Question

I am using python’s feedparser module, to parse an RSS feed. Once parsed, feedparser returns dates in a python 9 tuple time format (time.struct_time).

I am want to store these values in my mysql database so I can later check the Last-Modified headers of the feed. It’s import that if the times tuples are converted, that when converted back they stay the same, so I can later use them for comparison.

I tried this to convert the time tuple to datetime and then back, but it wasn’t the same when converted back:

dt = datetime.fromtimestamp(time.mktime(struct))
time_tuple = dt.timetuple()

What do you think is the method to do this?

Answer 1

I believe the reason why your method is not preserving the time tuple is because the is_dst value was changed. time.mktime respected the is_dst in struct, but dt.timetuple changes is_dst to -1.

One way to avoid this error would be to interprets the time tuple as representing a UTC time. That may not be strictly correct, but it may be good enough for your purposes.

In [1]: import datetime as dt
In [2]: import time
In [3]: import calendar

In [17]: time_tuple=(1970, 1, 1, 0, 0, 0, 3, 1, 1)

In [18]: timestamp=calendar.timegm(time_tuple)
In [19]: timestamp
Out[19]: 0

In [20]: date=dt.datetime.utcfromtimestamp(timestamp)
In [21]: date
Out[21]: datetime.datetime(1970, 1, 1, 0, 0)

In [22]: tuple(date.timetuple())
Out[22]: (1970, 1, 1, 0, 0, 0, 3, 1, -1)

PS. Here is an example showing how the method you posted might fail to preserve the time tuple. Suppose the remote server is in a locale where is_dst = 1, while is_dst = 0 in your locale:

In [11]: time_tuple=(1970, 1, 1, 0, 0, 0, 3, 1, 1)

In [12]: timestamp=time.mktime(time_tuple)
In [13]: timestamp
Out[13]: 14400.0

In [14]: date=dt.datetime.fromtimestamp(timestamp)
In [15]: date
Out[15]: datetime.datetime(1969, 12, 31, 23, 0)

In [16]: tuple(date.timetuple())
Out[16]: (1969, 12, 31, 23, 0, 0, 2, 365, -1)