Ben Finney wrote:
> I have written a context manager to save and restore a name binding::
>
> import contextlib
>
> @contextlib.contextmanager
> def preserve_value(namespace, name):
> """ A context manager to preserve, then restore, the specified
> binding.
>
> :param namespace: The namespace object (e.g. a class or dict)
> containing the name binding.
> :param name: The name of the binding to be preserved.
> :yield: None.
>
> When the context manager is entered, the current value bound
> to `name` in `namespace` is saved. When the context manager is
> exited, the binding is re-established to the saved value.
>
> """
> saved_value = getattr(namespace, name)
> yield
> setattr(namespace, name, saved_value)
>
> The use case is <URL: http://stackoverflow.com/a/6811921/70157>, where
> it's used like this::
>
> with preserve_value(sys, 'dont_write_bytecode'):
> sys.dont_write_bytecode = True
> module = imp.load_module(…)
>
> That way, I can set ‘sys.dont_write_bytecode’ to the value I need in
> this part of the code, knowing that however the code continues the
> previous value of that setting will be restored to whatever it was
> before I touched it.
>
> Have I re-invented a context manager which already exists? Is there a
> better way to do what ‘preserve_value’ is doing?
You should wrap yield in a try ... finally. You might allow setting the new
value in the manager (untested):
import contextlib
missing = object()
@contextlib.contextmanager
def preserve_attr(namespace, name, value=missing):
saved_value = getattr(namespace, name)
if value is not missing:
setattr(namespace, name, value)
try:
yield
finally:
setattr(namespace, name, saved_value)
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