[issue41904] datetime.datetime.today makes no sense and should be removed

[Alexander Belopolsky]

Change by Alexander Belopolsky :


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resolution:  -> wont fix
stage:  -> resolved
status: open -> closed

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[issue41904] datetime.datetime.today makes no sense and should be removed

[Raymond Hettinger]

Raymond Hettinger  added the comment:

I concur with the other respondents.  This ship has sailed.

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nosy: +rhettinger

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[issue41904] datetime.datetime.today makes no sense and should be removed

[Alexander Belopolsky]

Alexander Belopolsky  added the comment:

I agree that having some of datetime.now([tz]) functionality replicated in 
datetime.today() makes little sense.  However, the presence of this method in 
datetime class is a consequence of datetime being a subclass of the date class. 
The latter was a design mistake IMO, but as other pointed out it is too late to 
do anything about it.

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assignee:  -> belopolsky

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[issue41904] datetime.datetime.today makes no sense and should be removed

[Damian Yurzola]

Damian Yurzola  added the comment:

Sorry I got my "current" wrong and I can't find the edit button
Here again:


> "How long is it until Christmas?"

# Current implementation
In [23]: datetime.datetime(2020, 12, 25) - datetime.datetime.today()
Out[23]: datetime.timedelta(days=71, seconds=53018, microseconds=941806)

# Hassan's
In [16]: datetime.datetime(2020, 12, 25) - 
datetime.datetime(datetime.datetime.today().year, 
datetime.datetime.today().month, datetime.datetime.today().day)
Out[16]: datetime.timedelta(days=72)

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[issue41904] datetime.datetime.today makes no sense and should be removed

[Damian Yurzola]

Damian Yurzola  added the comment:

It took me a while to collect my thoughts but here you go.

Advanced users don't have a problem. They'll trade in date or datetime objects 
explicitly. The "proof" is I could not find any github repo with more than one 
start that'll call datetime.today().

The less advanced users are sometime doing datetime.today and then all kinds of 
weird things.

HassanAbouelela has a good point: datetime.today() is a straight forward way to 
get today in a datetime object.


On the topic of:

> "How long is it until Christmas?"

# Current
In [7]: datetime.datetime.today() - datetime.datetime.now()
Out[7]: datetime.timedelta(days=-1, seconds=86399, microseconds=91)

# Hassan's
In [16]: datetime.datetime(2020, 12, 25) - 
datetime.datetime(datetime.datetime.today().year, 
datetime.datetime.today().month, datetime.datetime.today().day)
Out[16]: datetime.timedelta(days=72)


Optimizing for the less advanced user, I believe Hassan's proposal yields the 
more intuitive result.

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[issue41904] datetime.datetime.today makes no sense and should be removed

[Hassan Abouelela]

Hassan Abouelela  added the comment:

> "How long is it until Christmas?" the answer should be different if I ask on 
> one minute past midnight on December 24 or one minute to midnight.

No disagreement there, but that doesn't change based on having the current 
precise time (the current implementation) vs the start of the current day (my 
recommendation), especially if you are just calculating the difference in days. 
In both cases, running it one minute before midnight would return the 23rd, 
running it one minute after would return the 24th.

The only difference between both is that a direct subtraction from the 
beginning of the day, with the current implementation, would result in 
one-point-something days, instead of one day. If that is the case, what is the 
point of having datetime.today? It should be, like the original issue 
suggested, phased out.

The benefit doesn't lie there, rather it lies in having direct access to 
date.today in a datetime format, so you can perform math on it with datetime 
objects. This, in my opinion, would be more consistent with other languages, 
and more importantly would make datetime.today logical, as it would be 
date.today in datetime form, instead of datetime.now.

> many (maybe a majority) of uses of datetime.today are conceptually better as 
> date.today, but not all of them.

In what cases would having the current time be better? Should those cases 
instead use datetime.now for legibility?

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[issue41904] datetime.datetime.today makes no sense and should be removed

[Steven D'Aprano]

Steven D'Aprano  added the comment:

On Wed, Oct 14, 2020 at 12:45:55AM +, Damian Yurzola wrote:

> And you also see people doing date math on datetime.date.today which 
> will result in different answers through out the day.

Yes? Is this a problem? If I ask the question "How long is it until 
Christmas?" the answer should be different if I ask on one minute past 
midnight on December 24 or one minute to midnight.

I daresay that you are correct that many (maybe a majority) of uses of 
datetime.today are conceptually better as date.today, but not all of 
them.

> I like HassanAbouelela's idea that datetime.datetime.today should 
> return an arbitrary fixed time rather than an arbitrary variable time.

But it's not an arbitrary variable time. It is just a high-resolution 
(down to the microsecond) version of "today", instead of the 
low-resolution (down to a single day) date.today.

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[issue41904] datetime.datetime.today makes no sense and should be removed

[Damian Yurzola]

Damian Yurzola  added the comment:

I searched all of github and there seem to be ~350K entries for datetime.today
I think this supports steven.daprano point against removal.

I could not spot any major library in a quick cursory look.

However I do see many calls that look a lot like they should have been

datetime.date.today rather than datetime.datetime.today.

You see people basically dropping the hours, minutes, secs.

And you also see people doing date math on datetime.date.today which will 
result in different answers through out the day.

I like HassanAbouelela's idea that datetime.datetime.today should return   an 
arbitrary fixed time rather than an arbitrary variable time.

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[issue41904] datetime.datetime.today makes no sense and should be removed

[Hassan Abouelela]

Hassan Abouelela  added the comment:

If having an arbitrary time returned from the function is an issue, why not use 
a fixed time, say 0 UTC, representing the start of the day. This would mean 
that datetime.today is date.today, with an (arguably) meaningless timestamp to 
make it a datetime object.

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nosy: +HassanAbouelela

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[issue41904] datetime.datetime.today makes no sense and should be removed

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


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nosy: +belopolsky, p-ganssle

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[issue41904] datetime.datetime.today makes no sense and should be removed

[Steven D'Aprano]

Steven D'Aprano  added the comment:

On Thu, Oct 01, 2020 at 04:37:28PM +, Damian Yurzola wrote:
> I was inspired to file this bug after reading through a multiplicity 
> of bugs introduced by folks confused by the library's behavior.

Are these public bug reports or private anecdotes?

I'm not saying that the method cannot be deprecated or removed, but it 
needs to be justified. We don't break people's code gratuitiously, at 
least not intentionally. If you make a good enough case for deprecating 
the method, we can do so.

> What do you say about the unnecessarily redefined properties?
> 
> Lines Lib/datetime.py#L1606 to datetime.py#L1620

I don't know. Do all the tests still pass if you take them out?

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[issue41904] datetime.datetime.today makes no sense and should be removed

[Damian Yurzola]

Damian Yurzola  added the comment:

Thanks for your prompt answer Steven.

I was inspired to file this bug after reading through a multiplicity of bugs 
introduced by folks confused by the library's behavior. So there's good 
precedent.

While granted, the documentation is explicit and the inheritance chain 
substantiates it. There's nothing more explicit than the function/type names 
and saying datetime.today() brings, as you say, arbitrary time to the 
conversation. Which I claim, subjectively, that it should not.


Gratuitous breakage, is debatable. It would not be the first or last. It could 
be a chance to remove a lot of code that works around potentially incorrect 
mental models.

But since both points are to some extent subjective. I'm OK to have left this 
on the record and move on.


What do you say about the unnecessarily redefined properties?

Lines Lib/datetime.py#L1606 to datetime.py#L1620

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[issue41904] datetime.datetime.today makes no sense and should be removed

[Steven D'Aprano]

Steven D'Aprano  added the comment:

Even if I agreed that this method "makes no sense", and I don't, removing it 
would be gratuitous breakage.

Why should we break potentially thousands of people's code who are happily 
using this method, merely because you say that people who use it have "the 
wrong semantic mental model"?

Considered as a datetime, having `today` return the date and time *now* makes 
good sense to me. Obviously the date components must be today's date, not 
yesterday or tomorrow; and the time component could be any arbitrary time, with 
the current time an obvious choice.

And since datetime inherits from date, it would be very odd if it didn't 
inherit the `today` method. It would also violate the Liskov Substitution 
Principle.

But even if you disagree with my opinions, you still have to justify breaking 
backwards compatibility.

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nosy: +steven.daprano

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[issue41904] datetime.datetime.today makes no sense and should be removed

[Damian Yurzola]

New submission from Damian Yurzola :

Last night I discovered we have datetime.datetime.today alongside
datetime.datetime.now and datetime.date.today.

- datetime.now
- date.today

Both make semantic sense.

datetime.datetime.today returns a datetime, which make no semantic sense and 
causes confusion.

On further inspection of the code, this is due to the fact that datetime 
inherits from date.

so datetime.today is practically an implementation of datetime.now minus the 
"tz".

I think we should implement a datetime.today only to rise an AttributeError or 
some other way to stop people from using the wrong semantic mental model.
We'd also need to remove the documentation entry: 
https://docs.python.org/3/library/datetime.html#datetime.datetime.today


>From this inspection we also find that:

datetime.hour/minute/second are unnecessarily redefined.
lines Lib/datetime.py#L1606 to datetime.py#L1620

could be removed without any ill effect.






date.today:
https://github.com/python/cpython/blob/256e54acdbdb26745d4bbb5cf366454151e42773/Lib/datetime.py#L833



https://docs.python.org/3/library/datetime.html#datetime.datetime.today

--
components: Library (Lib)
messages: 377768
nosy: yurzo
priority: normal
severity: normal
status: open
title: datetime.datetime.today makes no sense and should be removed

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