Comment by sdispater | Hacker News Reader

Comment by sdispater | Hacker News Reader

Pendulum is a new library for Python to ease datetimes, timedeltas and timezones manipulation.

It is heavily inspired by [Carbon](http://carbon.nesbot.com) for PHP.

Basically, the Pendulum class is a replacement for the native datetime one with some useful and intuitive methods, the Interval class is intended to be a better time delta class and, finally, the Period class is a datetime-aware timedelta.

Timezones are also easier to deal with: Pendulum will automatically normalize your datetime to handle DST transitions for you.

    import pendulum
    
    pendulum.create(2013, 3, 31, 2, 30, 0, 0, 'Europe/Paris’)
    # 2:30 for the 31th of March 2013 does not exist
    # so pendulum will return the actual time which is 3:30+02:00
    '2013-03-31T03:30:00+02:00’
    
    dt = pendulum.create(2013, 3, 31, 1, 59, 59, 999999, 'Europe/Paris’)
    '2013-03-31T01:59:59.999999+01:00’
    dt = dt.add(microseconds=1)
    '2013-03-31T03:00:00+02:00’
    dt.subtract(microseconds=1)
    '2013-03-31T01:59:59.999998+01:00’

To those wondering: yes I know [Arrow](http://crsmithdev.com/arrow/) exists but its flaws and strange API (you can throw almost anything at get() and it will do its best to determine what you wanted, for instance) motivated me to start this project. You can check why I think Arrow is flawed here: https://pendulum.eustace.io/faq/#why-not-arrow

Link to the official documentation: https://pendulum.eustace.io/docs/

Link to the github project: https://github.com/sdispater/pendulum

Cool library, I'll definitely be looking into this more closely in the future. The automatic handling of ambiguous time during timezone transitions is curious. As an example, Django punts on automatic handling and requires a user to choose between pre and post transition (I wrote the is_dst handling): https://github.com/django/django/blob/19e20a2a3f763388bba926...

What if you were to begin with a time at 3:30 and then subtract an hour. Would your library properly return 1:30? What if a user had a naive time at 3:30, subtracted an hour, and then converted to an aware time using your library? For what it's worth, I think moving to 3:30 is the correct behaviour in the vast amount of cases. Requiring a user to provide a direction to move and throwing an exception if they don't is dangerous. How often are users going to see this exception, if at all? Just wondering how much you've considered cases, if they exist, that would be better off moving to 1:30.

I'll add one more thing. As soon as I started browsing the page I thought "why would I use this over arrow?". Great to see you addressed that by default. I didn't know about some of arrows shortcomings/bugs, so they were really useful.

Handling dates, times, and timezones in particular is a tricky problem, as evident by the large number of libraries in each language trying to get it right. If you haven't already, I'd really recommend reading the blog posts of Jon Skeet regarding Noda Time http://blog.nodatime.org/ and https://codeblog.jonskeet.uk/category/nodatime/ even if it turns up some corner cases you haven't considered, or validates ones you have.

Thanks!

PEP 495 - Local Time Disambiguation (folding) handles this in Python 3.6 quite elegantly:

https://www.python.org/dev/peps/pep-0495/

It adds an extra attribute 'fold' that can be '0' (the default) for normal times and '1' for the later version with that same local time representation in the case of the clock going backwards.

I can't decide if auto-normalizing my times to DST is a feature or anti-feature. If I didn't know it was there, I would consider it a surprising default.

I see what you mean but as soon as you deal with timezone-aware datetimes you don't really have a choice, if an hour has been skipped, it simply doesn't exist.

What is important here, I think, is that any time arithmetic is not affected by this normalization, so if you add an hour the difference will be an hour, so you don't really have to think about it.

> I see what you mean but as soon as you deal with timezone-aware datetimes you don't really have a choice, if an hour has been skipped, it simply doesn't exist.

So somebody clearly made a mistake (either the user or the programmer not checking the input) which you can't automagically fix. Well, at least, it's a terrible idea.

I understand the intent of helping them, but you make more harm this way.

It's also against the ZEN of Python.

I tend to agree.

Obviously automatic normalisation is the correct thing to do when doing arithmetic that crosses DST-rollover boundaries (this is what the Python stdlib gets wrong), but I don't think it should be done (by default) upon creation with a location-based TZ specification:

pendulum.create(2016, 10, 30, 2, 30, 00, 0, 'Europe/Paris') is ambiguous, and pendulum.create(2016, 3, 27, 2, 30, 00, 0, 'Europe/Paris') is non-existent. pytz raises AmbiguousTimeError or NonExistentTimeError respectively in cases where you try to construct local times like that and specifiy is_dst=None: http://pytz.sourceforge.net/#problems-with-localtime

With pendulum I don't see a possibility to enforce a "strict mode" that turns of those automatic assumptions.

If the author is still reading this, I agree.

I am not too kind on raising an error by default (which pytz does not do by default either but rather return the pre-DST transition time).

I think the best thing to do here is keep the current behavior (post-DST) but with an option to choose what you want exactly (post-DST, pre-DST)

A cursory glance at Django's timezone docs says pytz (which is optional but recommended) raises an exception when a time doesn't exist due to DST.

> Unfortunately, during DST transitions, some datetimes don’t exist or are ambiguous. In such situations, pytz raises an exception. Other tzinfo implementations, such as the local time zone used as a fallback when pytz isn’t installed, may raise an exception or return inaccurate results. That’s why you should always create aware datetime objects when time zone support is enabled.

https://docs.djangoproject.com/en/1.10/topics/i18n/timezones...

I already faced some of mentioned problems using Arrow, I'll give pendulum a try tomorrow.

Good, because arrow is effectively dead at this point

Why not submit fixes for the existing library instead?

Can't talk about the specific complaints the author has, but there are a bunch of datetime libraries for python with often intentionally different behaviors. Submitting "fixes" requires the other libraries' authors to see what you want to fix as a defect and not a design decision/feature/to irrelevant to justify breaking changes.

At least that was my impression from trying to find a combination of libraries I like and reading various bug trackers in the process. In the end I pick and choose the pieces that do what I like for each part of the process, luckily the datatypes are mostly compatible or easily adapted.

What mysql lib would you recommend?

I'm doing a `pip install https://dev.mysql.com/get/Downloads/Connector-Python/mysql-c... on older centos boxes. Because I found `pip install mysql-connector-python` seems to fail on older boxes.

I also wonder if I should not start to look at SQLAlchemy. Not so much the ORM. But rather the simple DBAPI[0] interface.

I'm already using Postgres FDW to integrate PostGIS into our mysql dbs. So I am going to drag a Postgres python lib around in the near future, any recommendations.

PS: I Believe I just convinced myself, to go invest some time into learning SQLAlchemy.

http://aosabook.org/en/sqlalchemy.html

Simply because I do not really like the API of the existing libraries (Arrow or Delorean) and I just can't fix the design decisions made by the authors.

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