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Context manager to save/restore a name binding

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  Context manager to save/restore a name binding Ben Finney <ben+python@benfinney.id.au> - 2012-08-31 12:36 +1000
    Re: Context manager to save/restore a name binding Peter Otten <__peter__@web.de> - 2012-08-31 08:27 +0200
      Re: Context manager to save/restore a name binding Ben Finney <ben+python@benfinney.id.au> - 2012-08-31 19:32 +1000
    Re: Context manager to save/restore a name binding Chris Withers <chris@python.org> - 2012-08-31 12:06 +0100

#28157 — Context manager to save/restore a name binding

Howdy all,

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?

-- 
 \       “When a well-packaged web of lies has been sold to the masses |
  `\    over generations, the truth will seem utterly preposterous and |
_o__)                    its speaker a raving lunatic.” —Dresden James |
Ben Finney

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#28160

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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#28166

Peter Otten <__peter__@web.de> writes:

> You should wrap yield in a try ... finally. You might allow setting
> the new value in the manager (untested):

Thank you, both good advice.

I would still like to know if Python already has something to make this
unnecessary.

-- 
 \      “Compulsory unification of opinion achieves only the unanimity |
  `\        of the graveyard.” —Justice Roberts in 319 U.S. 624 (1943) |
_o__)                                                                  |
Ben Finney

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#28168

Hi Ben,

On 31/08/2012 03:36, Ben Finney wrote:
> 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?

Depends on the context (ho ho..), but if it's testing, then have a look at:

http://packages.python.org/testfixtures/mocking.html#the-context-manager

There's plenty of other goodies you may like in there too...

cheers,

Chris

-- 
Simplistix - Content Management, Batch Processing & Python Consulting
             - http://www.simplistix.co.uk

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