[ python-Bugs-1448640 ] datetime.__init__ cannot be overridden

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Bugs item #1448640, was opened at 2006-03-13 04:54
Message generated for change (Comment added) made by gbrandl
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Category: Python Library
Group: None
Status: Closed
Resolution: Invalid
Priority: 5
Submitted By: Martin Blais (blais)
Assigned to: Michael Hudson (mwh)
Summary: datetime.__init__ cannot be overridden

Initial Comment:
Hi

The following code does not work properly:

#!/usr/bin/env python
"""
Test overriding constructor of datetime classes.
"""

import sys, datetime

class MyDate(datetime.date):

    def __init__( self, year, month, day ):
        print >> sys.stderr, 'lose data'

d = MyDate(2006, 11, 29)
print d

class MyDate(datetime.date):

    def __new__( self, year, month, day ):
        print 'lose data'

    def __init__( self, year, month, day ):
        print 'lose data again'

d = MyDate(2006, 11, 29)
print d



The output is:

lose data
2006-11-29
lose data
None



The problem is that the initialization of the object is
done in its time_new() method which is registered for
__new__ rather than using an __init__ method.  This
prevent one from overriding the date/datetime/time
constructors.

cheers,


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>Comment By: Georg Brandl (gbrandl)
Date: 2006-03-19 12:25

Message:
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blais: your mistake was that you didn't call
datetime.date.__new__() in your overridden __new__() and did
return None from __new__ whereas __new__ must return a new
object of type MyDate.

----------------------------------------------------------------------

Comment By: Martin Blais (blais)
Date: 2006-03-13 21:45

Message:
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Hmmmm... that's not quite true.  If I derive from datetime
in order to add new data members (e.g. in my case I add
"seq" which is used as a sequence number on top of datetime
for storing the names of photographs, the sequence number is
in case I have a panorama or multiple photographs in burst
mode--all within one second), I might want a different
constructor.

I tried overriding __new__ and had some troubles, cannot
remember what exactly, will reproduce and send code soon.


----------------------------------------------------------------------

Comment By: Michael Hudson (mwh)
Date: 2006-03-13 16:42

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datetime.date objects are immutable, so this is the same as not being able to 
override __init__ in a subclass of int.  Override __new__ instead.

----------------------------------------------------------------------

Comment By: splitscreen (splitscreen)
Date: 2006-03-13 15:40

Message:
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Isn't this an abuse of __new__ ?

Taken from the documentation:

"__new__ must return an object. There's nothing that
requires that it return a new object that is an instance of
its class argument, although that is the convention. If you
return an existing object, the constructor call will still
call its __init__ method. If you return an object of a
different class, its __init__ method will be called. If you
forget to return something, Python will unhelpfully return
None, and your caller will probably be very confused."

So, you're actually calling __init__ with None?

Or have i misunderstood?

----------------------------------------------------------------------

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