Can't subclass datetime.datetime?

[Grant Edwards]

Is it true that a datetime object can convert itself into a
string, but not the other way around? IOW, there's no simple
way to take the output from str(d) and turn it back into d?

So, I tried to create a class that knows how to do that, but I
don't seem to be able to subclass datetime.dateti me:

import datetime

class MyDatetime(date time.datetime):
def __init__(self,s ):
s1,s2 = s.split(' ')
v = s1.split('-') + s2.split(':')
v = map(int,v)
datetime.dateti me.__init__(sel f,v[0],v[1],v[2],v[3],v[4],v[5])

s = '2005-02-14 12:34:56'
d = MyDatetime(s)

Running the above yields:

Traceback (most recent call last):
File "dt.py", line 11, in ?
d = MyDatetime(s)
TypeError: function takes at least 3 arguments (1 given)

What's going on?

--
Grant Edwards grante Yow! I'm in a twist
at contest!! I'm in a
visi.com bathtub! It's on Mars!! I'm
in tip-top condition!

[Steven Bethard]

Re: Can't subclass datetime.dateti me?

Grant Edwards wrote:[color=blue]
> Is it true that a datetime object can convert itself into a
> string, but not the other way around? IOW, there's no simple
> way to take the output from str(d) and turn it back into d?[/color]

I assume this is true because there is not one standard format for a
date-time string. But I don't use the module enough, so I'll let
someone else answer this part of the question.
[color=blue]
> import datetime
>
> class MyDatetime(date time.datetime):
> def __init__(self,s ):
> s1,s2 = s.split(' ')
> v = s1.split('-') + s2.split(':')
> v = map(int,v)
> datetime.dateti me.__init__(sel f,v[0],v[1],v[2],v[3],v[4],v[5])
>
> s = '2005-02-14 12:34:56'
> d = MyDatetime(s)
>
> Running the above yields:
>
> Traceback (most recent call last):
> File "dt.py", line 11, in ?
> d = MyDatetime(s)
> TypeError: function takes at least 3 arguments (1 given)[/color]

datetime.dateti me objects are immutable, so you need to define __new__
instead of __init__:

py> class DateTime(dateti me.datetime):
.... def __new__(cls, s):
.... s1, s2 = s.split(' ')
.... v = map(int, s1.split('-') + s2.split(':'))
.... return datetime.dateti me.__new__(cls, *v)
....
py> DateTime('2005-02-14 12:34:56')
DateTime(2005, 2, 14, 12, 34, 56)

Steve

[Grant Edwards]

Re: Can't subclass datetime.dateti me?

On 2005-02-14, Steven Bethard <steven.bethard @gmail.com> wrote:[color=blue]
> Grant Edwards wrote:[color=green]
>> Is it true that a datetime object can convert itself into a
>> string, but not the other way around? IOW, there's no simple
>> way to take the output from str(d) and turn it back into d?[/color]
>
> I assume this is true because there is not one standard format
> for a date-time string.[/color]

There seems to be a de-facto standard format: that which is
returned by the str(d).
[color=blue][color=green]
>> class MyDatetime(date time.datetime):
>> def __init__(self,s ):[/color][/color]
[...]
[color=blue]
> datetime.dateti me objects are immutable, so you need to define
> __new__ instead of __init__:[/color]

Thanks. I should have known that. I guess I've never subclassed
an immutible type before.

--
Grant Edwards grante Yow! It's hard being
at an ARTIST!!
visi.com

[Kent Johnson]

Re: Can't subclass datetime.dateti me?

Grant Edwards wrote:[color=blue]
> Is it true that a datetime object can convert itself into a
> string, but not the other way around? IOW, there's no simple
> way to take the output from str(d) and turn it back into d?[/color]

According to this thread, a patch has been checked in that adds strptime() to datetime. So there is
something to look forward to...
http://tinyurl.com/4fbkb

Kent