CODEWITHSUNDEEP
pythonpython-2.7parsingpython-dateutil
Whitecat
Whitecat

Reputation: 4020

Dateutil parse bug in python returns the wrong value

I have looked at many possible ways to parse python times. Using parse seems link the only method that should work. While trying to use datetime.strptime causes an error because %z does not work with python 2.7. But using parse.parse incorrectly recognizes the time zone.

I parse both Fri Nov 9 09:04:02 2012 -0500 and Fri Nov 9 09:04:02 2012 -0800 and get the exact same timestamp in unix time. 1352480642

Here is my code that runs the test.

#!/usr/bin/python
import time
from dateutil import parser

def get_timestamp(time_string):
    timing = parser.parse(time_string)
    return time.mktime(timing.timetuple())

test_time1 = "Fri Nov 9 09:04:02 2012 -0500"
test_time2 = "Fri Nov 9 09:04:02 2012 -0800"
print get_timestamp(test_time1)
print get_timestamp(test_time2)

Output

1352480642.0
1352480642.0

Expected output

1352469842.0
1352480642.0

Upvotes: 1

Views: 715

Answers (1)

Paul
Paul

Reputation: 10863

This has nothing to do with the parser, you'll see the same behavior just from mktime() alone, since datetime.timetuple() doesn't have any time zone offset information, and mktime() is the inverse of localtime. You can correct this by converting it to localtime before calling timetuple():

from time import mktime
from datetime import datetime
from dateutil import tz

dt_base = datetime(2012, 11, 9, 9, 4, 2)

dt_est = dt_base.replace(tzinfo=tz.tzoffset('EST', -5 * 3600))
dt_pst = dt_base.replace(tzinfo=tz.tzoffset('PST', -8 * 3600))

def print_mktime(dt):
    print(mktime(dt.timetuple()))

# Run in UTC
print_mktime(dt_est)   # 1352469842.0
print_mktime(dt_pst)   # 1352469842.0

# Convert to local time zone first first
print_mktime(dt_est.astimezone(tz.tzlocal())) # 1352469842.0
print_mktime(dt_pst.astimezone(tz.tzlocal())) # 1352480642.0

Note that there is a chart on the documentation for time() (python 2.x docs) that tells you how to convert between these representations:

From                        To                           Use
---------------------------------------------------------------------------
seconds since the epoch   | struct_time in UTC        |  gmtime()
seconds since the epoch   | struct_time in local time |  localtime()
struct_time in UTC        | seconds since the epoch   |  calendar.timegm()
struct_time in local time | seconds since the epoch   |  mktime()

My personal preference would be to convert the parsed date to UTC, in which case calendar.timegm() would be the appropriate function:

from calendar import timegm
def print_timegm(dt):
    print(timegm(dt.timetuple()))

print_timegm(dt_est.astimezone(tz.tzutc())) # 1352469842.0
print_timegm(dt_pst.astimezone(tz.tzutc())) # 1352480642.0

Upvotes: 1

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