flake8-datetimez

A plugin for flake8 to ban the usage of unsafe naive datetime class.

The checks of this plugin have been adopted into Ruff as its flake8-datetimez (DTZ) rule set. Many thanks to the Ruff project for carrying these rules forward — their implementation is faster, more actively maintained, and handles more cases than this plugin does.

If you are starting a new project, or already run Ruff, please prefer it:

# pyproject.toml
[tool.ruff.lint]
extend-select = ["DTZ"]

This plugin remains for codebases that are still on flake8, and is maintained on a best-effort basis for backward compatibility — keeping the existing rules working across the supported Python and flake8 versions, rather than growing new ones.

• DTZ001 : The use of datetime.datetime() without tzinfo argument is not allowed.

• DTZ002 : The use of datetime.datetime.today() is not allowed. Use datetime.datetime.now(tz=) instead.

• DTZ003 : The use of datetime.datetime.utcnow() is not allowed. Use datetime.datetime.now(tz=) instead.

• DTZ004 : The use of datetime.datetime.utcfromtimestamp() is not allowed. Use datetime.datetime.fromtimestamp(, tz=) instead.

• DTZ005 : The use of datetime.datetime.now() without tz argument is not allowed.

• DTZ006 : The use of datetime.datetime.fromtimestamp() without tz argument is not allowed.

• DTZ007 : The use of datetime.datetime.strptime() without %z must be followed by .replace(tzinfo=).

• DTZ011 : The use of datetime.date.today() is not allowed. Use datetime.datetime.now(tz=).date() instead.

• DTZ012 : The use of datetime.date.fromtimestamp() is not allowed. Use datetime.datetime.fromtimestamp(, tz=).date() instead.

• DTZ901 : The use of datetime.datetime.min or datetime.datetime.max without .replace(tzinfo=) is not allowed.

Calling .astimezone() on a naive datetime attaches the local timezone to it, so it is accepted as a way to resolve DTZ001, DTZ002, DTZ005, DTZ006 and DTZ007:

datetime.datetime.now().astimezone()  # ok
datetime.datetime(2000, 1, 1).astimezone()  # ok

It is deliberately not accepted for the remaining warnings:

• DTZ003, DTZ004 : utcnow() and utcfromtimestamp() return a UTC wall clock. Reading that value as local time shifts the instant, so utcnow().astimezone() is off by the local UTC offset, which is exactly what these two warnings are meant to prevent.

• DTZ011, DTZ012 : datetime.date has no .astimezone() at all.

• DTZ901 : datetime.min and datetime.max overflow when converted to another timezone.

Install with pip

$ pip install flake8-datetimez

• Python 3.8 or above (tested on 3.8 through 3.14)
• flake8 3.0.0 or above

The test suite runs against every supported Python version with tox. No interpreter has to be installed beforehand, they are downloaded automatically by uv:

$ uvx --with tox-uv tox

That covers py38 through py314 plus a lint environment, and it is exactly what CI runs, one job per environment. To run a single environment, or the tests with the current interpreter only:

$ uvx --with tox-uv tox -e py314
$ python -m unittest discover -p "test_*.py" -v

The code is formatted and linted with ruff:

$ uvx ruff format .
$ uvx ruff check .

MIT