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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

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" <> wrote in message
news:. 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

"Kenneth McDonald" <> wrote in message
news:. 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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