incorrect timedelta yielded by two on-the-fly nows subtraction #52889

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1. mykhal commented on May 7, 2010

   

   mykhalmannequin

   on May 7, 2010

   MannequinAuthor

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   now() - now() from datetime.datetime produces not-nearly-zero timedelta:

   >>> import datetime
   >>> (datetime.datetime.now() - datetime.datetime.now()).seconds
   86399

   (i can't in the moment figure out why this is happening, sice the datetime library is written in C)
2. mykhal

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on May 7, 2010
3. mykhal

   changed the title [-]incorrect timedelta yielded by two on-the-fly nows[/-] [+]incorrect timedelta yielded by two on-the-fly nows subtraction[/+] on May 7, 2010
4. mdickinson commented on May 7, 2010

   

   mdickinson

   on May 7, 2010

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   This is correct behaviour. Note that .seconds is only one of the three attributes that contribute to the value of a timedelta: the others are .microseconds and .days. A timedelta is normalised in such a way that td.seconds and td.microseconds are always nonnegative; this means that td.days will be strictly negative for negative timedeltas. In your case the timedelta will have a .days attribute of -1, and the total time represented (e.g., in seconds, 86400.0 * td.days + td.seconds + 0.000001 * td.microseconds) will be small and negative.

   Here's what I get on my system (in the py3k branch):

   Python 3.2a0 (py3k:80840:80842, May  7 2010, 12:29:35) 
   [GCC 4.2.1 (Apple Inc. build 5659)] on darwin
   Type "help", "copyright", "credits" or "license" for more information.
   >>> import datetime
   >>> td = datetime.datetime.now() - datetime.datetime.now()
   >>> td.microseconds, td.seconds, td.days
   (998977, 86399, -1)
   >>> print(td)
   -1 day, 23:59:59.998977
   >>> 1e-6 * td.microseconds + td.seconds + 86400 * td.days
   -0.0010230000043520704
   >>> td.total_seconds() # new in Python 2.7, 3.2
   -0.0010230000043520704
5. mdickinson

   closed this as completedon May 7, 2010
6. mdickinson

   added

   invalid

   on May 7, 2010
7. mykhal commented on May 7, 2010

   

   mykhalmannequin

   on May 7, 2010

   MannequinAuthor

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   ok, my fault, i should have tried

   >>> (abs(datetime.datetime.now() - datetime.datetime.now())).seconds
   0

   sorry :)
8. mdickinson commented on May 7, 2010

   

   mdickinson

   on May 7, 2010

   Member

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   Hmm. My example did make me realize that the total_seconds() method isn't as accurate as it could be. I'll open another issue for this.
9. ezio-melotti

   transferred this issue fromon Apr 10, 2022