Convert PascalCase string into snake_case

DESCRIPTION:

Complete the function/method so that it takes a PascalCase string and returns the string in snake_case notation. Lowercase characters can be numbers. If the method gets a number as input, it should return a string.

Examples

"TestController"  -->  "test_controller"
"MoviesAndBooks"  -->  "movies_and_books"
"App7Test"        -->  "app7_test"
1                 -->  "1"

---

def to_underscore(s):
    s=str(s)
    ans=""
    for i in s:
        if i in "ABCDEFGHIJKLMNOPQRSTUVWXYZ":
            ans+="_"
            ans+=i.lower()
        else:
            ans+=i
    if ans[0]=="_":
        return ans[1:]
    return ans