Commit eda1672

Online Python Tutor (thereafter abbreviated as OPT). View it online at:

web_exec.py : example CGI script for deploying backend on CGI-enabled webservers

To modify the backend, you will mainly need to understand `pg_logger.py` and `pg_encoder.py`.

First, run `generate_json_trace.py` to see the trace that the backend generates for a given input Python program.

This is the main way to do an "end-to-end" test on your backend modifications. For example, if you wrote a Python

program stored in `example.py`, then running:

python generate_json_trace.py example.py

will print a JSON-formatted execution trace to stdout. This is exactly what the backend sends to the frontend.

(Actually not quite: the sent trace is actually compressed to eliminate all extraneous spaces and newlines.

But for testing, I've made the trace more human-readable.)

Second, when you're "print debugging" in the backend, you can't simply print to stdout, since `pg_logger.py`

*redirects* stdout to a buffer. Instead, you need to write all of your print statements as:

print >> sys.stderr, <debug message to print>

so that the output goes to stderr.

The easiest way to debug is to insert in print statements (to stderr) and then run `generate_json_trace.py` on

small code examples.