How to mark a global as deprecated in Python?

Question

I've seen decorators that let you mark a function a deprecated so that a warning is given whenever that function is used. I'd like to do the same thing but for a global variable, but I can't think of a way to detect global variable accesses. I know about the globals() function, and I could check its contents, but that would just tell me if the global is defined (which it still will be if the function is deprecated and not all out removed) not if it's actually being used. The best alternative I can think of is something like this:

# myglobal = 3
myglobal = DEPRECATED(3)

But besides the problem of how to get DEPRECATED to act exactly like a '3', I'm not sure what DEPRECATED could do that would let you detect every time it's accessed. I think the best it could do is iterate through all of the global's methods (since everything in Python is an object, so even '3' has methods, for converting to string and the like) and 'decorate' them to all be deprecated. But that's not ideal.

Any ideas? Has anyone else tackled this problem?

Answer 1

Is this just for your dev environment? If so, perhaps tweaking Python itself may be your best option, in which case pypy (the Python interpreter written in Python) may be helpful, but even that may be difficult.

Answer 2

You could make your module into a class (see e.g this SO question) and make that deprecated global into a property, so you can execute some of your code when it's accessed and provide the warning you desire. However, this does seem a bit of an overkill.

Answer 3

You can't do this directly, since theres no way of intercepting the module access. However, you can replace that module with an object of your choosing that acts as a proxy, looking for accesses to certain properties:

import sys, warnings

def WrapMod(mod, deprecated):
    """Return a wrapped object that warns about deprecated accesses"""
    deprecated = set(deprecated)
    class Wrapper(object):
        def __getattr__(self, attr):
            if attr in deprecated:
                warnings.warn("Property %s is deprecated" % attr)

            return getattr(mod, attr)

        def __setattr__(self, attr, value):
            if attr in deprecated:
                warnings.warn("Property %s is deprecated" % attr)
            return setattr(mod, attr, value)
    return Wrapper()

oldVal = 6*9
newVal = 42

sys.modules[__name__] = WrapMod(sys.modules[__name__], 
                         deprecated = ['oldVal'])

Now, you can use it as:

>>> import mod1
>>> mod1.newVal
42
>>> mod1.oldVal
mod1.py:11: UserWarning: Property oldVal is deprecated
  warnings.warn("Property %s is deprecated" % attr)
54

The downside is that you are now performing two lookups when you access the module, so there is a slight performance hit.

Answer 4

Behold:

Code

from types import *

def wrapper(f, warning):
    def new(*args, **kwargs):
        if not args[0].warned:
            print "Deprecated Warning: %s" % warning
            args[0].warned = True
        return f(*args, **kwargs)
    return new

class Deprecated(object):
    def __new__(self, o, warning):
        print "Creating Deprecated Object"
        class temp(o.__class__): pass
        temp.__name__ = "Deprecated_%s" % o.__class__.__name__
        output = temp.__new__(temp, o)

        output.warned = True
        wrappable_types = (type(int.__add__), type(zip), FunctionType)
        unwrappable_names = ("__str__", "__unicode__", "__repr__", "__getattribute__", "__setattr__")

        for method_name in dir(temp):
            if not type(getattr(temp, method_name)) in wrappable_types: continue
            if method_name in unwrappable_names: continue

            setattr(temp, method_name, wrapper(getattr(temp, method_name), warning))

        output.warned = False

        return output

Output

>>> a=Deprecated(1, "Don't use 1")
Creating Deprecated Object
>>> a+9
Deprecated Warning: Don't use 1
10
>>> a*4
4
>>> 2*a
2

This can obviously be refined, but the gist is there.