datetime-RFC2822 roundtripping

[doerwalter]

BPO	665194
Nosy	@tim-one, @warsaw, @akuchling, @doerwalter, @abalkin, @devdanzin, @merwok, @bitdancer
Dependencies	bpo-5094: datetime lacks concrete tzinfo implementation for UTC
Files	datetime-mimeformat.diff formatdate_datetime_support.patch util_datetime.patch issue665194.diff localtime.patch localtime.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/abalkin'
closed_at = <Date 2012-08-23.04:10:12.456>
created_at = <Date 2003-01-09.18:24:53.000>
labels = ['easy', 'type-feature', 'library', 'expert-email']
title = 'datetime-RFC2822 roundtripping'
updated_at = <Date 2012-08-23.04:10:12.455>
user = 'https://github.com/doerwalter'

bugs.python.org fields:

activity = <Date 2012-08-23.04:10:12.455>
actor = 'belopolsky'
assignee = 'belopolsky'
closed = True
closed_date = <Date 2012-08-23.04:10:12.456>
closer = 'belopolsky'
components = ['Library (Lib)', 'email']
creation = <Date 2003-01-09.18:24:53.000>
creator = 'doerwalter'
dependencies = ['5094']
files = ['8214', '21886', '22649', '26094', '26954', '26955']
hgrepos = []
issue_num = 665194
keywords = ['patch', 'easy']
message_count = 36.0
messages = ['53725', '53726', '53727', '53728', '53729', '53730', '53731', '53732', '53733', '81810', '107436', '135154', '135211', '135222', '135226', '135228', '140317', '140747', '140748', '161644', '163486', '163516', '163620', '168834', '168835', '168837', '168838', '168840', '168841', '168912', '168913', '168914', '168916', '168919', '168920', '168922']
nosy_count = 10.0
nosy_names = ['tim.peters', 'barry', 'akuchling', 'doerwalter', 'belopolsky', 'ajaksu2', 'eric.araujo', 'r.david.murray', 'Alexander.Belopolsky', 'python-dev']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue665194'
versions = ['Python 3.3']

[doerwalter]

It would be good to have a simply way to convert
between datetime objects and RFC2822 style strings.
From string to datetime is easy with
datetime.datetime(*email.Utils.parsedate(m)[:7]) (but
this drops the timezone), but the other direction seems
impossible. email.Utils.formatdate takes a timestamp
argument, but AFAICT there's no way to get a timestamp
out of a datetime object.

Of course the best solution (ignoring backwards
compatibility) would be for parsedate und formatdate to
return/accept datetime objects or for datetime to have
the appropriate methods.

[tim-one]

Logged In: YES
user_id=31435

You can get a timestamp like so:

>>> time.mktime(datetime.date(2002, 1, 1).timetuple())
1009861200.0
>>>

The dates for which this works depends on the platform
mktime implementation, though.

BTW, this sounds a lot more like a new feature request
than a bug!

[doerwalter]

Logged In: YES
user_id=89016

OK, I'll mark this a feature request.

datetime has fromordinal() and toordinal(), it has
fromtimestamp(), so I'd say a totimestamp() method would be
a logical addition.

[tim-one]

Logged In: YES
user_id=31435

Define what totimestamp() should do. The range of
timestamps supported by the *platform* C library (and so
indirectly by Python's time module) isn't defined by any
standard, and isn't easily discoverable either. It may or
may not work before 1970, may or may not after 2038.
datetime covers days far outside that range. Note that
even a double doesn't have enough bits of precision to
cover the full range of datetime values, either.

In contrast, ordinals are wholly under Python's control, so
we can promise surprise-free conversion in both directions.
All we can promise about timestamps is that if the platform
supports a timestamp for a time in datetime's range,
datetime can make sense of it.

[doerwalter]

Logged In: YES
user_id=89016

totimestamp() should be the inverse of fromtimestamp(), i.e.
foo.totimestamp() should be the same as
time.mktime(foo.timetuple()).
datetime.datetime.totimestamp() should raise OverflowError
iff time.mktime() raises OverflowError.

But as this may lose precision, I'd say it would be better,
if datetime supported RFC1123 conversion directly, i.e. two
methods frommime() and tomime(), that parse and format
strings like "Sun, 06 Nov 1994 08:49:37 GMT" (what
rfc822.parsedate() and rfc822.formatdate() do)

[brettcannon]

Logged In: YES
user_id=357491

time.strptime doesn't solve your problem?

[doerwalter]

Logged In: YES
user_id=89016

time.strptime() is locale aware, but RFC1123 & RFC822
require english day and month names, so time.strptime()
can't be used as-is. (And of course time.strptime() only
works for formatting, not for parsing)

[akuchling]

Moving to feature requests.

[doerwalter]

Here is a patch that implements to formatting part. It adds a method mimeformat() to the datetime class. The datetime object must have a tzinfo for this to work.
File Added: datetime-mimeformat.diff

[devdanzin]

Patch includes tests, should be updated.