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.
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