Deciphering large program flow in Python

I'm in the process of learning how a large (356-file), convoluted Python program is set up. Besides manually reading through and parsing the code, are there any good methods for following program flow?

There are two methods which I think would be useful:

1. Something similar to Bash's "set -x"
2. Something that displays which file outputs each line of output

Are there any methods to do the above, or any other ways that you have found useful?

I don't know if this is actually a good idea, but since I actually wrote a hook to display the file and line before each line of output to stdout, I might as well give it to you…

import inspect, sys

class WrapStdout(object):
    _stdout = sys.stdout
    def write(self, buf):
        frame = sys._getframe(1)
        try:
            f = inspect.getsourcefile(frame)
        except TypeError:
            f = 'unknown'
        l = frame.f_lineno
        self._stdout.write('{}:{}:{}'.format(f, l, buf))
    def flush(self):
        self._stdout.flush()

sys.stdout = WrapStdout()

Just save that as a module, and after you import it, every chunk of stdout will be prefixed with file and line number.

Of course this will get pretty ugly if:

• Anyone tries to print partial lines (using stdout.write directly, or print magic comma in 2.x, or end='' in 3.x).
• You mix Unicode and non-Unicode in 2.x.
• Any of the source files have long pathnames.
• etc.

But all the tricky deep-Python-magic bits are there; you can build on top of it pretty easily.