How would you write a @debuggable decorator in python?

Use a debugger. Seriously. Decorating every function you want to keep track is a bad idea.

Python has a debugger included, so you don't need a good IDE.

If you don't want to use a debugger, you can use the trace function.

import sys

@sys.settrace
def trace_debug(frame, event, arg):
    if event == 'call':
        print ("calling %r on line %d, vars: %r" % 
                (frame.f_code.co_name, 
                 frame.f_lineno,
                 frame.f_locals))
        return trace_debug
    elif event == "return":
        print "returning", arg

def fun1(a, b):
    return a + b

print fun1(1, 2)

That prints:

calling 'fun1' on line 14, vars: {'a': 1, 'b': 2}
returning 3
3

Even easier would be to use Winpdb:

It is a platform independent graphical GPL Python debugger with support for remote debugging over a network, multiple threads, namespace modification, embedded debugging, encrypted communication and is up to 20 times faster than pdb.

Features:

• GPL license. Winpdb is Free Software.
• Compatible with CPython 2.3 or later.
• Compatible with wxPython 2.6 or later.
• Platform independent, and tested on Ubuntu Gutsy and Windows XP.
• User Interfaces: rpdb2 is console based, while winpdb requires wxPython 2.6 or later.

_{(source: winpdb.org)}

I think what you're after isn't really a debugging decorator, but more of a logging decorator.

It might make sense to use Python's logging module so you can have more fine grained control over the logging itself. For example you would be able to output to a file for later analysing the output.

The decorator might then look something more like:

import logging

logger = logging.getLogger('TraceLog')
# TODO configure logger to write to file/stdout etc, it's level etc


def logthis(level):
    def _decorator(fn):
        def _decorated(*arg,**kwargs):
            logger.log(level, "calling '%s'(%r,%r)", fn.func_name, arg, kwargs)
            ret=fn(*arg,**kwargs)
            logger.log(level, "called '%s'(%r,%r) got return value: %r", fn.func_name, arg, kwargs, ret)
            return ret
        return _decorated
    return _decorator

@logthis(logging.INFO)
def myfunc(this,that):
    return this+that

Then if you configure the logger to output to stderr you'd see:

>>> logger.setLevel(logging.INFO)
>>> handler=logging.StreamHandler()
>>> logger.addHandler(handler)
>>> myfunc(1,2)
calling 'myfunc'((1, 2),{})
called 'myfunc'((1, 2),{}) got return value: 3

I agree with nosklo using a debugger is much better than writing your own. I'll post an improvement to your code. But I still think you should follow nosklo's advice.

Use decorator classes to make your debugger neater:

class Debugger(object):
    enabled = False
    def __init__(self, func):
        self.func = func

    def __call__(self, *args, **kwargs):
        if self.enabled:
            print 'Entering', self.func.func_name 
            print '    args:', args, kwargs
        return self.func(*args, **kwargs)

Debugger.enabled = True

@Debugger
def myfunc(a, b, c, d):
    pass