def number_from_user( prompt ):
    while True:
        spec = input( prompt )
        try:
            return float( spec )
        except ValueError:
            s = "Sorry, '{}' is not a valid number spec. Try e.g. '3.14'."
            print( s.format( spec ) )
            print()

print( "This program computes the sum of two numbers A and B." )
print()
a = number_from_user( "Number A, please: " )
b = number_from_user( "Number B, please: " )
sum = a + b
print()
print( "{} + {} = {}".format( a, b, sum ) )