Easy to slap-on trace function.

trace.py

import inspect
import os

def trace(f, indent_level=[0]):
    """
    slap this on a function as a decorator and 
    it'll show you what it's called with 
    and what it returns.
    
    As an aside, if you put this on classes, be sure to
    *not* put it on the __repr__ method. Otherwise you'll
    end up in infinite recursion. ;)
    
    I can no longer find the SO answer that
    proposed setting up `indent_level` like this
    but using a mutable list as storage for indent level 
    is a great idea. (and cribbed from a StackOverflow answer)
    """
    _,_,line_number,_,_,_=\
            inspect.getouterframes(inspect.currentframe())[1]
    def wraps(*args, **kwargs):
        res = None
        ilevel = indent_level[0]
        _,file_name,calling_line,_,_,_=\
            inspect.getouterframes(inspect.currentframe())[1]
        file_name = os.path.split(file_name)[-1]
        called_by = "(called from %s:%d)" % (file_name, calling_line)
        src = "%04d:%s> %s" % (line_number, "--"*ilevel, f.__name__)
        ret = "%04d:<%s %s" % (line_number, "--"*ilevel, f.__name__)
        try:
            print("%s(%r, %r) %s" % (src, args, kwargs, called_by))
            indent_level[0] += 1
            res = f(*args, **kwargs)
            print("%s (%r)" % (ret, res))
            return res
        except Exception as e:
            print("%s EXCEPTION: %r %s" % (ret, e, called_by))
            raise
        finally:
            indent_level[0] -= 1
    wraps.__name__ = f.__name__
    wraps.__doc__ = f.__doc__
    return wraps

# if you want to quickly remove trace, just do
# trace=disabled  -- yay!
def disabled(f): return f