⏰ Whenever

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Sensible and typesafe datetimes

Do you cross your fingers every time you work with datetimes, hoping that you didn't mix naive and aware? or that you converted to UTC everywhere? or that you avoided the many pitfalls of the standard library <https://dev.arie.bovenberg.net/blog/python-datetime-pitfalls/>_? There's no way to be sure...

✨ Until now! ✨

Whenever is a datetime library designed from the ground up to enforce correctness. Mistakes become red squiggles in your IDE, instead of bugs in production.

📖 Docs <https://whenever.readthedocs.io> | 🐍 PyPI <https://pypi.org/project/whenever/> | 🐙 GitHub <https://github.com/ariebovenberg/whenever> | 🚀 Changelog <https://whenever.readthedocs.io/en/latest/changelog.html> | ❓ FAQ <https://whenever.readthedocs.io/en/latest/faq.html> | 🗺️ Roadmap | 💬 Issues & discussions <https://github.com/ariebovenberg/whenever/issues>_

Benefits

Distinct classes with well-defined behavior
Fixes pitfalls that arrow and pendulum don't <https://dev.arie.bovenberg.net/blog/python-datetime-pitfalls/>_
Enforce correctness without runtime checks
Based on familiar concepts <https://www.youtube.com/watch?v=saeKBuPewcU>_ and standards
Simple and obvious; no frills or surprises
Thoroughly documented <https://whenever.rtfd.io/en/latest/overview.html>_ and tested
One file; no third-party dependencies

Quickstart

.. code-block:: python

from whenever import ( ... # Explicit types for different use cases ... UTCDateTime, # -> Enforce UTC-normalization ... OffsetDateTime, # -> Simple localized times ... ZonedDateTime, # -> Full-featured timezones ... NaiveDateTime, # -> Without any timezone ... )

py311_release = UTCDateTime(2022, 10, 24, hour=17) UTCDateTime(2022-10-24 17:00:00Z) pycon23_start = OffsetDateTime(2023, 4, 21, hour=9, offset=-6) OffsetDateTime(2023-04-21 09:00:00-06:00)

Simple, explicit conversions

py311_release.as_zoned("Europe/Paris") ZonedDateTime(2022-10-24 19:00:00+02:00[Europe/Paris]) pycon23_start.as_local() # example: system timezone in NYC LocalSystemDateTime(2023-04-21 11:00:00-04:00)

Comparison and equality across aware types

py311_release > pycon23_start False py311_release == py311_release.as_zoned("America/Los_Angeles") True

Naive type that can't accidentally mix with aware types

hackathon_invite = NaiveDateTime(2023, 10, 28, hour=12)

Naïve/aware mixups are caught by typechecker

hackathon_invite - py311_release

Only explicit assumptions will make it aware

hackathon_start = hackathon_invite.assume_zoned("Europe/Amsterdam") ZonedDateTime(2023-10-28 12:00:00+02:00[Europe/Amsterdam])

DST-aware operators

hackathon_end = hackathon_start.add(hours=24) ZonedDateTime(2022-10-29 11:00:00+01:00[Europe/Amsterdam])

Lossless round-trip to/from text (useful for JSON/serialization)

py311_release.canonical_format() '2022-10-24T17:00:00Z' ZonedDateTime.from_canonical_format('2022-10-24T19:00:00+02:00[Europe/Paris]') ZonedDateTime(2022-10-24 19:00:00+02:00[Europe/Paris])

Conversion to/from common formats

py311_release.rfc2822() # also: from_rfc2822() "Mon, 24 Oct 2022 17:00:00 GMT" pycon23_start.rfc3339() # also: from_rfc3339() "2023-04-21T09:00:00-06:00"

Basic parsing

OffsetDateTime.strptime("2022-10-24+02:00", "%Y-%m-%d%z") OffsetDateTime(2022-10-24 00:00:00+02:00)

If you must: you can access the underlying datetime object

pycon23_start.py_datetime().ctime() 'Fri Apr 21 09:00:00 2023'

Read more in the feature overview <https://whenever.readthedocs.io/en/latest/overview.html> or API reference <https://whenever.readthedocs.io/en/latest/api.html>.

Why not...?

The standard library


The standard library is full of quirks and pitfalls.
To summarize the detailed `blog post <https://dev.arie.bovenberg.net/blog/python-datetime-pitfalls/>`_:

1.  Incompatible concepts of naive and aware are squeezed into one class
2.  Operators ignore Daylight Saving Time (DST)
3.  The meaning of "naive" is inconsistent (UTC, local, or unspecified?)
4.  Non-existent datetimes pass silently
5.  It guesses in the face of ambiguity
6.  False negatives on equality of ambiguous times between timezones
7.  False positives on equality of ambiguous times within the same timezone
8.  ``datetime`` inherits from ``date``, but behaves inconsistently
9.  ``datetime.timezone`` isn’t enough for full-featured timezones.
10. The local timezone is DST-unaware

Pendulum

Pendulum is full-featured datetime library, but it's hamstrung by the decision to inherit from the standard library datetime. This means it inherits most of the pitfalls mentioned above, with the notable exception of DST-aware addition/subtraction.

Arrow


Arrow is probably the most historically popular datetime library.
Pendulum did a good write-up of `the issues with Arrow <https://pendulum.eustace.io/faq/>`_.
It addresses fewer of datetime's pitfalls than Pendulum.

DateType

DateType mostly fixes the issue of mixing naive and aware datetimes, and datetime/date inheritance during type-checking, but doesn't address the other pitfalls. The type-checker-only approach also means that it doesn't enforce correctness at runtime, and it requires developers to be knowledgeable about how the 'type checking reality' differs from the 'runtime reality'.

python-dateutil


Dateutil attempts to solve some of the issues with the standard library.
However, it only *adds* functionality to work around the issues,
instead of *removing* the pitfalls themselves.
This still puts the burden on the developer to know about the issues,
and to use the correct functions to avoid them.
Without removing the pitfalls, it's still very likely to make mistakes.

Maya

It's unmaintained, but does have an interesting approach. By enforcing UTC, it bypasses a lot of issues with the standard library. To do so, it sacrifices the ability to represent offset, zoned, and local datetimes. So in order to perform any timezone-aware operations, you need to convert to the standard library datetime first, which reintroduces the issues.

Heliclockter


This library is a lot more explicit about the different types of datetimes,
addressing issue of naive/aware mixing with UTC, local, and zoned datetime subclasses.
It doesn't address the other datetime pitfalls though.

.. _roadmap:

Roadmap
-------

- 🧪 **0.x**: get to feature-parity, process feedback, and tweak the API:

  - ✅ Datetime classes
  - ✅ Deltas
  - ✅ Date and time of day (separate from datetime)
  - 🚧 Interval
  - 🚧 Improved parsing and formatting
- 🔒 **1.0**:
  - API stability and backwards compatibility
  - Implement Rust extension for performance
- 🐍 **future**: Inspire a standard library improvement

Not planned:

- Different calendar systems

Versioning and compatibility policy
-----------------------------------

**Whenever** follows semantic versioning.
Until the 1.0 version, the API may change with minor releases.
Breaking changes will be avoided as much as possible,
and meticulously explained in the changelog.
Since the API is fully typed, your typechecker and/or IDE
will help you adjust to any API changes.

  ⚠️ **Note**: until 1.x, pickled objects may not be unpicklable across
  versions. After 1.0, backwards compatibility of pickles will be maintained
  as much as possible.

Acknowledgements
----------------

This project is inspired by the following projects. Check them out!

- `Noda Time <https://nodatime.org/>`_
- `Temporal <https://tc39.es/proposal-temporal/docs/>`_
- `Chrono <https://docs.rs/chrono/latest/chrono/>`_

Contributing
------------

Contributions are welcome! Please open an issue or a pull request.

  ⚠️ **Note**: big changes should be discussed in an issue first.
  This is to avoid wasted effort if the change isn't a good fit for the project.

..

  ⚠️ **Note**: Some tests are skipped on Windows.
  These tests use unix-specific features to set the timezone for the current process.
  As a result, Windows isn't able to run certain tests that rely on the system timezone.
  It appears that `this functionality is not available on Windows <https://stackoverflow.com/questions/62004265/python-3-time-tzset-alternative-for-windows>`_.

Setting up a development environment

You'll need poetry <https://python-poetry.org/>_ installed. An example of setting up things up:

.. code-block:: bash

poetry install

To run the tests with the current Python version

pytest

if you want to build the docs

pip install -r docs/requirements.txt

Various checks

mypy src/ tests/ flake8 src/ tests/

autoformatting

black src/ tests/ isort src/ tests/

To run the tests with all supported Python versions

Alternatively, let the github actions on the PR do it for you

pip install tox tox -p auto

ariebovenberg / whenever

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