Handy debug thing for logging the entry and exit of functions and methods, showing arguments and return values. I found this useful for seeing how things work in an unfamiliar library (in this case, django-haystack).

debuggy_thinger.py

import logging
import sys
import types

# It's great to log the entering/leaving of methods and functions, but it would be way cool
# to see who gets called by the function you go into. That probably means hacking ASTs and
# finding the GOSUB bytecode.

def discover(obj, filtfunc=None):
    if filtfunc is None:
        filtfunc = lambda attr: True
    return dict([(attr, getattr(obj, attr)) for attr in dir(obj) if filtfunc(attr)])

def investigate(container, name, output=sys.stderr, indent=[0], debug=False, special=None, include=[]):
    obj = getattr(container, name)

    if isinstance(obj, types.ClassType):

        investigate_in(obj, include=include, output=output, debug=debug, indent=indent)

    elif callable(obj):

        indent_step = 8 * ' '

        file = lineno = None
        if isinstance(obj, types.MethodType):
            objname = str(obj.im_class) + '.' + obj.im_func.__name__
            file = obj.im_func.func_code.co_filename
            lineno = obj.im_func.func_code.co_firstlineno
        elif isinstance(obj, types.FunctionType):
            file = obj.func_code.co_filename
            lineno = obj.func_code.co_firstlineno
            objname = container.__name__ + '.' + name
        else:
            objname = str(obj)

        def emit(x, output=output):
            if isinstance(output, logging.Logger):
                output.debug(x)
            else:
                output.write(x + '\n')

        def enter(args, kwargs, output=output, file=file, line=lineno, special=special, objname=objname):
            if file is not None:
                emit('%s%s:%d' % (indent_step * indent[0], file, line))
            emit('%sEnter %s, args=%s, kwargs=%s' % (indent_step * indent[0], objname, args, kwargs))
            if special is not None:
                special(*args, **kwargs)
            indent[0] += 1

        def leave(result, output=output, objname=objname):
            indent[0] -= 1
            emit('%sLeave %s, returns %s' % (indent_step * indent[0], objname, result))

        if isinstance(container, types.InstanceType) or isinstance(container, types.ClassType):
            if getattr(container, name).im_self is None:
                def wrapped_callable(cls, *args, **kwargs):
                    enter(args, kwargs)
                    result = obj(cls, *args, **kwargs)
                    leave(result)
                    return result
                if debug:
                    emit('Instrumenting %s:%s' % (container, name))
                setattr(container, name, wrapped_callable)
            else:
                def wrapped_callable(cls, *args, **kwargs):
                    enter(args, kwargs)
                    result = obj(*args, **kwargs)
                    leave(result)
                    return result
                if debug:
                    emit('Instrumenting %s:%s' % (container, name))
                setattr(container, name, classmethod(wrapped_callable))

        else:
            def wrapped_callable(*args, **kwargs):
                enter(args, kwargs)
                result = obj(*args, **kwargs)
                leave(result)
                return result
            if debug:
                emit('Instrumenting %s:%s' % (container, name))
            setattr(container, name, wrapped_callable)


def investigate_in(container, include=[], output=sys.stderr, debug=False, indent=[0]):
    for attr in dir(container):
        if (not attr.startswith('_')) or attr in include:
            investigate(container, name=attr, output=output, debug=debug, indent=indent)


# example usage
from haystack.views import SearchView
from haystack.fields import SearchField 
investigate(SearchView, 'get_results')
investigate(SearchField, 'prepare_template')