How to inject variable into scope with a decorator?

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.

Here is a simple demonstration of using a decorator to add a variable into the scope of a function.

>>> def add_name(name):
...     def inner(func):
...         # Same as defining name within wrapped
...         # function.
...         func.func_globals['name'] = name
...
...         # Simply returns wrapped function reference.
...         return func
... 
...     return inner
...
>>> @add_name("Bobby")
... def say_hello():
...     print "Hello %s!" % name
...
>>> print say_hello()
Hello Bobby!
>>>

Python is lexically scoped, so I’m afraid there is no clean way to do what you want without some potentially nasty side effects. I recommend just passing var into the function via the decorator.

c = 'Message'

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

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

msg_printer()  # prints 'Message'

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_dec
return msg_decorator

@decorator_factory(c)
def msg_printer(*args, **kwargs):
    print kwargs["var"]

msg_printer()

Here’s a way of injecting multiple variables into a function’s scope in a manner somewhat similar to what @Martijn Pieters does in his answer. I’m posting it primarily because it’s a more general solution and would not need to be applied multiple times to do it — as would be required by his (and many of the other) answers.

It should be noted that a closure is formed between the decorated function and the namespace dictionary, so changing its contents — e.g. namespace['a'] = 42 — will affect subsequent calls to the function.

from functools import wraps

def inject_variables(context):
    """ Decorator factory. """

    def variable_injector(func):
        """ Decorator. """
        @wraps(func)
        def decorator(*args, **kwargs):
            func_globals = func.__globals__

            # Save copy of any global values that will be replaced.
            saved_values = {key: func_globals[key] for key in context
                                                        if key in func_globals}
            func_globals.update(context)
            try:
                result = func(*args, **kwargs)
            finally:
                func_globals.update(saved_values)  # Restore replaced globals.

            return result

        return decorator

    return variable_injector


if __name__ == '__main__':
    namespace = dict(a=5, b=3)

    @inject_variables(namespace)
    def test():
        print('a:', a)
        print('b:', b)

    test()

Assuming that in python functions are objects, you can do…

#!/usr/bin/python3


class DecorClass(object):
    def __init__(self, arg1, arg2):
        self.a1 = arg1
        self.a2 = arg2

    def __call__(self, function):
        def wrapped(*args):
            print('inside class decorator >>')
            print('class members: {0}, {1}'.format(self.a1, self.a2))
            print('wrapped function: {}'.format(args))
            function(*args, self.a1, self.a2)
        return wrapped


    @DecorClass(1, 2)
    def my_function(f1, f2, *args):
        print('inside decorated function >>')
        print('decorated function arguments: {0}, {1}'.format(f1, f2))
        print('decorator class args: {}'.format(args))


    if __name__ == '__main__':
        my_function(3, 4)

and the result is:

inside class decorator >>
class members: 1, 2
wrapped function: (3, 4)
inside decorated function >>
decorated function arguments: 3, 4
decorator class args: (1, 2)

more explanation here http://python-3-patterns-idioms-test.readthedocs.io/en/latest/PythonDecorators.html

def merge(d1, d2):
    d = d1.copy()
    d.update(d2)
    return d

# A decorator to inject variables
def valueDecorator(*_args, **_kargs):
    def wrapper(f):
        def wrapper2(*args, **kargs):
            return f(*args, **kargs)
        wrapper2.__name__ = f.__name__
        wrapper2.__doc__ = f.__doc__
        oldVars = getattr(f, 'Vars', [])
        oldNamedVars = getattr(f, 'NamedVars', {})
        wrapper2.Vars = oldVars + list(_args)
        wrapper2.NamedVars = merge(oldNamedVars, _kargs)
        return wrapper2
    return wrapper

@valueDecorator(12, 13, a=2)
@valueDecorator(10, 11, a=1)
def func():
    print(func.Vars)
    print(func.NamedVars)

Instead of revising the global scope, changing the annotated function itself is more reasonable.

Update __globals__ works for me.

def f():
    print(a)


def with_context(**kw):
    def deco(fn):
        g = fn.__globals__
        g.update(kw)
        return fn

    return deco


with_context(a=3)(f)() # 3

I have catched problem with solution using globals.

Context of globals may be overwritten when you have several concurrent requests. I thought that impossible, but it is – after some time I have catched change of context(globals) if request wasn’t quick.
Better solution is to pass variable using kwargs:

def is_login(old_fuction):
    def new_function(request, *args, **kwargs):
        secret_token = request.COOKIES.get('secret_token')
        if secret_token:
            items = SomeModel.objects.get(cookie = secret_token)
            if len(items) > 0:
                item = items[0]
                kwargs['current_user'] = item
                return old_fuction(request, *args, **kwargs)
            else:
                return HttpResponse('error')
        return HttpResponse(status=404)
    return new_function

@is_login  
def some_func(request, current_user):
    return HttpResponse(current_user.name)

You’ll have to add extra parameter to each decorated function.

I found an interesting post provides a different solution by creating functions on the fly. Basically:

def wrapper(func):
    cust_globals = func.__globals__.copy()

    # Update cust_globals to your liking

    # Return a new function
    return types.FunctionType(
        func.__code__, cust_globals, func.__name__, func.__defaults__, func.__closure__
    )

See https://hardenedapple.github.io/stories/computers/python_function_override/