Problem in a str subclass
mwh at python.net
Tue Nov 27 12:25:42 CET 2001
Manus Hand <manus at bullfrog-tech.com> writes:
> Can anyone explain why I get the following result?
> Python 2.2b2 (#1, Nov 26 2001, 20:03:34)
> [GCC egcs-2.91.66 19990314/Linux (egcs-1.1.2 release)] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
> >>> class X(str):
> ... def __init__(self, xx = None):
> ... if not xx: xx = 'ye olde default value'
> ... str.__init__(self, xx)
> >>> x = X('yo yo yo')
> >>> x
> 'yo yo yo'
> >>> x = X()
> >>> x
> I can't see why X() is not set to 'ye olde default value'
You need to do your initialisation inside the *class method* __new__;
by the time __init__ is called, the string has been constructed, and
as it's immutable...
You want to do this instead:
>>> class X(str):
... def __new__(cls, xx="default"):
... return str.__new__(cls, xx)
This is getting to be a FAQ, and it's a confusing point...
7. It is easier to write an incorrect program than understand a
-- Alan Perlis, http://www.cs.yale.edu/homes/perlis-alan/quotes.html
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