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 decorator that does something like injecting a name into the scope of another function (such that the name does not leak outside the decorator's scope). For example, if I have a function that says to print a variable named var that has not been defined, I would like to define it within a decorator where it is called. Here is an example that breaks:

c = 'Message'

def decorator_factory(value):
    def msg_decorator(f):
        def inner_dec(*args, **kwargs):
            var = value
            res = f(*args, **kwargs)
            return res
        return inner_dec
    return msg_decorator

@decorator_factory(c)
def msg_printer():
    print var

msg_printer()

I would like it to print "Message", but it gives:

NameError: global name 'var' is not defined

The traceback even points to wher var is defined:

<ipython-input-25-34b84bee70dc> in inner_dec(*args, **kwargs)
      8         def inner_dec(*args, **kwargs):
      9             var = value
---> 10             res = f(*args, **kwargs)
     11             return res
     12         return inner_dec

So I don't understand why it can't find var.

Is there any way to do something like this?

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.