[Tutor] subclass problem: __names and type-checking

Python python at venix.com
Sat Oct 8 19:32:26 CEST 2005

I think that a sub-class *needs* to support the same programming
interface as the parent class.  Bertrand Meyer has written about
programming by contract and the implications for object oriented design.

If B inherits from A then every context where A or an A instance appears
should work correctly with B or a B instance.  Since the B constructor
*requires* more arguments, it violates that ideal.  In other words, it
would be OK to allow additional arguments.  It is not OK to require

In other words sub-class should really be a different class that adapts
or possibly acts as a proxy for the _BaseClass.  Obviously you have
picked names that presuppose inheritance.

I've abused inheritance in the past in an attempt to reuse code and have
usually regretted it.  An alternative is to create a new class that
"fronts" for the class with the code we want to reuse.  The __getattr__
method provides a real simple way to redirect references from our new
class to the original class.

class Base:
	def __init__(self, arg1,arg2):
class Adapt:
	def __init__(self,arg1,arg2,arg3,arg4):
		self._base = Base(arg1,arg2)
	# this provides reuse of those Base methods that can be reused directly
	def __getattr__(self,attrname):
		return getattr(self._base, attrname)

Without knowing more about what you are doing, I could be sending you
off in the wrong direction, but hopefully this is relevant.

Lloyd Kvam
Venix Corp

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