[Python-bugs-list] [ python-Bugs-468887 ] type(self) not preserved on some methods

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Bugs item #468887, was opened at 2001-10-07 18:20
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http://sourceforge.net/tracker/?func=detail&atid=105470&aid=468887&group_id=5470

Category: Type/class unification
>Group: Not a Bug
>Status: Closed
>Resolution: Invalid
Priority: 5
Submitted By: Mark J (average)
>Assigned to: Tim Peters (tim_one)
Summary: type(self) not preserved on some methods

Initial Comment:
Python-2.2a4

For user-defined types derived from built-in types, it
appears that built-in methods which return copies are
not preserving the type of self.  

>>> class Test(list): pass
>>> t=Test()
>>> map(type, [t, t[:], t+t])
[<class '__main__.Test'>, <type 'list'>, <type 'list'>]
>>> t += t
>>> type(t)
<class '__main__.Test'>

Same with types int, dictionary, etc.

Thanks,

Mark




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>Comment By: Tim Peters (tim_one)
Date: 2001-10-07 21:04

Message:
Logged In: YES 
user_id=31435

This is deliberate.  If, e.g., you don't override the 
slicing operator in Test, then t[:] is handled by the base 
class's slicing operator.  The base class can't know what 
invariants Test needs to preserve, so does the best it can 
by constructing a list.  If you want operators that return 
Test instances instead, you have to supply them.

Note that base-class operators *sometimes* returned 
instances of subclasses in earlier 2.2 alphas, but 
unpredictably (depending on internal optimizations).  This 
was properly reported as a bug, and was fixed for 2.2a4; 
see bug 460020 for details.

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