Re: Voluntary Abstract Base Classes
- From: James Stroud <jstroud@xxxxxxxxxxxx>
- Date: Fri, 29 Jun 2007 11:08:00 -0700
Daniel Nogradi wrote:
Hi list,
Well, the short question is: what are they? I've read Guido's python
3000 status report on
http://www.artima.com/weblogs/viewpost.jsp?thread=208549 where he
mentions ABC's but don't quite understand what the whole story is
about.
Anyone has good use cases?
Daniel
My interpretation of his description is that this is a way to check for quack likes a duck behavior.
Say I want a function to check if something behaves like a slice, but don't want to resort to checking for explicit inheritence, I might now have to do something like (factoring out the checking):
def is_slice_like(candidate):
for att in 'start', 'stop', 'step', 'indices':
if not hasattr(candidate, att):
return False
return True
def doit(anarg):
if not is_slice_like(anarg):
raise Exception, 'Must give something like a slice.'
do_rest_of_it()
However, with an ABC, you could define what it means to be slice-like by defining a slice-like ABC (e.g. SliceLike) and then virtually inheriting from this ABC:
# assuming abstract inheritence syntax is the same as regular
class MySlice(SliceLike):
# etc.
def doit(anarg):
if not issubclass(anarg.__class__, SliceLike):
raise Exception, 'Must give something SliceLike.'
def main():
anarg = MySlice(1,2,3)
doit(anarg)
With ABCs, the concept of slice-like is more formalized and transparent than the attribute checking done in is_slice_like and is far more flexible than explicit type checking.
The concept seems to be borrowed from Java interfaces.
But I'm ready to be corrected on my interpretation.
James
.
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