Question

I’m writing some code, in which a class defines a few nested classes (to keep things neater and grouped by purpose). I want the nested classes to use the attribute of the enclosing class (see below). This attribute would provide a default value that is set on top of the enclosing class and not a ‘magic number’.

 class Progress_indicator:
     #use this attribute to provide a default value
     number_of_dots=5
     class Dots:
         #the attribute above provides a default value here:
         def __init__(self, number = Progress_indicator.number_of_dots):
             self._current=0
             self._number_of_dots=number

Now, when the above code is run I get:

NameError: name 'Progress_indicator' is not defined

from the line containing __init__.
I understand that this is due to python defining a namespace as it sees a class declaration but actually assigning that namespace to a name (Progress_indicator in this case) after the full class declaration is processed (i.e. the indented block of code is left). Redefining __init__ to use self.number_of_dots instead of Progress_indicator.number_of_dots generates the same error (as there is no inheritance here). All references I tried (books, and lots of web) suggest that the above should work. But it doesn’t. Is there a neat way of accessing the attribute defined in the enclosing class to provide a default value for a function parameter as above?
I’m using python3.2.

Answer 1

Use None as a placeholder value, and substitute for default if it hasn’t been provided.

class Indicator:
    number_of_dots = 5

    class Dots:
        def __init__(self, number = None):
            if number is None:
                number = Indicator.number_of_dots

            self._current        = 0
            self._number_of_dots = number

It might be bit wordier, but you’ll avoid namespacing issues. Alternatively you can use self._number_of_dots = number or Indicator.number_of_dots if 0 is not a valid value that can be used as an argument.

You could also do def __init__(self, **kwargs) and then kwargs.pop('number', Indicator.number_of_dots), but that might be less clear.