unwitting
imitator of nature.
-- George A. Cowan, "A Natural Fission Reactor"

Unbound method [ In reply to ]

jam at newimage

Jun 22, 1999, 2:48 PM

Post #3 of 3 (569 views)

Permalink

On Tue, Jun 22, 1999 at 03:31:31PM -0300, faria wrote:
>
> Hi everyone!!!
>
> I'm new in Python and in Brasil we don't wave any book of Python (and
> it's very expensive to import!), so I have to ask you all this boring
> questions!
>
> Why it doens't work ?
>
> --- test.py
> |
> |class A:
> | a = 'Greetings from Brasil!!!'
> | def pa(self):
> | print self.a
>
>
> Python 1.5.2 (#0, Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)] on win32
> Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
> >>> from test import *
> >>> x = A
> >>> x.a
> 'Greetings from Brasil!!!'
> >>> x.pa
> <unbound method A.pa>
> >>>
>
>
>

hi.

I'm going to take a stab at explaining this, since I ran into similar
confusion in some code I'm working on for Image.

what you did above was to lookup and use *unbound* methods and attributes of
the class called 'A'.

let's simplify this, and rename the class to 'spam':

class spam:
a = "greetings from brasil!"
def pa(self):
print self.a

now, instantiate (or 'create') an *instance* of type class 'spam':

[~] [5:44pm] [jam@toast-pts/4] % python
Python 1.5.2 (#1, Apr 18 1999, 16:03:16) [GCC pgcc-2.91.60 19981201 (egcs-1.1.1 on linux2
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> class spam:
... a = "greetings from brasil!"
... def pa(self):
... print self.a
...
>>> egg = spam()
>>> egg.a
'greetings from brasil!'
>>> egg.pa
<method spam.pa of spam instance at 80e1250>
>>> egg.pa()
greetings from brasil!
>>> type(egg)
<type 'instance'>
>>> type(spam)
<type 'class'>

does that help to explain what is going on a little better? I'm sorry I
don't have the precise terms used to describe these things.. what you used
in your example was a perfectly valid expression in python (it's *ok* to
reference unbound methods of objects.. I'm sure there are uses for them),
but it's probably not what you expected.

regards,
J
--
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