Python - Access class member without initialization of object

Question

In the Python module "Wave" I can use the following syntax:

import wave
wave.open("test.wav", "rb")

This works perfectly fine.Let's say I wanted to use my own initialization of a class:

class Wave():

   def __init__(self):
       pass; 

   def Open(self, fileName, Type):
       return True; # Just testing

Now If I have "main" which calls this class, why can't I do the following?:

if Wave.Open("testing.wav", "rb"): 
   print "The file is open"

TypeError: unbound method Open() must be called with Wave instance as first argument (got str instance instead)

`Wave().Open("testing.wav", "rb")` call it through an instance or convert `Open` to a classmethod. — Ashwini Chaudhary, Nov 12 '13 at 11:25
It's a `classmethod` not `staticmethod` there is a very good explanation [here](http://stackoverflow.com/a/12179752/1982962) — Kobi K, Nov 12 '13 at 11:26

score 4 · Answer 1 · answered Nov 12 '13 at 11:25

4

To be able to call it like this you need to make Open a static method:

@staticmethod
def Open(fileName, Type):
   ...

The difference between your code and the example you give at the start is that wave is a module, and Wave is a class. You could turn Wave into a module and have Open be a top-level function within that module.

answered Nov 12 '13 at 11:25

NPE

486,780
108
951
1,012

@NPE Thanks I'm learning Python and did not know such things existed. :) – Phorce Nov 12 '13 at 11:39
@alko: Actually, on second thoughts you could use either. I've removed my earlier comment. – NPE Nov 12 '13 at 11:43

Lix · Answer 2 · 2013-11-12T11:46:49.253

As the error states, you need an instance of the Wave class to call the Open method. Something like this should work:

if Wave().Open("testing.wav", "rb"): 
   print "The file is open"

Note the parenthesis after Wave - that's what creates the new instance.

If you want to be able to call this method without having an instance of the Wave class you could add a @staticmethod decorator to the Open function. This will allow you to call the method as you do in the code you provided.

alko · Answer 3 · 2013-11-12T11:48:05.650

Other option, if you want your open method be class-specific, is @classmethod decorator, i.e.

class Wave():
   def __init__(self):
       pass;
   @classmethod 
   def open(cls, filename):
       print '{}.open method with args "{}"'.format(cls, filename)
       return True; # Just testing 

class SubWave(Wave): pass

and use as follows:

>>> Wave.open('filename')
__main__.Wave.open method with args "filename"
True

>>> SubWave.open('filename')
__main__.SubWave.open method with args "filename"
True

Python - Access class member without initialization of object

3 Answers3