Parametrized module import

[Oliver Fromme]

Heather Coppersmith <me at privacy.net> wrote:
 > Jacek Generowicz <jacek.generowicz at cern.ch> wrote:
 > > Oliver Fromme <olli at haluter.fromme.com> writes:
 > > > You could create another module called "config" or "cfg" which
 > > > contains some global variables.  You import it into your
 > > > configurable module as well as into your main program.  Then
 > > > you can configure the module's behaviour via those global
 > > > variables before importing the module.
 > > 
 > > Yes, my initial crappy prototype idea was to add configuration
 > > information to the sys module, but this variation is much neater
 > > ... in fact, after the first 2 minutes of thinking about it, it
 > > looks perfect :-)
 > > 
 > > However, one thing which keeps bothering me about the whole
 > > business, is the possibilty of importing the module (with your
 > > chosen configuration) after it has already been imported,
 > > without you knowing it, with a different configuration. Ideally
 > > there should be some warning about the fact that the
 > > configuration you specified is being ignored as a result of the
 > > module already being imported ... and I don't see how to achieve
 > > this.
 > 
 > Upon import, a module's "top level" code is executed, so try a
 > variation on this theme at the top level of your module:

It's only executed when the module is imported for the
_first_ time, so that wouldn't work.

However, the problem can be solved by not modifying a
global variable in the "config" module directly, but by
using a function which allows only one call.

    #---  File config.py:  ---
    foo_interface_mode = None
    def set_foo_interface_mode (mode):
        if foo_interface_mode is None:
            foo_interface_mode = mode
        else:
            raise "foo_interface_mode may only be set once"

    #---  File your_module.py:  ---
    import config
    if config.foo_interface_mode is None:
        raise "foo_interface_mode has not been set"
    elif config.foo_interface_mode == 0:
        ... this interface
    else:
        ... that interface

    #---  File main_program.py:  ---
    import config
    config.set_foo_interface_mode (1)
    import your_module

Of course, you could use assert instead of raise, or just
print a warning and go on.  Whatever you prefer.

Best regards
   Oliver

-- 
Oliver Fromme, Konrad-Celtis-Str. 72, 81369 Munich, Germany

``All that we see or seem is just a dream within a dream.''
(E. A. Poe)