Python assert function confirms an expression. If the expression fails, the script will exit with an error.

>>> x = 3
>>> assert(x>2)

>>> assert(x<2)

Traceback (most recent call last):
File "", line 1, in 
AssertionError

def cm2inch(h):
assert(h>=0), "height must be positive";
return(h*0.3937);

>>> cm2inch(2)

0.7874

>>> cm2inch(-2)

Traceback (most recent call last):
File "", line 1, in 
File "", line 1, in 
AssertionError: height must be positive