decorator to fetch arguments from global objects

[andrea crotti]

Using a CouchDB server we have a different database object potentially for
every request.

We already set that db in the request object to make it easy to pass it
around form our django app, however it would be nice if I could set it once
in the API and automatically fetch it from there.

Basically I have something like

class Entity:
     def save_doc(db)
        ...

I would like basically to decorate this function in such a way that:
- if I pass a db object use it
- if I don't pass it in try to fetch it from a global object
- if both don't exist raise an exception

Now it kinda of works already with the decorator below.
The problem is that the argument is positional so I end up maybe passing it
twice.
So I have to enforce that 'db' if there is passed as first argument..

It would be a lot easier removing the db from the arguments but then it
would look too magic and I didn't want to change the signature.. any other
advice?

def with_optional_db(func):
    """Decorator that sets the database to the global current one if
    not passed in or if passed in and None
    """
    @wraps(func)
    def _with_optional_db(*args, **kwargs):
        func_args = func.func_code.co_varnames
        db = None
        # if it's defined in the first elements it needs to be
        # assigned to *args, otherwise to kwargs
        if 'db' in func_args:
            assert 'db' == func_args[0], "Needs to be the first defined"
        else:
            db = kwargs.get('db', None)

        if db is None:
            kwargs['db'] = get_current_db()

        assert kwargs['db'] is not None, "Can't have a not defined database"
        ret = func(*args, **kwargs)
        return ret

    return _with_optional_db
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