Printing debug

This tech note is a simple tip for debugging your Flask app.

If you are trying to figure out what is going on in your program, especially when it isn't working, put some print() statements in there.

Use print()

Say you're not sure if you've set qty_wheels correctly. Don't guess! Print it out:

print("FIXME: qty_wheels is ", qty_wheels)

Sometimes you just need to know Python really is getting to the bit of the program you think it is:

print("---------------->>> FIXME hello! I am here!")

...and look out for when your webserver next gets a request. Maybe hello does or it does not appear: either way you now know something is or is not happening.

This inside information is especially helpful when you are working on a webserver where (if you look at the browser) you only see the start (click!) and end (page or error) of the process.

Where does this show up?

In the terminal where you ran your program!

You're already seeing flask's prints coming out when your webserver logs the record of its response.

Test it by putting this in your home() procedure:

@app.route('/')
def home():
    print("FIXME hello world!")
    return render_template('index.html', server_url=BUGGY_RACE_SERVER_URL)

...and hit the home page in your browser.

See "hello world!" in your terminal output?

When you've got things working, take them out

When you're done carefully delete these print() lines.

Be careful because you don't want to delete the actual program code you were debugging!

If you're not sure if you've finished, comment them out so you can quickly switch them on and off again.

    # print("FIXME hello world!")

While you are debugging, use any mechanism you can to get more information.

Why "FIXME"?

You can put anything you like in your debugging print statements.

That ---------------->>> makes it easy to spot. No other reason.

These print statements are to help you debug so obviously they shouldn't end up in your final program... you take them out when you've figured out what you were trying to debug.

FIXME is a special word some programmers use because it's a word that you'll never normally use. So if you have the discipline of always using the word in your debug statements, later you can search your program for the word "FIXME" to check you haven't left any debug statements in your work.

In fact, some programmers add a commit hook to their Git so it will refuse to let them commit any code that has the word FIXME in it.

This page is special...

...because it really does contain the word "FIXME" :-/