Splitting a word at the uppercase without using regex or a function leads to error cannot assign the expression or not enough value to unpack

I am trying to learn python. In this context I need to split a word when encountering an upper case, convert it as a lower case and then insert a "_".

I have seen various quite complex answer here or here or here, but I am trying to do it as simply as possible.

So far, here is my code:

word = input("What is the camelCase?")

for i , k in word:
    if i.isupper() and k.islower():
        word2 = k + "_" + i.lower + k
        print(word2)
    else:
        print(word)

This leads to "not enough values to unpack (expected 2, got 1)" after I input my word.

Another attempt here:

word = input("What is the camelCase?")

for i and k in word:
    if i.isupper() and k.islower():
        word2 = k + "_" + i.lower + k
        print(word2)
    else:
        print(word)

In this case I cannot even write my word, I directly have the error: "cannot assign to expression"

Try it this way.

a=input("enter word:")
b=""
for i in a:
    if i.isupper():
        b=b+(i.lower()+"_")
    else:b=b+i
print(b)

Hope it helped :)