How to inject variable into scope with a decorator?

by

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.

More Related Contents:

  1. How to make function decorators and chain them together?
  2. Don’t understand why UnboundLocalError occurs (closure) [duplicate]
  3. Local variables in nested functions
  4. How does the @property decorator work in Python?
  5. How do I create a list of lambdas (in a list comprehension/for loop)?
  6. Why does this UnboundLocalError occur (closure)? [duplicate]
  7. Decorator execution order
  8. How can a function access its own attributes?
  9. How do I make function decorators and chain them together?
  10. How to make function decorators and chain them together in Python?
  11. Python decorator? – can someone please explain this? [duplicate]
  12. How to do a conditional decorator in python?
  13. How to create a custom decorator in Django?
  14. Decorator error: NoneType object is not callable
  15. How to skip a pytest using an external fixture?
  16. How do lexical closures work?
  17. Python lambda’s binding to local values
  18. How to get method parameter names?
  19. What’s the scope of a variable initialized in an if statement?
  20. Nested classes’ scope?
  21. Block scope in Python
  22. Read/Write Python Closures
  23. Python dynamic function creation with custom names
  24. Introspection to get decorator names on a method?
  25. The scope of names defined in class block doesn’t extend to the methods’ blocks. Why is that?
  26. What is the scope of a defaulted parameter in Python?
  27. Schrödinger’s variable: the __class__ cell magically appears if you’re checking for its presence?
  28. Python decorator as a staticmethod
  29. Applying a decorator to every method in a class?
  30. How can I access global variable inside class in Python

Leave a Comment