On Sun, 12 Sep 2010 04:01:39 am Walter Prins wrote:
> I guess the question to ask/consider is: How can be establish whether
> a particular object supports a particular interface/set of behaviours
> that we require? E.g. how do we most pythonically check whether some
> object "walks like a list" and "quacks like a list" without tying
> such code to explicit type checking?
isinstance(object, base_class) becomes even more useful in Python 2.6
and beyond, because when you create a class, you can register it as a
list even if you didn't inherit from list. You're essentially
promising "this class will walk and quack and swim like a list".
But often you don't care that something walks, quacks, and swims like a
list. You might only care that it walks. We can combine "Look Before
You Leap" with duck-typing:
def swap_items(sequence):
try:
sequence[0] = sequence[0]
except Exception:
print("does not support item lookup or item assignment")
else:
for i in range(0, len(sequence)-1, 2):
sequence[i], sequence[i+1] = sequence[i+1], sequence[i]
return sequence
>>> swap_items("abcd")
does not support item lookup or item assignment
>>> swap_items(['a', 'b', 'c', 'd', 'e', 'f'])
['b', 'a', 'd', 'c', 'f', 'e']
--
Steven D'Aprano
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