Accessing 'mangled' class attrbutes

[Gerard Flanagan]

Hello all

I would like to do the following:

from elementtree.Sim pleXMLWriter import XMLWriter

class HtmlWriter(XMLW riter, object):
def write_raw(self, text):
super( HtmlWriter, self ).flush()
super( HtmlWriter, self ).__write(text)

but because of the name-mangling caused by '__write' I get:

AttributeError: 'super' object has no attribute '_HtmlWriter__w rite'.

Is there any simple way round this situation in general?

(I just want to write out a HTML 'DOCTYPE' declaration)

Thanks

Gerard

[Steve Juranich]

Re: Accessing 'mangled' class attrbutes

Gerard Flanagan wrote:
[color=blue]
> I would like to do the following:
>
> from elementtree.Sim pleXMLWriter import XMLWriter
>
> class HtmlWriter(XMLW riter, object):
> def write_raw(self, text):
> super( HtmlWriter, self ).flush()
> super( HtmlWriter, self ).__write(text)
>
> but because of the name-mangling caused by '__write' I get:
>
> AttributeError: 'super' object has no attribute '_HtmlWriter__w rite'.
>
> Is there any simple way round this situation in general?
>
> (I just want to write out a HTML 'DOCTYPE' declaration)[/color]

Try: (not the Python keyword, but a directive to you)

super(HtmlWrite r, self)._XMLWrite r__write(text)

In general, to access the name-mangled members, simply add
_<class_name> to the front of the member name and you should be able to get
at it. But be careful, since this is a reference to the base class, so if
it's inherited from some other class, you'll need to know from which class
the member is inherited.

HTH

--
Steve Juranich
Tucson, AZ
USA

[danmcleran@yahoo.com]

Re: Accessing 'mangled' class attrbutes

>>Is there any simple way round this situation in general?

It might be safer to use composition instead of inheritance in this
case. Assuming that XMLWriter has a write method to write what you
want, you could hold a reference to an XMLWriter within your class and
pass along write command like:

writer = XMLWriter()
writer.write(st uff)

[Gerard Flanagan]

Re: Accessing 'mangled' class attrbutes


Steve Juranich wrote:[color=blue]
> Gerard Flanagan wrote:
>[color=green]
> > I would like to do the following:
> >
> > from elementtree.Sim pleXMLWriter import XMLWriter
> >
> > class HtmlWriter(XMLW riter, object):
> > def write_raw(self, text):
> > super( HtmlWriter, self ).flush()
> > super( HtmlWriter, self ).__write(text)
> >
> > but because of the name-mangling caused by '__write' I get:
> >
> > AttributeError: 'super' object has no attribute '_HtmlWriter__w rite'.
> >
> > Is there any simple way round this situation in general?
> >
> > (I just want to write out a HTML 'DOCTYPE' declaration)[/color]
>
> Try: (not the Python keyword, but a directive to you)
>
> super(HtmlWrite r, self)._XMLWrite r__write(text)
>
> In general, to access the name-mangled members, simply add
> _<class_name> to the front of the member name and you should be able to get
> at it. But be careful, since this is a reference to the base class, so if
> it's inherited from some other class, you'll need to know from which class
> the member is inherited.
>
> HTH
>
> --
> Steve Juranich
> Tucson, AZ
> USA[/color]

I tried that Steve but it didn't work, and i don't think I can do what
I want in any case. There is no method '__write' in the base class, it
is only declared as an instance attribute in the constructor, like so:

def __init__(self, file, encoding="us-ascii"):
...
self.__write = file.write
...

I tried putting '__write = None' at the class level (in the base class
XMLWriter) but then, although '_XMLWriter__wr ite' appears in
'dir(HtmlWriter )', I get 'NoneType is not callable'.

I also tried 'def __write(self, text) : pass ' in the base class, but
then the code runs but doesn't write the text I want - and anyway, if
I'm going to change the base class, then i may as well just add the
'write_raw' method to the base directly!

It's just some toy code at any rate, and I've learnt something new!
Thanks for your reply.

Gerard

[Gerard Flanagan]

Re: Accessing 'mangled' class attrbutes

danmcleran@yaho o.com wrote:[color=blue][color=green][color=darkred]
> >>Is there any simple way round this situation in general?[/color][/color]
>
> It might be safer to use composition instead of inheritance in this
> case. Assuming that XMLWriter has a write method to write what you
> want, you could hold a reference to an XMLWriter within your class and
> pass along write command like:
>
> writer = XMLWriter()
> writer.write(st uff)[/color]

No, XMLWriter doesn't have a 'write' method, if it did I could have
done:

super(HtmlWrite r, self).write(stu ff)

I think the XMLWriter class has been designed so you can't just write
any old text because this better ensures that tags are properly closed
and so on. There is a public 'data' method:

writer.data( text )

but it escapes angle brackets, and what i wanted was to write
'<!DOCTYPE..... '.

Thanks

Gerard

[Steve Juranich]

Re: Accessing 'mangled' class attrbutes

Gerard Flanagan wrote:
[color=blue]
> I tried that Steve but it didn't work, and i don't think I can do what
> I want in any case. There is no method '__write' in the base class, it
> is only declared as an instance attribute in the constructor, like so:
>
> def __init__(self, file, encoding="us-ascii"):
> ...
> self.__write = file.write
> ...
>
> I tried putting '__write = None' at the class level (in the base class
> XMLWriter) but then, although '_XMLWriter__wr ite' appears in
> 'dir(HtmlWriter )', I get 'NoneType is not callable'.
>
> I also tried 'def __write(self, text) : pass ' in the base class, but
> then the code runs but doesn't write the text I want - and anyway, if
> I'm going to change the base class, then i may as well just add the
> 'write_raw' method to the base directly!
>
> It's just some toy code at any rate, and I've learnt something new!
> Thanks for your reply.
>
> Gerard
>[/color]

Make sure you're calling the super's constructor before you try and access
the mangled member. Then (I forgot this part), you can just call the
mangled member from `self'. Example follows.

<foo.py>
class A(object):
def __init__(self):
self.__foo = lambda x, y : x + y

class B(A):
def __init__(self, x, y):
# Make sure you're calling the super's constructor first.
super(B, self).__init__( )
self.sum = self._A__foo(x, y)
</foo.py>
[color=blue][color=green][color=darkred]
>>> import foo
>>> b = foo.B(3, 4)
>>> b.sum[/color][/color][/color]
7[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

--
Steve Juranich
Tucson, AZ
USA

[Gerard Flanagan]

Re: Accessing 'mangled' class attrbutes

Steve Juranich wrote:[color=blue]
> Gerard Flanagan wrote:
>[color=green]
> > I tried that Steve but it didn't work, and i don't think I can do what
> > I want in any case. There is no method '__write' in the base class, it
> > is only declared as an instance attribute in the constructor, like so:
> >
> > def __init__(self, file, encoding="us-ascii"):
> > ...
> > self.__write = file.write
> > ...
> >
> > I tried putting '__write = None' at the class level (in the base class
> > XMLWriter) but then, although '_XMLWriter__wr ite' appears in
> > 'dir(HtmlWriter )', I get 'NoneType is not callable'.
> >
> > I also tried 'def __write(self, text) : pass ' in the base class, but
> > then the code runs but doesn't write the text I want - and anyway, if
> > I'm going to change the base class, then i may as well just add the
> > 'write_raw' method to the base directly!
> >
> > It's just some toy code at any rate, and I've learnt something new!
> > Thanks for your reply.
> >
> > Gerard
> >[/color]
>
> Make sure you're calling the super's constructor before you try and access
> the mangled member. Then (I forgot this part), you can just call the
> mangled member from `self'. Example follows.
>
> <foo.py>
> class A(object):
> def __init__(self):
> self.__foo = lambda x, y : x + y
>
> class B(A):
> def __init__(self, x, y):
> # Make sure you're calling the super's constructor first.
> super(B, self).__init__( )
> self.sum = self._A__foo(x, y)
> </foo.py>
>[color=green][color=darkred]
> >>> import foo
> >>> b = foo.B(3, 4)
> >>> b.sum[/color][/color]
> 7[color=green][color=darkred]
> >>>[/color][/color]
>
> --
> Steve Juranich
> Tucson, AZ
> USA[/color]


It's all becoming clear! Yes, calling the base constructor was all I
needed to do:

class HtmlWriter(elem enttree.SimpleX MLWriter.XMLWri ter, object):

def __init__(self, file):
super( HtmlWriter, self).__init__( file)

def write_raw(self, text):
self.flush()
self._XMLWriter __write(text)

-works a charm. Appreciate your help Steve, thanks again.

Gerard