Issue 1289118: timedelta multiply and divide by floating point (original) (raw)
Created on 2005-09-12 21:41 by agthorr, last changed 2022-04-11 14:56 by admin. This issue is now closed.
Messages (33)
Author: Daniel Stutzbach (agthorr)
Date: 2005-09-12 21:41
In python 2.4.1, the datetime.timedelta type allows for the multiplication and division by integers. However, it raises a TypeError for multiplication or division by floating point numbers. This is a counterintutive restriction and I can't think of any good reason for it.
For example:
import datetime datetime.timedelta(minutes=5)/2 datetime.timedelta(0, 150) datetime.timedelta(minutes=5)*0.5 Traceback (most recent call last): File "", line 1, in ? TypeError: unsupported operand type(s) for *: 'datetime.timedelta' and 'float'
Author: Raymond Hettinger (rhettinger) * 
Date: 2005-09-13 04:11
Logged In: YES user_id=80475
Tim, do you prefer the current behavior?
Author: Michael Chermside (mcherm) 
Date: 2005-09-15 16:03
Logged In: YES user_id=99874
I, too, would like to know what Tim thinks, but for what it's worth (not much) I find Daniel's point fairly convincing... multiplication by floats is an operation that makes sense, has only one possible obvious meaning, and is not particularly likely to cause errors (the way multiplying Decimal's with floats does). So IF it's easy to implement, I say go for it.
Author: Tim Peters (tim.peters) *
Date: 2005-09-15 21:04
Logged In: YES user_id=31435
timedelta arithmetic is 100% portable now, and wholly explainable in terms of universally understood integer arithmetic. Throw floats into it, and that's lost.
That said, I don't have a strong objection to complicating the
implementation if there are strong use cases. The OP's
example isn't "a use case": it's not worth anything to let
someone multiply a timedelta by 0.5 instead of dividing by 2.
I don't have a use case to offer in its place (never felt a need
here).
If someone wants to work on it, note that a timedelta can contain more than 53 bits of information, so, e.g., trying to represent a timedelta as an IEEE double-precision number of microseconds can lose information. This makes a high- qualty "computed as if to infinite precision with one rounding at the end" implementation of mixed datetime/float arithmetic tricky to do right.
Author: Daniel Stutzbach (agthorr)
Date: 2005-09-15 22:00
Logged In: YES user_id=6324
Let me elaborate on the use-case where I originally ran into this.
I'm conducting a series of observation experiments where I measure the duration of an event. I then want to do various statistical analysis such as computing the mean, median, etc. Originally, I tried using standard functions such as lmean from the stats.py package. However, these sorts of functions divide by a float at the end, causing them to fail on timedelta objects. Thus, I have to either write my own special functions, or convert the timedelta objects to integers first (then convert them back afterwards).
Basically, I want timedelta objects to look and act like fixed-point arithmetic objects so that I can reuse other functions on them that were originally developed to operate on floats. More importantly, I'd rather not have to maintain two different versions of the functions to deal with different types.
For implementation, why not multiply the float times .day, .seconds, and .microseconds separately, then propagate and fraction parts into the next smallest group (e.g., 0.5 days becomes 246060*0.5 seconds).
I agree it'd be possible to lose information with the wrong sequence of operations, but that's always the case when using floating point numbers. In other words, that, too, is what I would expect from the timedelta implementation.
Author: Skip Montanaro (skip.montanaro) *
Date: 2005-09-17 01:48
Logged In: YES user_id=44345
Thus, I have to either write my own special functions, or convert the timedelta objects to integers first (then convert them back afterwards).
How about adding tolong() that returns the number of microseconds in the timedelta and fromlong() that accepts a long representing microseconds and returns a timedelta object? That way the timedelta object does a reasonably simple thing and the user is still responsible for overflow the normal arithmetic stuff. You can do any sort of arithmetic operations on the long (including converting to other numeric types) with all the attendant caveats, then convert back to a timedelta object at the end.
Author: Skip Montanaro (skip.montanaro) *
Date: 2007-09-02 01:54
Attached is a diff to the datetime module that implements floating point division. Comments? Is it worthwhile to pursue? If so, I'll implement the other floating point arithmetic operations.
Author: Skip Montanaro (skip.montanaro) *
Date: 2007-09-02 02:59
Ummm... make that: "I'll implement multiplication."
Author: STINNER Victor (vstinner) *
Date: 2008-12-22 14:25
I like this idea, it's the opposite of the issue #2706.
Author: Alexander Belopolsky (Alexander.Belopolsky)
Date: 2010-04-20 21:04
This is in a way more similar to than to . I am -1 on this RFE for the same reason as I am opposing allowing true division of timedelta by an int. The timedelta type is fundamentally an integer type. A type delta is just a certain number of microseconds. A timedelta divided by a number or multiplied by a float is logically a fractional number of microseconds and python does not have a type to represent it.
Daniel's use case of passing timedeltas to a statistical packages is neatly addressed by 's timedelta / timedelta (true) division. Just strip the dimensionality from your data by dividing each time delta by a chosen unit interval (depending on the problem, a second, a microsecond or even a day may be appropriate). The result will be a set of floats that your number crunching package will be happy to process.
Another advantage of this approach is that floats can be processed more efficiently than timedeltas with FP arithmetics and intermediate results will be more accurate in most cases.
I recommend accepting and rejecting this issue together with .
Author: Alexander Belopolsky (Alexander.Belopolsky)
Date: 2010-04-20 21:06
I meant rejecting , of course.
Author: Mark Dickinson (mark.dickinson) *
Date: 2010-04-20 21:36
The timedelta type is fundamentally an integer type.
I disagree strongly with this, and find this a bizarre point of view. Regardless of how the timedelta is stored internally, it's used to represent physical times. I doubt there are many applications that care about the fact that each timedelta is an integral number of microseconds.
Multiplication or division of a time by a float or int makes perfect sense physically, and I think it should be a legal operation here.
Author: Mark Dickinson (mark.dickinson) *
Date: 2010-04-20 21:41
Not sure why this is marked for 3.3.
Author: Mark Dickinson (mark.dickinson) *
Date: 2010-04-20 21:44
I'll take a look at Skip's patch.
Author: Mark Dickinson (mark.dickinson) *
Date: 2010-04-20 21:45
Whoops. I meant to assign this to me, not Skip.
Author: Mark Dickinson (mark.dickinson) *
Date: 2010-05-20 16:35
Sorry, dropping this again. I've got caught up with too many non-datetime related issues.
Author: Alexander Belopolsky (belopolsky) *
Date: 2010-05-20 20:10
dt.diff does not apply to current SVN version anymore. I am attaching a quick update that does not change the actual calculation performed. See -py3k.diff.
I am still -1 for the reason I stated before, but I would like to review a working patch first before proposing a resolution.
Author: Mark Dickinson (mark.dickinson) *
Date: 2010-05-21 08:42
Alexander, I still don't understand your objection. What's the downside of allowing the multiplication or division of a timedelta by a float?
Perhaps it's true that there are applications where timedeltas are best viewed as integers (with an implicitt 'microsecond' unit), but I think it's also true that there are plenty of applications where they're just regarded as a representation of a physical quantity, and there this proposal seems entirely appropriate.
I would want the timedelta * float and timedelta / float operations to be correctly rounded, so that behaviour is entirely predictable; the current patch doesn't do that. But it wouldn't be hard to implement: there are functions available to express a float as a quotient of two integers, and after that the computation can be performed in integer arithmetic.
Author: Mark Dickinson (mark.dickinson) *
Date: 2010-05-21 09:22
Python reference implementation showing how to do correct rounding.
Author: Mark Dickinson (mark.dickinson) *
Date: 2010-05-21 10:05
N.B. There's already logic for doing div_nearest (i.e., divide one integer by another, returning the closest integer to the result) in the long_round function in Objects/longobject.c. It might be worth pulling that logic out and making it available in a _Py function so that it can be reused in other modules.
Author: Alexander Belopolsky (belopolsky) *
Date: 2010-05-24 18:46
I am attaching a patch that implements Mark's timedelta_arith.py algorithms in C.
With rounding well defined, I am close to withdrawing my opposition to supporting mixed timedelta with float operations. There is still one issue that I believe is worth discussing before this feature is accepted. Time, unlike most other physical quantities has a non-arbitrary notion of direction. Therefore, in many applications, rounding towards past or towards future may be preferable to rounding to nearest.
For example, one of the likely applications of floating point division would be to construct time series from continuous or differently sampled functions. If such series are used to measure correlations between cause and effect, it is important that effect is measured at a time following the cause and not at an early or the same moment.
As Mark noted in private correspondence, this issue is mitigated by the fact that "with correct rounding, for timedeltas t and s, and a positive float x, it is guaranteed that t <= s implies t op x <= s op x" (where op is either * or /). It is still possible however, that even the case of t < s and t op x == s op x present a problem in some applications.
Despite this issue, I would support round to nearest even choice over round to past or to future mainly because it is less likely to lead to surprises where (d1/d2) * d2 != d1. This choice also conforms with the round() builtin definition and is somewhat more difficult to implement right using existing means.
Daniel, would you like to chime in on the questions of how the results of these operations should be rounded?
If I don't hear principle objections from the "nosy" list, I'll add a documentation patch.
Author: Daniel Stutzbach (stutzbach)
Date: 2010-05-24 19:50
I don't have a strong feeling about the method of rounding. My thinking is: If my application is sensitive to how the last microsecond is rounded, then I shouldn't be using a type that only gives me 1-microsecond precision.
(Likewise, if my application is sensitive to how the last binary digital of the floating point mantissa is rounded ... I'm in trouble)
That said, round-to-nearest strikes me as the least-surprising approach.
Author: Mark Dickinson (mark.dickinson) *
Date: 2010-05-24 19:58
Re rounding: I'll just note that timedelta / timedelta -> float currently does round to nearest; I'd find it quite surprising if float * timedelta -> timedelta didn't round to nearest.
Author: Alexander Belopolsky (belopolsky) *
Date: 2010-05-24 20:44
It looks like we have a consensus on the rounding mode. Note, however that timedelta constructor rounds away from zero at least on Intel/MacOS X:
[-10, -9, -8, -7, -6, -5, -4, -3, -2, -1, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
Should this be considered a bug?
For comparison,
[-10, -8, -8, -6, -6, -4, -4, -2, -2, 0, 0, 2, 2, 4, 4, 6, 6, 8, 8, 10]
[-10, -8, -8, -6, -6, -4, -4, -2, -2, 0, 0, 2, 2, 4, 4, 6, 6, 8, 8, 10]
Author: Mark Dickinson (mark.dickinson) *
Date: 2010-05-24 20:48
Alexander, it looks like Roundup ate some of your message there. :)
Yes, ideally I'd say that the constructor should be doing round-half-to-even. Though last time I looked, the constructor looked quite complicated (especially for float inputs); it may not be feasible to fix this easily.
At any rate, we should open a separate issue for this.
Author: Alexander Belopolsky (belopolsky) *
Date: 2010-05-24 20:54
Indeed. Here is what I intended:
"""
from datetime import timedelta as d [d(microseconds=i + .5)//d.resolution for i in range(-10,10)] [-10, -9, -8, -7, -6, -5, -4, -3, -2, -1, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
Should this be considered a bug?
For comparison,
[d.resolution*(i+0.5)//d.resolution for i in range(-10,10)] [-10, -8, -8, -6, -6, -4, -4, -2, -2, 0, 0, 2, 2, 4, 4, 6, 6, 8, 8, 10]
and
[round(i+0.5) for i in range(-10,10)] [-10, -8, -8, -6, -6, -4, -4, -2, -2, 0, 0, 2, 2, 4, 4, 6, 6, 8, 8, 10] """
I checked the documentation and while it says: "If any argument is a float and there are fractional microseconds, the fractional microseconds left over from all arguments are combined and their sum is rounded to the nearest microsecond." it does not specify how half-integers should be handled.
While it may not be a bug in strict sense, it looks like the code in question can be improved. I'll open a separate issue for this.
Author: Mark Dickinson (mark.dickinson) *
Date: 2010-05-25 09:10
By the way, does your patch do the right thing for
timedelta(microseconds=1) / -4.0
? Because my Python code doesn't. :) [If n is negative, then the 2r > n condition in div_nearest should be 2r < n instead.]
Author: Mark Dickinson (mark.dickinson) *
Date: 2010-05-25 12:57
There's a patch in issue 8817 that exposes a round-to-nearest form of divmod in a function called _PyLong_Divmod_Near; this would save on duplication of code.
Author: Alexander Belopolsky (belopolsky) *
Date: 2010-05-25 14:31
By the way, does your patch do the right thing for
timedelta(microseconds=1) / -4.0
No.
timedelta(microseconds=1) / -4.0 datetime.timedelta(-1, 86399, 999999)
(I just copied your python algorithm ...)
I will merge with issue 8817 patch and that should fix the problem.
Author: Alexander Belopolsky (belopolsky) *
Date: 2010-05-25 15:10
Attaching a combined + patch. Datetime code now uses 's _PyLong_Divmod_Near.
Author: Alexander Belopolsky (belopolsky) *
Date: 2010-05-25 23:28
Attaching a new patch with documentation changes, additional tests, updated patch and a reference leak fix.
Author: Mark Dickinson (mark.dickinson) *
Date: 2010-05-29 19:48
The patch looks good to me. Please replace the tab characters in datetimemodule.c with spaces, though. :)
Author: Alexander Belopolsky (belopolsky) *
Date: 2010-05-31 19:01
Committed in r81625. Fixed white space and added a note to "new in 3.2" section of the RST doc.
History
Date
User
Action
Args
2022-04-11 14:56:13
admin
set
github: 42364
2010-06-06 01:25:31
belopolsky
link
2010-05-31 19:01:14
belopolsky
set
status: open -> closed
resolution: accepted
messages: +
stage: commit review -> resolved
2010-05-29 19:48:37
mark.dickinson
set
messages: +
2010-05-27 22:19:49
belopolsky
set
files: + issue1289118-withdoc.diff
stage: commit review
2010-05-25 23:28:54
belopolsky
set
files: + issue1289118+issue8817-withdoc.diff
messages: +
2010-05-25 15:10:11
belopolsky
set
files: + issue1289118+issue8817-nodoc.diff
messages: +
2010-05-25 14:31:35
belopolsky
set
messages: +
2010-05-25 12:57:41
mark.dickinson
set
messages: +
2010-05-25 09:10:10
mark.dickinson
set
messages: +
2010-05-24 20:54:59
belopolsky
set
messages: +
2010-05-24 20:48:29
mark.dickinson
set
messages: +
2010-05-24 20:44:40
belopolsky
set
messages: +
2010-05-24 19:58:57
mark.dickinson
set
messages: +
2010-05-24 19:50:01
stutzbach
set
messages: +
2010-05-24 18:46:46
belopolsky
set
files: + issue1289118-nodoc.diff
messages: +
2010-05-21 10:05:23
mark.dickinson
set
messages: +
2010-05-21 09:22:21
mark.dickinson
set
files: + timedelta_arith.py
messages: +
2010-05-21 08:42:44
mark.dickinson
set
messages: +
2010-05-20 20:41:36
skip.montanaro
set
nosy: - skip.montanaro
2010-05-20 20:22:49
stutzbach
set
nosy: - agthorr
2010-05-20 20:10:14
belopolsky
set
files: + issue1289118-py3k.diff
keywords: + patch
messages: +
2010-05-20 18:13:05
belopolsky
set
assignee: belopolsky
nosy: + belopolsky, - Alexander.Belopolsky
2010-05-20 16:35:12
mark.dickinson
set
assignee: mark.dickinson -> (no value)
messages: +
2010-04-27 14:08:12
stutzbach
set
nosy: + stutzbach
2010-04-20 21:45:02
mark.dickinson
set
assignee: skip.montanaro -> mark.dickinson
messages: +
2010-04-20 21:44:40
mark.dickinson
set
assignee: skip.montanaro
messages: +
2010-04-20 21:41:52
mark.dickinson
set
messages: +
versions: + Python 3.2, - Python 3.3
2010-04-20 21:36:32
mark.dickinson
set
nosy: + mark.dickinson
messages: +
2010-04-20 21:06:25
Alexander.Belopolsky
set
messages: +
2010-04-20 21:04:55
Alexander.Belopolsky
set
versions: + Python 3.3, - Python 2.6
nosy: + Alexander.Belopolsky
messages: +
type: enhancement
2008-12-22 14:25:54
vstinner
set
nosy: + vstinner
messages: +
2007-09-02 02:59:53
skip.montanaro
set
messages: +
2007-09-02 01:54:54
skip.montanaro
set
files: + dt.diff
messages: +
versions: + Python 2.6
2005-09-12 21:41:10
agthorr
create