How to inject variable into scope with a decorator?

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.

There is a clean way to do what you want without using global variable. If you want to be stateless and threads safe, you don't really have the choice.

Use the "kwargs" variable:

c = 'Message'def decorator_factory(value):    def msg_decorator(f):    def inner_dec(*args, **kwargs):        kwargs["var"] = value        res = f(*args, **kwargs)        return res    return inner_decreturn msg_decorator@decorator_factory(c)def msg_printer(*args, **kwargs):    print kwargs["var"]msg_printer()

You can't. Python has lexical scoping. That means the meaning of an identifier is determined solely based on the scopes that physically surround it when you look at the source code.