Nesting Custom Errors in Classes

[Rob Gaddi]

On 7/22/19 7:54 PM, DL Neil wrote:
> Do you use nested classes?
> 
> [following-on from the earlier, "Namespaces: memory vs 'pollution'" discussion 
> thread, wherein a certain 'someone' remembered to from ... import ... as ... an 
> 'action' class but forgot to also import the related custom error class! The 
> original quest was for a wild-card import device. This discussion may obviate 
> continuing the quest and/or needing to remember...]
> 
> 
> Python manages nested classes.
> 
> I've NEVER seen such 'in the wild'.
> (but perhaps I lead a sheltered life?)
> 
> 
> What are proposed as use-cases for such a technique?
> - other languages may not offer decent "inheritance", and this is an alternative 
> method/hack
> - the class is a 'class factory', generating/returning an object
> ? any others
> 
> 
> Why not use it as an "encapsulation" device?
> (please be gentle - reminder: I am too old to have been an OO-native!)
> 
> 
> ***** stub definitions
> 
>  >>> class PythonEnvironment():
> ...     class PythonVersionError( EnvironmentError ):
> ...             pass
> ...     def iscompatible( self ):
>          ''' Ensure that the Python in-use will support
>          all of the facilities employed by the application.
>          '''
> ...             # stub to simulate failure
> ...             raise self.PythonVersionError
> ...
> 
> (code would require an import or from ... import ...)
>  >>> pe = PythonEnvironment()
> 
> 
> ***** its basic application becomes:-
> 
>  >>> pe.iscompatible()
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
>    File "<stdin>", line 6, in compatibility
> __main__.PythonVersionError
> 
> 
> ***** somewhat more realistic use:-
> 
>  >>> try:
> ...     pe.iscompatible()
> ... except PythonEnvironment.PythonVersionError:
> ...     print( "Trapped! -> informative errmsg" )
> ...
> Trapped! -> informative errmsg
> 
> 
> With this construct, one only has to import the 'outer' class, rather than both 
> the class AND its ancillary error class!
> 
> 
> Why haven't I seen it before? Can you see anything 'wrong' with this picture?

I've used them sometimes for basic encapsulation principles without really 
gaining anything.  I use inheritance of nested classes in my 
https://pypi.org/project/indexedproperty/ project, where subclasses of 
IndexedProperty have a _Trampoline inner class subclassed from Trampoline that 
can be redefined as needed, but that's a bit of an obscure use case.

-- 
Rob Gaddi, Highland Technology -- www.highlandtechnology.com
Email address domain is currently out of order.  See above to fix.