A terrible thing: print the source of every line of Python code as it executes in your scripts. Please don't actually use it. Please.

terrible_trace.py

import inspect
import sys

def tracefunction(frame, event, arg):
    if event == "line":
        info = inspect.getframeinfo(frame)
        fname, lineno, fn = info.filename, info.lineno, info.function
        with open(fname, 'rb') as f:
            line = [line.rstrip() for line in f][lineno - 1]
            print "Function: {} (in file {}:{}) | {}".format(fname, fn, lineno, line)
    return tracefunction

def registertracefunction():
    sys.settrace(tracefunction)

registertracefunction()

def mainfunction():
    for x in xrange(10):
        print x * 5

if __name__ == "__main__":
    mainfunction()