Python, exceptions

[KingKhan248]

I'm trying to practice using exceptions in python programming but I can't seem to figure out how it works.
I'm trying to create a basic program that asks the user to enter the side lengths of a triangle and then calculates and outputs the perimeter. I want to use exceptions to give error messages when any of the side lengths are entered incorrectly, like if instead of a number they enter "akjdhg", and it should give an error when they enter a negative number.

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.")
except:
    if len1 <= 0 or len2 <= 0 or len3 <= 0:
    
        print("Please enter a positive number.")

When I enter anything other than a number I do get the error message I want but when I enter a negative or 0, the error message does not appear and the code continues.

Still pretty new to this forum I hope I followed all the rules and guidelines on posting here. If not please let me know and I will fix it. Thanks in advance!