Is there a way to access a function's attributes/parameters within a ContextDecorator?

Question

I'm trying to write a context manager decorator using Python's contextlib.ContextDecorator class.

Is there a way to access the decorated function's parameters within the context manager?

Here's an example of what I'm doing:

from contextlib import ContextDecorator

class savePen(ContextDecorator):
    def __enter__(self):
        self.prevPen = self.dc.GetPen()     # AttributeError
        return self

    def __exit__(self, *exc):
        self.dc.SetPen(self.prevPen)
        return False

Given the above, this:

@savePen()
def func(dc, param1, param2):
    # do stuff, possibly changing the pen style

should be equivalent to:

def func(dc, param1, param2):
    prevPen = dc.GetPen()
    # do stuff, possibly changing the pen style
    dc.SetPen(prevPen)

---

I've scoured the docs for contextlib and haven't found anything useful.

Does anyone know how to access the decorated functions' attributes from within a ContextDecorator class?

EDIT1:

As @chepner said in this response, ContextDecorator is sugar for

def func(dc, param1, param2):
    with savePen():
        ...

and that it cannot access the functions' parameters.

However, in this case, whatever runs inside of with savePen() has access to the function parameters dc, param1, and param2. This makes me think that I should be able to access them using ContextDecorator.

For example, this is valid:

def func(dc, param1, param2):
    with savePen():
        print(param1)

Answer 1

contextlib.contextmanager seems more appropriate here. Note that, like anything else, you can't access the local variables of a function body from outside the function (short of introspection hacks, anyway).

@contextlib.contextmanager
def savePen(dc):
    prevPen = dc.GetPen()
    yield
    dc.SetPen(prevPen)

with savePen(dc):
    func(dc, param1, param2)

Note that with a ContextDecorator, the context manager is instantiated with no arguments, that is

@savePen()
def func(dc, param1, param2):
    # do stuff, possibly changing the pen style

is syntactic sugar (according to the documentation) for

def func(dc, param1, param2):
    with savePen():
        ...

and so there's no way to tell savePen which object (dc) to work with.