Returning same type as self for arithmetic in subclasses

[Max M]

# -*- coding: latin-1 -*-
"""
I subclass datetime and timedelta

dt = myDatetime(1970 ,1,1)
type(dt) <class 'dtime.myDateti me'>
td = myTimedelta(hou rs=1)
type(td) <class 'dtime.myTimede lta'>

But when I do arithmetic with these classes, they return datetime and
timedelta,
where I want them to return myDatetime and myTimedelta
new_time = dt + td
new_time datetime.dateti me(1970, 1, 1, 1, 0)
type(new_time)

<type 'datetime.datet ime'>

So I wondered if there was a simlpler way to coerce the result into my
desired
types rather than overwriting the __add__, __sub__ etc. methods?

"""

from datetime import datetime, timedelta

class myDatetime(date time):
pass

class myTimedelta(tim edelta):
pass
if __name__ == "__main__":

import os.path, doctest, dtime
# import and test this file
doctest.testmod (dtime)


--

hilsen/regards Max M, Denmark

http://www.mxm.dk/
IT's Mad Science

[Tim Peters]

[Max M]

"""
I subclass datetime and timedelta

dt = myDatetime(1970 ,1,1)
type(dt) <class 'dtime.myDateti me'>
td = myTimedelta(hou rs=1)
type(td) <class 'dtime.myTimede lta'>

But when I do arithmetic with these classes, they return datetime and
timedelta, ....

new_time = dt + td
new_time datetime.dateti me(1970, 1, 1, 1, 0)
type(new_time)

<type 'datetime.datet ime'>

Yes, and all builtin Python types work that way. For example,
int.__add__ or float.__add__ applied to a subclass of int or float
will return an int or float; similarly for a subclass of str. This
was Guido's decision, based on that an implementation of any method in
a base class has no idea what requirements may exist for invoking a
subclass's constructor. For example, a subclass may restrict the
values of constructor arguments, or require more arguments than a base
class constructor; it may permute the order of positional arguments in
the base class constructor; it may even be "a feature" that a subclass
constructor gives a different meaning to an argument it shares with
the base class constructor. Since there isn't a way to guess, Python
does a safe thing instead.
where I want them to return myDatetime and myTimedelta

So I wondered if there was a simlpler way to coerce the result into my
desired types rather than overwriting the __add__, __sub__ etc. methods?

Generally speaking, no. But I'm sure someone will torture you with a
framework that purports to make it easy <wink>.

[Max M]

Tim Peters wrote:

Yes, and all builtin Python types work that way. For example,
int.__add__ or float.__add__ applied to a subclass of int or float
will return an int or float; similarly for a subclass of str. This
was Guido's decision...
I will not discuss it with him. He is usually right :-s

Generally speaking, no. But I'm sure someone will torture you with a
framework that purports to make it easy <wink>.

Apparently not... But here is my solution.

If anybody is interrested. It should also be obvious what I am working on.

Btw. I really love doctests ... Unittests are a nice idea. But doctest
is a really practical solution.

############### ############### #

class vDatetime(datet ime):
"""
A subclass of datetime, that renders itself in the iCalendar datetime
format.

dt = vDatetime(1970, 1,1, 12, 30, 0)
str(dt) '19700101T12300 0'
dt2 = vDatetime(1970, 1,1, 0, 0, 0)
str(dt - dt2) 'PT12H30M'

Adding is not allowed dt + dt2

Traceback (most recent call last):
...
AttributeError: 'NotImplemented Type' object has no attribute 'days'
"""

def __init__(self, *args, **kwargs):
datetime.__init __(self, *args, **kwargs)
self.params = Params()

def __add__(self, other):
return self._to_vdatet ime(datetime.__ add__(self, other))

def __sub__(self, other):
return self._to_vdatet ime(datetime.__ sub__(self, other))

def _to_vdatetime(s elf, result):
if hasattr(result, 'timetuple'):
return vDatetime(*resu lt.timetuple()[:6])
return vDuration(resul t.days, result.seconds)

def fromstring(st):
"Class method that parses"
try:
timetuple = map(int, ((
st[:4], # year
st[4:6], # month
st[6:8], # day
st[9:11], # hour
st[11:13], # minute
st[13:15], # second
)))
except:
raise ValueError, 'Wrong format'
return vDatetime(*time tuple)
fromstring = staticmethod(fr omstring)

def __str__(self):
return self.strftime(" %Y%m%dT%H%M%S")

--

hilsen/regards Max M, Denmark

http://www.mxm.dk/
IT's Mad Science

[Steve Holden]

Tim Peters wrote:

[Max M]

"""
I subclass datetime and timedelta

[...]
Generally speaking, no. But I'm sure someone will torture you with a
framework that purports to make it easy <wink>.

Clearly the easy way is to have the type declaration introspect on the
definitions of datetime and timedelta and then auto-create methods
wrapping the base types' methods in a "myxxx" conversion.

left-as-an-exercise-for-the-reader-ly y'rs - steve
--
Steve Holden http://www.holdenweb.com/
Python Web Programming http://pydish.holdenweb.com/
Holden Web LLC [phone] [phone]