Picky floats

[smontanaro]

BPO	1478364
Nosy	@tim-one, @loewis, @smontanaro
Files	picky.diff

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2007-04-29.12:57:59.000>
created_at = <Date 2006-04-28.12:33:21.000>
labels = ['interpreter-core']
title = 'Picky floats'
updated_at = <Date 2007-04-29.12:57:59.000>
user = 'https://github.com/smontanaro'

bugs.python.org fields:

activity = <Date 2007-04-29.12:57:59.000>
actor = 'loewis'
assignee = 'none'
closed = True
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2006-04-28.12:33:21.000>
creator = 'skip.montanaro'
dependencies = []
files = ['7204']
hgrepos = []
issue_num = 1478364
keywords = ['patch']
message_count = 6.0
messages = ['50121', '50122', '50123', '50124', '50125', '50126']
nosy_count = 3.0
nosy_names = ['tim.peters', 'loewis', 'skip.montanaro']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue1478364'
versions = []

[smontanaro]

I came across a bug at work yesterday where I had
written:

    if not delta:
        return 0.0

where delta was a floating point number. After a
series of calculations piling up round-off error it
took on a value on the order of 1e-8. Not zero, but it
should have been. The fix was easy enough:

    if abs(delta) < EPSILON:
        return 0.0

for a suitable value of EPSILON.

That got me to thinking... I'm sure I have plenty of
other similar mistakes in my code. (Never was much of
a numerical analysis guy...) What if there was a
picky-float configure option that generated warnings if
you compared a float with either another float or an
int using "=="? Does that make sense to try for
testing purposes? The initial implementation seemed
straightforward enough, though I'm sure if this idea
merits any serious consideration it will need some more
work (at the least this behavior should probably be
user-enabled).

[smontanaro]

Logged In: YES
user_id=44345

FYI, regarding my original note and the attached patch,
I realized that comparison with ints is mostly (always?)
okay, so I took that out of the patch. Forgot to amend
the wording above though.

[loewis]

Logged In: YES
user_id=21627

I don't think it should be configure-time option. If
implemented, it should be a run-time option, e.g. through a
interpreter command line option, or even a flag in the sys
module.

OTOH, I doubt that it is very useful in practice. How many
warnings do you get out of the test suite when this is on?

[smontanaro]

Logged In: YES
user_id=44345

Yeah, I don't think configure-time selection is the ultimate choice either.
Ideally, I think it's something you might want to enable on a per-module
basis, but other than __future__ settings I'm not aware of anything.

I just ran "make test" which runs everything twice. Looking at the output
it seems to be complaining about 93 locations. All but 16 of them are in
the test suite itself. I've not checked any of them carefully yet. A quick
scan suggests most are comparisons with small integer-valued floats,
so most are probably innocuous.

I'm not surprised there's not a lot of problem in the standard library
though, since it doesn't really do much in the way of numerical
algorithms. It might be more instructive to look at some more
floating point-intensive code (scipy?, MayaVi?).

I'm less interested in finding specific bugs at this point though. I'm
more concerned with deciding whether a suitably constrained
implementation can be provided.

(BTW, off-list I got a somewhat related report of lots of float comparison
warnings in the Python C code by Intel's C compiler. I've asked him to
run it again and file a bug report.)

Skip

[tim-one]

Logged In: YES
user_id=31435

The problem with looking "at some more floating
point-intensive code (scipy?, MayaVi?)" is that people
writing scientific code generally don't use float comparison
incorrectly -- wading through false positives isn't all that
much fun ;-)

[loewis]

Since there were some objections to the patch, and apparently no progress in addressing these objections, I'm now rejecting the patch (on its first birthday).