Python, exceptions

[buran]

Looking at your code I think you misunderstand how the try/except works. When there is error in the try block it looks at the exceptions. You handle ValueError and you also have all-catching blank except`. When you enter a negative number, no exception is raised so no except block is entered.

Probably you want to have else part. It will run after the try block if no except` block was entered

try:
    len1 = (float(input("Enter the length of one side of the triangle: ")))
    len2 = float(input("Enter the length of a different side: "))
    len3 = float(input("Enter the length of the final side: "))
 
except ValueError:
    print("Please enter a number.")
else:
    if len1 <= 0 or len2 <= 0 or len3 <= 0:
     
        print("Please enter a positive number.")

Note that it will print the message and just continue with program execution.
For completeness you should know there can be also finally - i.e. this code will be executed always, regardless of except part being executed or not.