Is it possible to have instance variables in subclasses of builtins?
Russell Blau
russblau at hotmail.com
Wed Jun 16 09:02:18 EDT 2004
"Kenneth McDonald" <kenneth.m.mcdonald at sbcglobal.net> wrote in message
news:slrnccutjp.3qk.kenneth.m.mcdonald at g4.gateway.2wire.net...
> I've recently used subclasses of the builtin str class to good effect.
> However, I've been unable to do the following:
>
> 1) Call the constructor with a number of arguments other than
> the number of arguments taken by the 'str' constructor.
>
> 2) Create and use instance variables (eg. 'self.x=1') in
> the same way that I can in a 'normal' class.
>
> As an example of what I mean, here's a short little session:
>
> >>> class a(str):
> ... def __init__(self, a, b):
> ... str.__init__(a)
> ... self.b = b
> ...
> >>> x = a("h", "g")
> Traceback (most recent call last):
> File "<stdin>", line 1, in ?
> TypeError: str() takes at most 1 argument (2 given)
> >>>
The problem is that "str" is an immutable type. Therefore, to change the
constructor, you need to override the __new__ method. See
http://www.python.org/2.2.1/descrintro.html#__new__
Here's an example:
>>> class two(str):
def __new__(cls, a, b):
return str.__new__(cls, a)
def __init__(self, a, b):
self.b = b
>>> z = two("g", "h")
>>> z
'g'
>>> z.b
'h'
Note that both arguments (a and b) get passed to both the __new__ and
__init__ methods, and each one just ignores the one it doesn't need.
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