On Wednesday, April 24, 2013 6:20:36 AM UTC+5:30, alex23 wrote: > On Apr 24, 9:13 am, vasudevram <vasudev...@gmail.com> wrote: > > > On Wednesday, April 24, 2013 3:52:57 AM UTC+5:30, Ian wrote: > > > > On Tue, Apr 23, 2013 at 3:50 PM, vasudevram wrote: > > > > > I saw an example of defining a class within another class > > > > > In what way is this useful? > > > > > > > In that particular case they're just using it as a namespace. > > > > > > Not clear. An example or more explanation might help, if you can. Thanks. > > > > Namespaces are used to allow for the same label to be applied to > > different concepts without the labels conflicting with each other. If > > I was writing a program that dealt with the mathematics of morality, I > > might want to use the sine function and refer to it in the standard > > way as 'sin', and I might also want to store a value representing your > > lack of goodness as 'sin'. As you can't use the same label in the same > > scope to refer to two different objects, one way of dealing with this > > that lets you still use what you feel are the most appropriate names > > is to put them into a namespace. So you could express this as: > > > > class Math(object): > > sin = function() > > > > class Morality(object): > > sin = True > > > > Then in your code you can clearly distinguish between the two by using > > Math.sin and Morality.sin. Modules & packages are also namespaces, so > > in this example we'd replace the Math class with `import math`, which > > has a sin function defined within it. > > > > A nested class definition will be defined as an attribute of the class > > its defined within: > > > > >>> class Outer(object): > > ... foo = 'FOO' > > ... class Inner(object): > > ... bar = 'BAR' > > ... > > >>> Outer.Inner > > <class '__main__.Inner'> > > >>> Outer.Inner.bar > > 'BAR' > > > > With peewee, the Model class looks for a Meta attribute and uses > > attributes on it to perform some tasks, like where to retrieve/store > > the model data. This allows for a way of distinguishing between > > attributes used to define fields, and attributes needed for those > > tasks. It also means your Models can use field names that the class > > would otherwise reserve for its own internal purposes: > > > > class DatabaseDetails(Model): > > # these attributes are fields > > database = CharField() > > owner = CharField() > > > > # ...but the Meta attribute isn't > > class Meta: > > # these attributes are used by the Model class > > database = db > > > > Here, database as a field is a text string that could contain a > > database name, while DatabaseDetails.Meta.database contains a > > reference to an actual database where the DatabaseDetails record would > > be stored.
Actually, I did know what namespaces are in general. What I didn't get was how the inner class Meta in the peewee example was being used as a namespace. Your explanation makes things very clear. Thank you. Just one other doubt: > >>> Outer.Inner > > <class '__main__.Inner'> > In the above output, I would have thought Python would print __main__.Outer.Inner or Outer.Inner instead of __main__.Inner, since Inner is an attribute of Outer? -- http://mail.python.org/mailman/listinfo/python-list
