Is it possible to have instance variables in subclasses of builtins?

[Kenneth McDonald]

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)
(hmm, on reflection, I shouldn't have used 'a' as a
parameter to __init__--but that's merely bad
style, not the cause of the error :-) )

On the other hand, this example works:
class b(str): .... def __init__(self, x):
.... str.__init__(x)
.... x = b("h")
x 'h'

Is it possible to circumvent the above restrictions?

Thanks,
Ken

[Larry Bates]

The following might work for you:

from UserString import UserString
class b(UserString):
def __init__(self, x, y, z):
self.y=y
self.z=z
self.x=x
UserString.__init__(self, x)
return

x=b('h', 1, 2)
x.z 2 x.y 1 x.x 'h' len(x)

1

Uses old UserString class.

HTH,
Larry Bates
Syscon, Inc.
"Kenneth McDonald" <ke****************@sbcglobal.net> wrote in message
news:sl*******************************@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)
(hmm, on reflection, I shouldn't have used 'a' as a
parameter to __init__--but that's merely bad
style, not the cause of the error :-) )

On the other hand, this example works:
class b(str): ... def __init__(self, x):
... str.__init__(x)
... x = b("h")
x 'h'

Is it possible to circumvent the above restrictions?

Thanks,
Ken

[Russell Blau]

"Kenneth McDonald" <ke****************@sbcglobal.net> wrote in message
news:sl*******************************@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.
--
I don't actually read my hotmail account, but you can replace hotmail with
excite if you really want to reach me.