Question

I’m having trouble understanding the following line:

result = (status!=None and "off" or "on")

Now when this is called, only status has been assigned a value (the result of a regular expression search). However, after this command, result takes the value of “on” or “off”.

To my untrained eye, this just looks like a comparison. How is this assignment happening?

Answer 1

From the documentation:

The expression x and y first evaluates x; if x is false, its value is returned; otherwise, y is evaluated and the resulting value is returned.

The expression x or y first evaluates x; if x is true, its value is returned; otherwise, y is evaluated and the resulting value is returned.

Furthermore, any non-empty string is considered to be "true" when it appears in a boolean context.

With this, we can tackle your question.

Due to operator precedence rules, the following:

status != None and "off" or "on"

is equivalent to:

(status != None and "off") or "on"

The first part, status != None and "off" evaluates to:

• False if status is None;
• "off" otherwise

The entire expression, (status != None and "off") or "on" evaluates to:

• "on" if status is None;
• "off" otherwise.

An arguably more Pythonic way to write this statement is:

result = "on" if status is None else "off"