[Python-Dev] Negative times behaviour in itertools.repeat for Python maintenance releases (2.7, 3.3 and maybe 3.4)

[Vajrasky Kok]

Dear comrades,

I would like to bring to your attention my disagreement with Larry
Hastings in this ticket: http://bugs.python.org/issue19145
(Inconsistent behaviour in itertools.repeat when using negative
times).

Let me give you the context:

>>> from itertools import repeat
>>> repeat('a')
repeat('a')
>>> repeat('a', times=-1)
repeat('a')
>>> repeat('a', -1)
repeat('a', 0)
>>> repeat('a', times=-4)
repeat('a', -4)
>>> repeat('a', -4)
repeat('a', 0)

Right now, the only way you can tell repeat to do endless repetitions
is to omit the `times` argument or by setting `times` argument to -1
via keyword.

Larry intends to fix this in Python 3.5 by making None value to
`times` argument as a representative of unlimited repetitions and
negative `times` argument (either via keyword or positional) ALWAYS
means 0 repetitions. This will ensure repeat has the appropriate
signature. I have no qualms about it. All is well.

My disagreement is related to Larry's decision not to fix this bug in
Python 2.7, 3.3, and 3.4. Both of us agree that we should not let
Python 2.7, 3.3, and 3.4 happily accepts None value because that is
more than bug fix. What we don't agree is whether we should make
negative `times` argument via keyword behaviour needs to be changed or
not. He prefer let it be. I prefer we change the behaviour so that
negative `times` argument in Python 2.7, 3.3, and 3.4 ALWAYS means 0
repetitions.

My argument is that, on all circumstances, argument sent to function
via positional or keyword must mean the same thing.

Let's consider this hypothetical code:

# 0 means draw, positive int means win, negative int means lose
result = goals_result_of_the_match()

# For every goal of the winning match, we donate money to charity.
# Every donation consists of 10 $.
import itertools
itertools.repeat(donate_money_to_charity(), result)

Later programmer B refactor this code:

# 0 means draw, positive int means win, negative int means lose
result = goals_result_of_the_match()

# For every goal of the winning match, we donate money to charity.
# Every donation consists of 10 $
from itertools import repeat
repeat(object=donate_money_to_charity(), times=result)

They use Python 2.7 (remember Python 2.7 is here to stay for a long
long time). And imagine the match is lost 0-1 or 1-2 or 2-3 (so the
goal difference is negative one / -1). It means they donate money to
charity endlessly!!! They can go bankrupt.

So I hope my argument is convincing enough. We need to fix this bug in
Python 2.7, 3.3, and 3.4, by making `times` argument sent via
positional or keyword in itertools.repeat ALWAYS means the same thing,
which is 0 repetitions.

If this is not possible, at the very least, we need to warn this
behaviour in the doc.

Whatever decision that comes up from this discussion, I will make peace with it.

Vajrasky