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Voluntary Abstract Base Classes

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/viewpo...?thread=208549 where he
mentions ABC's but don't quite understand what the whole story is
about.

Anyone has good use cases?

Daniel
Jun 29 '07 #1
1 1089
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/viewpo...?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
Jun 29 '07 #2

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