python timedelta behaviour on subtraction

问题

This question originated when I came upon (another thread) about python's datetime and timedelta instances

I followed the update by jimgardener and read the comments by eyquem ,and tried out some python code ..I am having a bad time understanding the way things work here(due to my newbie to python status)..I thought it was proper to ask a new question

import datetime
#for t1=23:30:00 PM
t1 = datetime.time(23,30,00)

#for t1=00:15:30 AM
t2 = datetime.time(0,15,30)

td1 = datetime.timedelta(hours=t1.hour,minutes = t1.minute,seconds=t1.second)

td2 = datetime.timedelta(hours=t2.hour,minutes = t2.minute,seconds=t2.second)

#substarcting timedeltas
tdiff = td2-td1

printing these variables yielded

td1 ==> datetime.timedelta(0, 84600)
td1.seconds ==> 84600

td2 ==> datetime.timedelta(0, 930)
td2.seconds ==> 930

tdiff ==> datetime.timedelta(-1, 2730)

When I looked at these results,I noticed that

td1.seconds (ie 84600) is equivalent to 
84600/60 ==> 1410 minutes
1410/60 ==> 23.5 hours
or in short,td1 represents the duration **from previous midnight** to 23:30 PM

now ,

td2.seconds (ie 930) is equivalent to
930/60 ==> 15.5 minutes or 15 minutes and 30 seconds
which means td2 represents the duration from **that midnight**
 to 00:15:30 AM

when tdiff is examined,I found that

tdiff ==> timedelta(-1,2730)
tdiff.seconds ==> 2730
tdiff.seconds/60 ==>45 minutes

this is the same as duration between t1(23:30:00 PM) and t2(00:15:30 AM) assuming that t2 follows t1

My question is,since td1 is the duration from previous midnight to 23:30:00 PM and td2 is the duration from that midnight to 00:15:30 AM , how can their difference represent the duration between t2 and t1 ?

Can some python gurus explain

回答1:

The timedeltas don't represent time from a midnight, just an amount of time.

If it took me three hours minus four hours to do something, it would take me negative one hour.

The difference between the two timedeltas is negative one day plus 45 minutes.

This is the same as negative 23 hours and 15 minutes.

15 minutes minus 23 hours and 30 minutes is negative 23 hours and 15 minutes.

来源：https://stackoverflow.com/questions/7091616/python-timedelta-behaviour-on-subtraction

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python

datetime

timedelta