CS50P - Problem Set 2 - Problem 1 (camelCase): Did I overthink this?

Question

This was the first problem and it took me a few hours to figure out an answer. The task to is implement a program that prompts the user for the name of a variable in camel case and outputs the corresponding name in snake case. It took me less than 3 minutes to come up with this, which satisfied any input where only one capital existed:

camelcase = input("camelCase: ")
for letter in camelcase:
    if letter.isupper():
        first, second, last = camelcase.partition(letter)
        second = "_" + second.lower()
print("snakecase: ", first + second + last)

However to come up with the following to satisfy inputs with multiple capitals I spent a solid 2-3 hours:

camelcase = input("camelCase: ")
for letter in camelcase:
    if letter.isupper():
        _ = camelcase.split(letter)
        _.insert(1, "_" + letter.lower())
        camelcase = "".join(_)
print("snake_case:", camelcase)

I feel like given that this was the first problem, there is a simpler solution that I completely overlooked. Any advice? Please remember that my knowledge scope is only as far as lecture 3 which introduced loops.

I first tried adding a second if letter.isupper() loop to the final variable resulting from the .partition operation, however, that wasn't working as intended. Then I tried using the s.insert() method on the original input but learnt through the documentation that strings are immutable. Eventually after reading and experimenting with a plethora of different str functions and their results I managed to get to my submission.