Simple debug function that uses introspection to display the file name, function name and line number of the caller.

debug.py

def debug(msg):
    '''
    Display a debug message with the file name, function
    name and line number.
    '''
    import inspect
    parent_frame = inspect.currentframe().f_back
    lineno = parent_frame.f_lineno
    fctname = parent_frame.f_code.co_name
    filename = parent_frame.f_code.co_filename
    sys.stderr.write('DEBUG:{}:{}:{} {}\n'.format(filename, fctname, lineno, msg))

def fct1():
  '''
  Dummy test function.
  '''
  debug('a debug message')