On 3/11/2011 1:43 AM, Steven D'Aprano wrote:
The iter() built-in takes two different forms, the familiar
iter(iterable) we all know and love, and an alternative form:
iter(callable, sentinel)
E.g.:
T = -1
def func():
... global T
... T += 1
... return T
...
it = iter(func, 3)
next(it)
0
next(it)
1
next(it)
2
next(it)
Traceback (most recent call last):
File "<stdin>", line 1, in<module>
StopIteration
I've never seen this second form in actual code. Does anyone use it, and
if so, what use-cases do you have?
Looking through Peter's nice list of real use cases, I see two
categories: adapting an iterator-like function to Python's protocol;
stopping an iterator before its normal exhaustion.
An example of the latter: suppose a file consists of header and body
separated by a blank line. An opened file is an iterable, but its
iterator only stops at eof. To process the two parts separately (using
example from Peter's list):
for l in iter(f.readline,''):
<process header line>
for l in f:
<process body line>
(I am pretty sure that there are no buffer screwups with stop and
restart since f iterator should call f.readline internally.)
Alternative is explicit "if l == '': break" in first loop.
I suspect the two param form, still rather new and unusual, is used less
that is could, and perhaps should be. I am glad you raised the question
to get me thinking about it.
--
Terry Jan Reedy
--
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