flake8-ban-utcnow

flake8 plugin which checks that datetime.utcnow() is not used. It suggests using datetime.now(timezone.utc) instead.

note: timezone must be imported from datetime first:

from datetime import datetime
from datetime import timezone

datetime.now(timezone.utc)

installation

pip install flake8-ban-utcnow

flake8 code

as a pre-commit hook

See pre-commit for instructions

Sample .pre-commit-config.yaml:

- repo: https://github.com/pycqa/flake8
  rev: 5.0.4
  hooks:
    - id: flake8
      additional_dependencies: [flake8-ban-utcnow==0.1.0]

rationale

One could expect that when explicitly calling datetime.utcnow() the datetime object would be timezone aware, but it's not! A common pitfall is, deriving a timestamp from the datetime object created using datetime.utcnow().

example

• the computer is in CEST and we want to derive a datetime in UTC formatted as a timestamp hence calling utcnow().timestamp().

  >>> from datetime import datetime
  >>> datetime.utcnow()
  datetime.datetime(2022, 8, 7, 23, 40, 17, 7858)
  >>> datetime.utcnow().timestamp()
  1659908656.048843
  

• if we convert the timestamp, it says this, which is obviously incorrect.

  GMT: Sunday, 7. August 2022 21:44:16
  Your time zone: Sunday, 7. August 2022 23:44:16 GMT+02:00 DST
  Relative: 2 hours ago
  

• converting it using python and datetime.fromtimestamp, we by accident get the correct datetime in UTC

  >>> datetime.fromtimestamp(1659908656.048843)
  datetime.datetime(2022, 8, 7, 23, 44, 16, 48843)
  

• being aware that the timestamp should be in UTC we call utcfromtimestamp instead and get the result as above, since the timestamp actually is in local time, but unaware of this.

  >>> datetime.utcfromtimestamp(1659908656.048843)
  datetime.datetime(2022, 8, 7, 21, 44, 16, 48843)
  

the correct way

• the computer is in CEST and we want to actually derive a datetime in UTC formatted as a timestamp .

  >>> from datetime import timezone
  >>> from datetime import datetime
  >>> datetime.now(timezone.utc).timestamp()
  1659916399.651218
  

• we now get what we actually expect

  GMT: Sunday, 7. August 2022 23:53:19
  Your time zone: Monday, 8. August 2022 01:53:19 GMT+02:00 DST
  Relative: A few seconds ago
  

• the next thing to keep in mind is, that only timezone aware datetime objects can be compared hence using this forces us to always make sure all objects are timezone aware.