[Python-ideas] datetime.timedelta literals

Giampaolo Rodola' g.rodola at gmail.com
Mon Jun 4 09:08:09 EDT 2018


On Sat, Jun 2, 2018 at 2:21 PM, Pål Grønås Drange <paal.drange at gmail.com>
wrote:

> Elevator pitch:
>
> (2.5h - 14min + 9300ms).total_seconds()
> # 8169.3
>
> from datetime import datetime as dt
> start = dt.now()
> end = dt.now()
> (end-start) < 5s
> # True
>
>
> chrono::duration:
>
> In C++ 14 the std::chrono::duration was introduced which corresponds
> somewhat to
> datetime.timedelta.
>
> C++ 14 introduced so-called chrono literals[1], which are literals
> specified as
> [number][h|min|s|ms|us|ns], e.g.
>
> * 2.5h
> * 14min
> * 9300ms
>
> These literals should correspond to
>
> * datetime.timedelta(0, 9000)  # 2.5h = 2.5*3600 = 9000 seconds
> * datetime.timedelta(0, 840)   # 14min = 14*60 = 840 seconds
> * datetime.timedelta(0, 9, 300000)  # 9300ms = 9 seconds + 3*10^5
> microseconds
>
>
> If a literal was interpreted as a datetime.timedelta, the following would
> work
> out of the box:
>
> 2.5h - 14min + 9300ms * 2
>
> which would correspond to
>
> from datetime import timedelta as D
>
> D(hours=2.5) - D(minutes=14) + D(milliseconds=9300) * 2
> # datetime.timedelta(0, 8178, 600000)  # (*2 precedes, so that's to be
> expected)
>
>
> (D(hours=2.5) - D(minutes=14) + D(milliseconds=9300)) * 2
> # datetime.timedelta(0, 16338, 600000)
>
>
>
> Notes:
>
> * C++ uses `min` instead of `m`.  `min` is a keyword in Python.
> * In C++, `1d` means the first day of a month [2].
> * In C++, `1990y` means the year 1990 (in the Proleptic Gregorian
> calendar) [3].
> * C++ have the types signed integers and not floats, so 2.5h would not be
> valid.
> * My apologies if this has been discussed before; my search-fu gave me
> nothing.
>
>
>
> References:
>
>
> [1] std::literals::chrono_literals::operator""min
> http://en.cppreference.com/w/cpp/chrono/operator%22%22min
>
> [2] http://en.cppreference.com/w/cpp/chrono/day
>
> [3] http://en.cppreference.com/w/cpp/chrono/year
>
>
> Best regards,
> Pål Grønås Drange
>

IMO datetimes are not common enough to deserve their own literals. It would
make the language more complex and harder to learn for a relatively little
benefit. This would probably make more sense as a third party lib:

>>> import datetimeutils
>>> datetimeutils.interpretstr("2.5h - 14min + 9300ms")
datetime(...)

Both the string and the possibility to specify function arguments would
give you way more expressiveness than language literals.


-- 
Giampaolo - http://grodola.blogspot.com
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