On Sun, 18 Jan 2009 07:36:53 -0800, Paul Rubin wrote:
> Steven D'Aprano <st...@remove-this-cybersource.com.au> writes:
>> def foo(self, x, y=None):
>> if y is None:
>> y = self.a
>>
>> I don't find that clumsy in the least. I find it perfectly readable and
>> a standard idiom.
>
> That has the same problem as the earlier version.
No it doesn't. The earlier version had the problem that *any* false
object is replaced by self.a. I'm just following the standard Python
idiom of using None as a sentinel. Have a look through the standard
library and see how many times it is used.
Built-ins rarely accept None as a sentinel, slice() being a conspicuous
exception. This is sometimes a nuisance when writing wrappers:
def my_find(S, sub, start=None, end=None):
"""Like string.find() only with pre-processing."""
pre_process() # stub for something complicated
if end is None and start is None:
return S.find(sub)
elif end if None:
return S.find(sub, start)
else:
return S.find(sub, start, end)
or if you prefer:
def my_find(S, sub, start=None, end=None):
"""Like string.find()"""
pre_process()
args = [sub]
if start is not None:
args.append(start)
if end is not None:
args.append(end)
return S.find(*args)
Having said that, there are times where you need to pass None as a
legitimate argument and not as a sentinel. In that case, your solution:
> If the person passes
> None, they get self.a. I prefer:
>
> sentinel = object()
> ...
>
> def foo(x, y=sentinel):
> if y is sentinel:
> y = self.a
is an excellent one.
--
Steven
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