Context manager to temporarily change the variable of a register [aka write swap(a,b)]

Diez B. Roggisch deets at nospam.web.de
Tue Aug 25 22:14:48 CEST 2009


Evan Driscoll schrieb:
> (If you don't want to read the following, note that you can answer my
> question by writing a swap function.)
> 
> I want to make a context manager that will temporarily change the
> value of a variable within the scope of a 'with' that uses it. This is
> inspired by a C++ RAII object I've used in a few projects. Ideally,
> what I want is something like the following:
> 
>     x = 5
>     print x   # prints 5
>     with changed_value(x, 10):
>         print x  # prints 10
>     print x  # prints 5
> 
> This is similar to PEP 343's example 5 ("Redirect stdout temporarily";
> see http://www.python.org/dev/peps/pep-0343/), except that the
> variable that I want to temporarily change isn't hard-coded in the
> stdout_redirected function.
> 
> What I want to write is something like this:
>     @contextmanager
>     def changed_value(variable, temp_value):
>         old_value = variable
>         variable = temp_value
>         try:
>             yield None
>         finally:
>             variable = old_value
> with maybe a check in 'finally' to make sure that the value hasn't
> changed during the execution of the 'with'.
> 
> Of course this doesn't work since it only changes the binding of
> 'variable', not whatever was passed in, and I kind of doubt I can
> stick a "&" and "*" in a couple places to make it do what I want. :-)
> 
> So my question is: is what I want possible to do in Python? How?

No. And unfortunately, the with-statement (as the for-statement and 
others) leaks it's name, so you can't even use it like this:


with changed_value(variable, temp_value) as new_name:
    ...

The new_name will be defined afterwards.

Diez



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