python 3, subclassing TextIOWrapper.

R. David Murray rdmurray at bitdance.com
Sun Mar 22 21:15:26 CET 2009


"Gabriel Genellina" <gagsl-py2 at yahoo.com.ar> wrote:
> En Sun, 22 Mar 2009 15:11:37 -0300, R. David Murray  
> <rdmurray at bitdance.com> escribió:
> > "Gabriel Genellina" <gagsl-py2 at yahoo.com.ar> wrote:
> >> En Sat, 21 Mar 2009 23:58:07 -0300, <lambertdw at corning.com> escribió:
> >> >
> >> > class file(io.TextIOWrapper):
> >> >
> >> >     '''
> >> >         Enhance TextIO.  Streams have many sources,
> >> >         a file name is insufficient.
> >> >     '''
> >> >
> >> >     def __init__(self,stream):
> >> >         #self.stream = stream
> >> >         super().__init__(stream.buffer)
> >> >
> >> >
> >> > print(file(open('p.py')).read())
> >>
> >>
> >> [...] So, if you're not interested in the TextIOWrapper object, don't  
> >> create it in the first place. That means, don't use the open() shortcut  
> >> and build
> >> the required pieces yourself.
> >>
> > I'm wondering if what we really need here is either some way to tell open
> > to use a specified subclass(s) instead of the default ones, or perhaps
> > an 'open factory' function that would yield such an open function that
> > otherwise is identical to the default open.
> >
> > What's the standard python idiom for when consumer code should be
> > able to specialize the classes used to create objects returned from
> > a called package?  (I'm tempted to say monkey patching the module,
> > but that can't be optimal :)
> 
> I've seen:
> - pass the desired subclass as an argument to the class constructor /  
> factory function.
> - set the desired subclass as an instance attribute of the factory object.
> - replacing the f_globals attribute of the factory function (I wouldn't  
> recomend this! but sometimes is the only way)
> 
> In the case of builtin open(), I'm not convinced it would be a good idea  
> to allow subclassing. But I have no rational arguments - just don't like  
> the idea :(

When 'file' was just a wrapper around C I/O, that probably made as much
sense as anything.  But now that IO is more Pythonic, it would be nice
to have Pythonic methods for using a subclass of the default classes
instead of the default classes.  Why should a user have to reimplement
'open' just in order to use their own TextIOWrapper subclass?

I should shift this thread to Python-ideas, except I'm not sure I'm
ready to take ownership of it (yet?). :)

--
R. David Murray           http://www.bitdance.com




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