adding a simulation mode

[andrea crotti]

I'm writing a program which has to interact with many external
resources, at least:
- mysql database
- perforce
- shared mounts
- files on disk

And the logic is quite complex, because there are many possible paths to
follow depending on some other parameters.
This program even needs to run on many virtual machines at the same time
so the interaction is another thing I need to check...

Now I successfully managed to mock the database with sqlalchemy and only
the fields I actually need, but I now would like to simulate also
everything else.

I would like for example that if I simulate I can pass a fake database,
a fake configuration and get the log of what exactly would happen.  But
I'm not sure how to implement it now..  One possibility would be to have
a global variable (PRETEND_ONLY = False) that if set should be checked
before every potentially system-dependent command.

For example

copytree(src, dest) becomes:
if not PRETEND_ONLY:
    copytree(src, dest)

But I don't like it too much because I would have to add a lot of
garbage around..

Another way is maybe to set the sys.excepthook to something that catchs
all the exceptions that would be thrown by these comands, but I'm not
sure is a good idea either..

Any suggestion?