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Python Context Management

From: "andrew cooke" <andrew@...>

Date: Sun, 27 Apr 2008 19:50:32 -0400 (CLT)

This is quite cute - it's similar to what I've seen in Scheme/Lisp and, I
think, Haskell, where you can specify a "finally" action:

from __future__ import with_statement
from myapp.context import close, dispose
[...]
    class ...
    def _create_database(self):
        (db, url) = split_database(self._url)
        log.debug(_('Creating database %s') % db)
        try:
            with dispose(self._create_engine(url)) as engine:
                with close(engine.connect()) as cnx:
                    # can't use text here as it quotes strings
                    cnx.execute('create database %s' % db)
        except ...

Note the "with close(...)" and "with dispose(...)" which call those
methods (close(), dispose()) on the associated objects at the end of the
block.

myapp.context:
from acooke.context import no_args
def close(client):
    return no_args(client, 'close')
def dispose(client):
    return no_args(client, 'dispose')

acooke.context:
class Context(object):
    def __init__(self, client=None, on_entry=lambda x: x,
                  on_exit=lambda (x, tp, vl, tb): False, **kargs):
        super(Context, self).__init__(**kargs)
        self._client = client
        self._on_entry = on_entry
        self._on_exit = on_exit
    def __enter__(self):
        return self._on_entry(self._client)
    def __exit__(self, tp, vl, tb):
        return self._on_exit(self._client, tp, vl, tb)
def no_args(client, attr):
    def f(x, tp, vl, tb):
        getattr(x, attr)()
        return False
    return Context(client=client, on_exit=f)

Andrew

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