Why aren't identical datetimes equal?

Question

I'm working on a simple Python3 script that considers data in five-minute increments. Thanks to this post, I have code which takes any Python datetime object and then rounds it down to the nearest five minutes. (:00, :05, :10, :15, etc.) Note that I cannot use pandas.

Now I need to be able to compare that "rounded-down" datetime with other datetimes, and here I'm running into a problem. Consider this test code:

import sys
from datetime import datetime
from datetime import timedelta

def roundDownDateTime(dt):
    # Arguments:
    #   dt      datetime object
    delta = timedelta(minutes=1) * (dt.minute % 5)
    return dt - delta

def testAlarm(testDate):
    # Arguments:
    #   testDate    datetime object
    currDate = roundDownDateTime( datetime.now() )      # currDate is a DateTime object, rounded down to 5 mins
    print("currDate:  "+currDate.strftime("%Y%m%d%H%M"))
    print("testDate:  "+testDate.strftime("%Y%m%d%H%M"))
    if(currDate == testDate):
        print("ALARM!!!!")

def main():
    testDate = datetime.strptime(sys.argv[1], "%Y%m%d%H%M")
    testAlarm(testDate)

if __name__ == "__main__":
    main()

The code does all of the following:

The main() function takes a string you enter on the command line, then converts it into a "%Y%m%d%H%M" datetime
Your datetime is rounded down to the last five minute increment
In testAlarm(), your date is compared with the current date, also in "%Y%m%d%H%M" format, also rounded down five minutes.
If the current date matches the cmd line argument, you should get an "ALARM!!! in the output.

Here's the actual output, run on my Ubuntu machine:

me@unbuntu1$ date
Tue Jan 17 14:27:41 UTC 2023
me@unbuntu1$ 
me@unbuntu1$ python3 toy04.py 202301171425
currDate:  202301171425
testDate:  202301171425
me@unbuntu1$

Okay: Although I'm rounding down my date to match the "rounded-down" version of the current date, the if(currDate == testDate): line of code is still evaluating to False. While both datetimes appear equal in the "%Y%m%d%H%M" format, they are somehow not equal.

My first thought was that maybe the "rounded down" datetime still retained some residual seconds or microseconds even after the rounding part? So I modified my function to this:

def roundDownDateTime(dt):
    # Arguments:
    #   dt      DateTime object
    delta = timedelta(minutes=1) * (dt.minute % 5)
    dt = dt - delta
    dt.replace(second=0, microsecond=0)
    return dt

But that makes no difference; I still get the exact same output as before.

Normally, you would only care if currDate > testDate for alarming purposes. But in my case, I must be able to compare datetimes for equality after one (or more) of them has been through the roundDownDateTime() function. What am I missing? Is my roundDownDateTime() function faulty? Thank you.

https://docs.python.org/3/library/datetime.html#datetime-objects - check if one of them isn't naive. According to docs, "For equality comparisons, naive instances are never equal to aware instances." — matszwecja, Jan 17 '23 at 14:59
You are printing the `strftime` but comparing the datetime object for `currDate`, which are not equal because of the decimals. — It_is_Chris, Jan 17 '23 at 15:02
@It_is_Chris That's a great point, probably the source of my confusion. What's the recommended way to print datetime's, I wonder? — Pete, Jan 17 '23 at 15:09

score 4 · Accepted Answer · answered Jan 17 '23 at 15:00

dt.replace returns a new datetime object; it does not modify dt in place.

def roundDownDateTime(dt):
    # Arguments:
    #   dt      DateTime object
    delta = timedelta(minutes=1) * (dt.minute % 5)
    dt = dt - delta
    return dt.replace(second=0, microsecond=0)

Why aren't identical datetimes equal?

1 Answers1