Static properties

Shalabh Chaturvedi shalabh at cafepy.com
Fri Aug 27 02:02:32 CEST 2004


Per Erik Stendahl wrote:

> Hello everyone,
> 
> Is it possible to define "static properties" in a class?

Yes and no, depending on what exactly you want :)

> Example:
> I have a class, Path, that wraps a lot of os.* and os.path.* functions 
> (and others), so I can write things like this:
> 
> x = Path('/usr/local/some/file')
> if x.IsFile:
>     contents = x.Read()
>     ...
> 
> Now, I would like to wrap os.getcwd(). I can do it like this:
> 
> class Path:
>     def CurrentDirectory():
>         return os.getcwd()
>     CurrentDirectory = staticmethod(CurrentDirectory)
> 
> pwd = Path.CurrentDirectory()
> 
> Question: Can I make CurrentDirectory a property? Just adding
> CurrentDirectory = property(CurrentDirectory) after the staticmethod 
> line didn't work, unsurprisingly. :-)

If you use raw descriptors and not properties, you can make computed 
class attributes:

class CurrentDirectoryDesc(object):
    def __get__(self, obj, cls=None):
       return os.getcwd()

class Path(object):
     CurrentDirectory = CurrentDirectoryDesc()

print Path.CurrentDirectory
print Path().CurrentDirectory

Note that:
- the classes are derived from object
- you cannot make a settable class attribute this way (you have to use a 
metaclass for that)
- if you subsequently assign Path.CurrentDirectory, you will overwrite 
the descriptor

HTH,
Shalabh




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