[Tutor] problem with time module

[Tim Peters]

[Dick Moores]
> From the time module doc at
> <http://docs.python.org/lib/module-time.html>,
> I expected mktime(localtime()) and time() to be approx. 7*3600 seconds
> apart (I'm on the U.S. west coast, in -0700).

I don't know how you're reading the docs.  There are two ways of
representing time in that module:

1. Timestamps, which are floats.  These are always seconds from the epoch,
   as measured in UTC.  No exceptions.

2. struct_time thingies.  These are broken out into year, month, etc.  They
   don't contain time zone info explicitly, but correct interpretation requires
   anyway that you know which time zone was intended.  mktime() assumes
   its input is expressed in local time; there's nothing in the time module that
   converts a UTC struct_time to a timestamp, although calendar.timegm()
   does that.

localtime() creates a #2.  mktime(localtime()) produces a #1, so must
be UTC.  time() produces a #1, so must also be UTC.  Therefore I
expect the same result from both, although mktime()'s input doesn't
have a fraction-of-a-second member so I expect an exact integer (in
floating format) form mktime().

> But they're not:

Good <wink>!

> >>> from time import *
> >>> mktime(localtime());time()
> 1092510279.0
> 1092510279.2650001
>
> Have I misunderstood something?

Yes, but since you didn't lay out your reasoning, I can't guess which
step got off track.