How to do this in Python? - A "gotcha"
jhg at acm.org
Wed Mar 18 18:06:50 CET 2009
S Arrowsmith wrote:
> Jim Garrison <jhg at acm.org> wrote:
>> It's a shame the iter(o,sentinel) builtin does the
>> comparison itself, instead of being defined as iter(callable,callable)
>> where the second argument implements the termination test and returns a
>> boolean. This would seem to add much more generality... is
>> it worthy of a PEP?
> class sentinel:
> def __eq__(self, other):
> return termination_test()
> for x in iter(callable, sentinel()):
> Writing a sensible sentinel.__init__ is left as an exercise....
If I understand correctly, this pattern allows me to create
an object (instance of class sentinel) that implements whatever
equality semantics I need to effect loop termination. In the
case in point, then, I end up with
return other=='' or other==b''
with open(filename, "rb") as f:
for buf in iter(lambda: f.read(1000), sentinel())):
i.e. sentinel is really "object that compares equal to both ''
and b''". While I appreciate how this works, I think the
introduction of a whole new class is a bit of overkill for
what should be expressible in iter()
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