On Jun 18, 5:25 am, "Gabriel Genellina" <gagsl-...@yahoo.com.ar>
wrote:
> En Thu, 17 Jun 2010 07:12:23 -0300, Fuzzyman <fuzzy...@gmail.com> escribi�:
>
> > On Jun 17, 10:29 am, "Gabriel Genellina" <gagsl-...@yahoo.com.ar>
> > wrote:
> >> En Thu, 17 Jun 2010 04:52:48 -0300, Alf P. Steinbach <al...@start.no>
> >> escribi�:
>
> >> > But who would have thunk that Python *isn't dynamic enough*? :-)
>
> >> Yep... There are other examples too (e.g. the print statement in 2.x
> >> bypasses sys.stdout.write;
>
> > What do you mean by this? The print statement in 2.x does *not* bypass
> > sys.stdout. It may use other methods besides write (writeln perhaps)
> > but you can *definitely* override sys.stdout to capture the output
> > from print statements.
>
> Suppose you want to implement a "tee" variant in Python: print output
> should go to stdout and also to some file (with timestamp added, just to
> be fancy). First attempt:
>
> py> import sys
> py> import time
> py>
> py> class tee(file):
> ... def write(self, data):
> ... file.write(self, '%s: %r\n' % (time.ctime(), data))
> ... sys.__stdout__.write(data)
> ...
> py> sys.stdout = tee('test.txt', 'w')
> py> print "Hello world"
> py> print "Bye"
> py> ^Z
>
> D:\TEMP>type test.txt
> Hello world
> Bye
>
> Note:
> - no output to stdout inside the interpreter
> - no timestamp in the file
>
> This modified version works fine:
>
> py> class tee():
> ... def __init__(self, filename, mode):
> ... self.file = open(filename, mode)
> ... def write(self, data):
> ... self.file.write('%s: %r\n' % (time.ctime(), data))
> ... sys.__stdout__.write(data)
>
> What happened? When sys.stdout is an instance of some class inheriting
> from file (that is, isinstance(sys.stdout, file) is true) then the print
> statement ignores sys.stdout.write() completely -- instead it calls
> directly some C stdio functions (fwrite).
> The only way to influence 'print' is *not* to inherit from file in the
> first place.
>
> It's an optimization, sure. I guess it is there before inheriting from
> builtin types was allowed (in such scenario, it's a perfectly valid
> optimization). Now, perhaps the test for 'file' should be more strict,
> only taking the C shortcut when using an actual file instance, not a
> subclass of it. This would allow the example above to work correctly.
>
Ah, so by "bypasses" you mean "under certain specific circumstances
bypasses". By all means file a bug report on this, I agree that
bypassing the optimization for file subclasses (assuming your
diagnosis is correct) would be a sensible approach.
All the best,
Michael
> --
> Gabriel Genellina
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