Raymond Hettinger added the comment:
"Surprising" is in the eye of the beholder. Having islice() always consume at
least "start" number of values seems reasonable enough and it is the documented
behavior: "If start is non-zero, then elements from the iterable are skipped
until start is reached. ... If stop is None, then iteration continues until the
iterator is exhausted, if at all; otherwise, it stops at the specified
position."
That covers a lot of cases including the existing consume() recipe and what
happens to the iterator if it supplies fewer than start elements:
it=iter('abc'); list(islice(it, 4, 4)); print(list(it)). Also, to my eyes, the
current behavior seems like a natural and smooth transition as the arguments
transition towards cross-over:
>>> from itertools import islice
>>> it = iter('ABCDEFG'); print(list(islice(it, 2, 4)), list(it))
['C', 'D'] ['E', 'F', 'G']
>>> it = iter('ABCDEFG'); print(list(islice(it, 2, 3)), list(it))
['C'] ['D', 'E', 'F', 'G']
>>> it = iter('ABCDEFG'); print(list(islice(it, 2, 2)), list(it))
[] ['C', 'D', 'E', 'F', 'G']
>>> it = iter('ABCDEFG'); print(list(islice(it, 2, 1)), list(it))
[] ['C', 'D', 'E', 'F', 'G']
>>> it = iter('ABCDEFG'); print(list(islice(it, 2, 0)), list(it))
[] ['C', 'D', 'E', 'F', 'G']
>>> it = iter('ABCDEFG'); print(list(islice(it, 0, 2)), list(it))
['A', 'B'] ['C', 'D', 'E', 'F', 'G']
>>> it = iter('ABCDEFG'); print(list(islice(it, 1, 2)), list(it))
['B'] ['C', 'D', 'E', 'F', 'G']
>>> it = iter('ABCDEFG'); print(list(islice(it, 2, 2)), list(it))
[] ['C', 'D', 'E', 'F', 'G']
>>> it = iter('ABCDEFG'); print(list(islice(it, 3, 2)), list(it))
[] ['D', 'E', 'F', 'G']
>>> it = iter('ABCDEFG'); print(list(islice(it, 4, 2)), list(it))
[] ['E', 'F', 'G']
Putting in your suggested optimization to treat start > stop as being fully
consumed from the outset would have some big disadvantages. It would break
existing code relying on the current reasonable and documented behavior of
always eating at least "start" elements. And it would incentivize a new and
weird idiom for "consume" as islice(it, n, 0).
As far as I can tell, the current behavior hasn't been problematic for anyone
in the almost 15 year history of the itertools module, so it is hard to get
excited enough about this to want to incur those disadvantages.
----------
priority: normal ->
resolution: -> not a bug
status: open -> closed
_______________________________________
Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue27384>
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