Jean-Michel Pichavant wrote:
Steven D'Aprano wrote:
On Fri, 14 Aug 2009 18:49:26 +0200, Jean-Michel Pichavant wrote:
Sorry guys (means guys *and* gals :op ), I realized I've not been able
to describe precisely what I want to do. I'd like the base class to be
virtual (aka abstract). However it may be abstract but it does not mean
it cannot do some usefull stuff.
Here is the schema of my abstract methods :
class Interface(object):
def method(self):
# ---------------------
# some common stuff executed here
# ---------------------
print 'hello world'
# ---------------------
# here shall stand child specific stuff (empty in the interface
method)
# ---------------------
if self.__class__.method == Interface.method:
raise NotImplementedError('You should have read the f******
manual ! You must override this method.')
Okay, so I want to sub-class your Interface class. As you said, the
methods in the abstract class are still useful, so in my class, I
don't need any extra functionality for some methods -- I'm happy with
just the "common stuff". So I use normal OO techniques and over-ride
just the methods I need to over-ride:
Sometimes the base is doing cool stuff but incomplete stuff which
requires knowledge only hold by the sub class. In my case the interface
is a high level interface for a software that can run on multiple
hardware platforms. Only the sub class has knowledge on how to operate
the hardware, but no matter the hardware it still produces the same effect.
Let's say I have 50 different hardwares, I'll have 50 sub classes of
Interface with the 'start' method to define. It wouldn't be appropriate
(OO programming)to write 50 times '_log.debug('Starting %s' % self)' in
each child start method when the simple task of logging the call can be
nicely handled by the base class.
In the meantime, I must make sure the user, who is not a python guru in
this case, has implemented the start method for his hardware, because
only him knows how to effectively start this hardware. I don't want him
to come to me saying, "I got no error, still my hardware does not
start". You can then blame him for not reading the docs, but it will
still be less expensive to throw a nice exception with an accurate
feedback.
[snip]
class VerboseGoodChild(Interface):
# forced to over-ride methods for no good reason
Definitely no !! This is the purpose of an interface class: to force
people to write these methods. They *are* required, if they were not,
they would not belong to the Interface.
JM
But there _is_ one moment when you can check those things, then avoid
checking thereafter: object creation. So you can complicate your
__init__ (or __new__) with those checks that make sure you instantiate
only fully defined subclasses:
# obviously not tested except in concept:
class Base(object_or_whatever):
def __init__(self, ...):
class_ = self.__class__
if class_ is Base:
raise TypeError('Attempt to instantiate Base class')
for name in 'one two three four':
if getattr(Base, name) is not getattr(Base, name):
raise NotImplementedError(
'%s implementation missing' % name)
...
--Scott David Daniels
scott.dani...@acm.org
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