flake8 plugin which bans the usage of datetime.datetime.utcnow
Project description
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
Code | Description |
---|---|
UTC001 | don't use datetime.utcnow() , use datetime.now(timezone.utc) instead |
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 adatetime
in UTC formatted as a timestamp hence callingutcnow().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 callutcfromtimestamp
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 adatetime
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.
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