Buffering control in python?
Bengt Richter
bokr at oz.net
Sat Oct 12 22:20:41 EDT 2002
On Sun, 13 Oct 2002 01:30:06 +0200, holger krekel <pyth at devel.trillke.net> wrote:
>Michal Wallace wrote:
>> On Sat, 12 Oct 2002, Fernando P=E9rez wrote:
>>=20
>> > In Perl each stream can be set to unbuffered via a simple=20
>> >=20
>> > STDOUT->autoflush(1);
>
>you can start with 'python -u ...' and get unbuffered IO.=20
>
>> Well, I don't know if there's a built in way to do this or
>> not, but python is an object oriented language, so you can
>> either subclass the file class, or, for a more general solution,
>> you can use the Proxy design pattern:
>>=20
>> class Proxy(object):
>> def __init__(self, subject):
>> self.__dict__["subject"] =3D subject
>> def __getattr__(self, attr):
>> return getattr(self.__dict__["subject"], attr)
>> def __setattr__(self, attr, value):
>> setattr(self.__dict__["subject"], attr, value)
>> def _subj(self):
>> return self.__dict__["subject"]
>>=20
>>=20
>> class AutoFlush(Proxy):
>> def write(self, data):
>> self._subj().write(data)
>> self._subj().flush()
>> print "LOOK MA! I'M FLUSHING! :)"
>>=20
>>=20
>> file =3D AutoFlush(open("file.txt", "w"))
>> print >> file, "go for it!"
>
>the 'print "LOOK ..."' statement probably calls 'write' twice.=20
>
Yup. Checking with my little watcher makes that easy to verify:
>>> from ut.tracewatch import TraceWatch as TW
>>> tw = TW()
>>> class Proxy(object):
[... as above]
>>> class AutoFlush(Proxy):
[... as above]
>>> tw.addwatch('write','#f')
>>> tw.on()
>>> file = AutoFlush(open("file.txt", "w"))
>>> print >> file, "go for it!"
--------------------------------------------------------------------
File: "<stdin>"
Line [scope] C:all, R:eturn eX:cept S:tack N:ew M:od E:quiv U:nbound
---- ------- -------------------------------------------------------
2 [write]: C: write(self=<__main__.AutoFlush object at 0x007D6310>, data='go for it!')
LOOK MA! I'M FLUSHING! :)
5 [write]: R: write(...) => None
2 [write]: C: write(self=<__main__.AutoFlush object at 0x007D6310>, data='\n')
LOOK MA! I'M FLUSHING! :)
5 [write]: R: write(...) => None
The '\n' goes out separately ;-)
>Yip, proxying is a nice pattern especially with python.
>
Is there a Python pattern repository somewhere?
Regards,
Bengt Richter
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