How to inject variable into scope with a decorator?

Question

[Disclaimer: there may be more pythonic ways of doing what I want to do, but I want to know how python\'s scoping works here]

I\'m trying to find a way to make a dec

Answer 1

You can't. Scoped names (closures) are determined at compile time, you cannot add more at runtime.

The best you can hope to achieve is to add global names, using the function's own global namespace:

def decorator_factory(value):
    def msg_decorator(f):
        def inner_dec(*args, **kwargs):
            g = f.__globals__  # use f.func_globals for py < 2.6
            sentinel = object()

            oldvalue = g.get('var', sentinel)
            g['var'] = value

            try:
                res = f(*args, **kwargs)
            finally:
                if oldvalue is sentinel:
                    del g['var']
                else:
                    g['var'] = oldvalue

            return res
        return inner_dec
    return msg_decorator

f.__globals__ is the global namespace for the wrapped function, so this works even if the decorator lives in a different module. If var was defined as a global already, it is replaced with the new value, and after calling the function, the globals are restored.

This works because any name in a function that is not assigned to, and is not found in a surrounding scope, is marked as a global instead.

Demo:

>>> c = 'Message'
>>> @decorator_factory(c)
... def msg_printer():
...     print var
... 
>>> msg_printer()
Message
>>> 'var' in globals()
False

But instead of decorating, I could just as well have defined var in the global scope directly.

Note that altering the globals is not thread safe, and any transient calls to other functions in the same module will also still see this same global.