170 – assert statements with custom message

The keyword assert takes an expression on its right and then raises an AssertionError if that expression doesn’t evaluate to True (or Truthy):

x = 73
y = 42

assert y > x

...
AssertionError

The error message can be customised if you add a string in front of the expression being checked, separating the two with a comma:

x = 73
y = 42

assert y > x, f"{y=} isn't > {x=}..."

...
AssertionError: y=42 isn't > x=73...

Clear error messages make life easier for the person who’s dealing with the error (which is likely to be “you” from the future), so take a second to write a useful error message that you’ll be thankful for.

Just remember to never use assert for security-related or sensitive checks, since there are ways to “turn off” the assertions and have Python run without verifying them.