comparing datetime with date

[Donnal Walter]

I was very surprised to discover that

import datetime
x = datetime.date(2 004, 9, 14)
y = datetime.dateti me(2004, 9, 14, 6, 43, 15)
print x == y

True

How can these two objects be considered equal? Is there a *general* way
to test for date != datetime as well as 4.5 != 4.6?

Donnal Walter
Arkansas Children's Hospital

[Peter Hansen]

Donnal Walter wrote:

I was very surprised to discover that

>>> import datetime
>>> x = datetime.date(2 004, 9, 14)
>>> y = datetime.dateti me(2004, 9, 14, 6, 43, 15)
>>> print x == y

True

How can these two objects be considered equal?

Guessing:

In http://docs.python.org/lib/datetime-datetime.html
it says this """Note: In order to stop comparison from falling back to
the default scheme of comparing object addresses, datetime comparison
normally raises TypeError if the other comparand isn't also a datetime
object. However, NotImplemented is returned instead if the other
comparand has a timetuple attribute. This hook gives other kinds of date
objects a chance at implementing mixed-type comparison. """
(the docs for the date class say the same).

I suspect this means simply that datetime.date *does* use this
behaviour to provide a mixed-type comparison that makes sense,
to _it_.
Is there a *general* way
to test for date != datetime as well as 4.5 != 4.6?

Couldn't say, sorry.

-Peter

[Diez B. Roggisch]

Donnal Walter wrote:

I was very surprised to discover that

>>> import datetime
>>> x = datetime.date(2 004, 9, 14)
>>> y = datetime.dateti me(2004, 9, 14, 6, 43, 15)
>>> print x == y

True

I think thats perfectly legal - a date object should care only about if the
object it is compared to contains some sort of valid date information, and
restrict its comparision to that.

The matter is that you try to compare two different things here - so its up
to the implementation if how it deals with this, as there is no canonical
way to compare two things that aren't even structural equivalent.

The current implementation chose one way to do it - and thats as good as any
other one could think of, and certainly has some convenience advantage on
its side.
How can these two objects be considered equal? Is there a *general* way
to test for date != datetime as well as 4.5 != 4.6?

Try making a datetime-object from your date, with zeros for the time part -
then you'll get what you want.

--
Regards,

Diez B. Roggisch

[Peter Hansen]

Diez B. Roggisch wrote:

Donnal Walter wrote:

How can these two objects be considered equal? Is there a *general* way
to test for date != datetime as well as 4.5 != 4.6?

Try making a datetime-object from your date, with zeros for the time part -
then you'll get what you want.

While trying to provide my first answer, I thought of suggesting
this, but there doesn't appear to be a simple way of doing it.
It looked like the following was the best I could do (where 'x'
is a datetime.date object):

z = datetime.dateti me(x.year, x.month, x.day)

I just found timetuple(), which means this could be written
in the awful form:

z = datetime.dateti me(*x.timetuple ()[:3])

but that's hardly better, and definitely more inscrutable.

To answer Donnal's question above, however, it appears this
might be a reasonable approach:

x.timetuple() == y.timetuple()

False

Not sure there's a better approach readily available.

-Peter

[Tim Peters]

[Diez B. Roggisch]

Try making a datetime-object from your date, with zeros for the time part -
then you'll get what you want.
[Peter Hansen]

While trying to provide my first answer, I thought of suggesting
this, but there doesn't appear to be a simple way of doing it.

datetime.dateti me has a class constructor for building a datetime out
of a date and a time, so this can be done via a simple one-liner:

import datetime
t = datetime.date.t oday()
t datetime.date(2 004, 9, 14) datetime.dateti me.combine(t, datetime.time() ) datetime.dateti me(2004, 9, 14, 0, 0)

[Donnal Walter]

Peter Hansen wrote:

Diez B. Roggisch wrote:

Donnal Walter wrote:

How can these two objects be considered equal? Is there a *general* way
to test for date != datetime as well as 4.5 != 4.6?

Try making a datetime-object from your date, with zeros for the time
part -
then you'll get what you want.

While trying to provide my first answer, I thought of suggesting
this, but there doesn't appear to be a simple way of doing it.
It looked like the following was the best I could do (where 'x'
is a datetime.date object):

z = datetime.dateti me(x.year, x.month, x.day)

I just found timetuple(), which means this could be written
in the awful form:

z = datetime.dateti me(*x.timetuple ()[:3])

but that's hardly better, and definitely more inscrutable.

To answer Donnal's question above, however, it appears this
might be a reasonable approach:

>>> x.timetuple() == y.timetuple()

False

Not sure there's a better approach readily available.

-Peter

Thanks for the suggestions. I have decided *not* to store date/datetime
objects, but rather store the corresponding time tuples: 3-tuple for
date only, and 5- or 6-tuple for date and time, depending on whether or
not seconds are given). Then the following method notifies observers if
a simple date is changed to the date with time (just like number changes
and string changes).

def set(self, value):
if value != self._state: # now this works!
self._state = value
self.notify_obs ervers(self)

This means, of course, that if an observer wants to calculate a
TimeDelta, the corresponding datetimes have to be constructed, but I
don't think this is too inefficient.

time1 = datetime.dateti me(*time1.get() ) # 3-, 5-, or 6-tuple
time2 = datetime.dateti me(*time2.get() )
tdelta = time2 - time1

The observer object (and user) would need to understand the loss of
precision when times are not actually given, but at least the relevant
methods work regardless.

Thanks again,

Donnal Walter
Arkansas Children's Hospital

[Graeme Matthew]

This is very interesting because if you continue further and coerce both to
strings you get:

print str(y) == str(y) True str(y) '2004-09-14 06:43:15' str(x) '2004-09-14'
Even though there string values are different they still match?

If we perform a CRC (Cyclic Redundancy Check) i.e compute a one way hash:

import binascii crc_x = binascii.crc32( str(x))
crc_y = binascii.crc32( str(y))
print crc_x == crc_y

False

So this confirms that there values do differ yet a comparision using ==
returns True?

All i can assume is that when a Date / DateTime comparision is made the
'lowest denominator' i.e Date is used but I may be very wrong

Will watch this thread closely

"Donnal Walter" <do****@donnal. net> wrote in message
news:ma******** *************** *************** @python.org...

I was very surprised to discover that
import datetime
x = datetime.date(2 004, 9, 14)
y = datetime.dateti me(2004, 9, 14, 6, 43, 15)
print x == y

True

How can these two objects be considered equal? Is there a *general* way to
test for date != datetime as well as 4.5 != 4.6?

Donnal Walter
Arkansas Children's Hospital

[Sam Holden]

On Wed, 15 Sep 2004 09:24:03 +1000,
Graeme Matthew <gr************ @contrado.com.a u> wrote:

This is very interesting because if you continue further and coerce both to
strings you get:

print str(y) == str(y) True

Of course if you actually do:

print str(x) == str(y) False

str(y) '2004-09-14 06:43:15' str(x) '2004-09-14'

Even though there string values are different they still match?

No, they only matched becuase you compared y with y :)
--
Sam Holden

[Pierre Fortin]

On Wed, 15 Sep 2004 09:24:03 +1000 Graeme wrote:

This is very interesting because if you continue further and coerce both
to strings you get:

print str(y) == str(y) of course: ^ ^ :^)

True Even though there string values are different they still match? today = datetime.date(2 004,9,14)
yesterday = datetime.date(2 004,9,13)
now = datetime.dateti me(2004,9,14,20 ,0)
now == today True now == yesterday False
Looks like date and datetime have 3 arguments and minimum 3 args
respectively, so the comparison appears to simply use the minimum data
available... when given more data to compare, we get:
minute_ago = datetime.dateti me(2004,9,14,19 ,59)
minute_ago == now

False

All i can assume is that when a Date / DateTime comparision is made the
'lowest denominator' i.e Date is used but I may be very wrong
I agree...
Will watch this thread closely

[Graeme Matthew]

yes it was late, apologies therefore a simple way to solve this problem
would be:

str(x) == str(y)

:=-)

"Sam Holden" <sh*****@flexal .cs.usyd.edu.au > wrote in message
news:slrnckf3e5 .ptn.sh*****@fl exal.cs.usyd.ed u.au...

On Wed, 15 Sep 2004 09:24:03 +1000,
Graeme Matthew <gr************ @contrado.com.a u> wrote:

This is very interesting because if you continue further and coerce both
to
strings you get:

> print str(y) == str(y)

True

Of course if you actually do:

> print str(x) == str(y)

False

> str(y)

'2004-09-14 06:43:15'

> str(x)

'2004-09-14'

>

Even though there string values are different they still match?

No, they only matched becuase you compared y with y :)
--
Sam Holden