Search code examples
pythondatetimetimedelta

Datetime wrong deduction

Problem statement

It seems like there is a bug in python datetime module, following code snippet should be self-explanatory:

import datetime 

dep = datetime.datetime(2021, 9, 11, 7, 25)
arr = datetime.datetime(2021, 9, 11, 12, 35)

print(f"expected: -5h10m, real: {dep - arr}")
print(f"expected: 5h10m, real: {arr - dep}")

What I get with python 3.10:

expected: -5h10m, real: -1 day, 18:50:00
expected: 5h10m, real: 5:10:00

Question

Is it a bug or a feature? If it is a bug, what can I do to fix it?

Solution

  • It is a feature - subtraction of datetime from datetime results in a timedelta object, which is normalised so that only negative value can be days. That behaviour is documented, eg. here: https://pl.python.org/docs/lib/datetime-timedelta.html