Where Python 3.10’s := Operator Can Be Used

Today I learned how the walrus operator works. I had assumed it could be placed anywhere within an expression; that was incorrect. It can’t be used within an expression. Its proper place is outside and to the left of an expression, but before the if, elif, =, or wherever the expression’s value is going to be used.

As a result, I found myself writing this:

if ua_match := (index < len(robots_lines) and ua_re.match(robotslines[index])):

I needed the if to fail if index had reached len(lines), but the boolean expression returns its last true value, so the .match() needs to be placed at the end of the expression for the := used before the expression to be able to capture its value. It reads oddly, to be sure.

An intuitive but incorrect approach might be to write:

if ua_match := ua_re.match(lines[index]) and index < len(lines):

but that sets ua_match to True, since the rightmost term of the boolean expression was a comparison. The other intuitive but incorrect approach would be:

if index < len(lines) and ua_match := ua_re.match(lines[index])):

But that doesn’t compile, since placing the := operator inside an expression is a syntax error. It belongs in a new extra layer outside and to the left of the expr but before the leftward if, elif, = or whatever will capture the value of the expression. Here and I thought it could be put anywhere. Its usage is a lot simpler than I took it for.

14 March 2023