I don't know if this is a line of code you want to keep in your program, maybe your method was more simple and readable.

What i do here is create a list comprehension with all the values in the spam list but i add the item as "{spam[i]}," if is not the last value (i < len(spam) - 1) and as "and {spam[i]}" if it is the last value, then you can directly join the element of the new list with .join() method.

def list_to_string(spam: list) -> str:

    return " ".join([f"{spam[i]}," if i < len(spam) - 1 else f"and {spam[i]}" for i in range(len(spam))])

This is equal to this code:

def list_to_string(spam: list) -> str:

    new_list = []
    for i in range(len(spam)):
        if i < len(spam) - 1:
            new_list.append(f"{spam[i]},")
        else:
            new_list.append(f"and {spam[i]}")
        
    return " ".join(new_list)

And equal to this code:

def list_to_string(spam: list) -> str:

    new_list = []
    for i in range(len(spam)):
        new_list.append(f"{spam[i]}," if i < len(spam) - 1 else f"and {spam[i]}")
        
    return " ".join(new_list)

Edit: note that if the list include only one item the resulting string will be "and item" like your initial request state:

with and inserted before the last item.

I don't know if this is a line of code you want to keep in your program, maybe your method was more simple and readable.

What I do here is create a list comprehension with all the values in the spam list but I add the item as "{spam[i]}," if is not the last value (i < len(spam) - 1) and as "and {spam[i]}" if it is the last value, then you can directly join the element of the new list with .join() method.

def list_to_string(spam: list) -> str:

    return " ".join([f"{spam[i]}," if i < len(spam) - 1 else f"and {spam[i]}" for i in range(len(spam))])

This is equal to this code:

def list_to_string(spam: list) -> str:

    new_list = []
    for i in range(len(spam)):
        if i < len(spam) - 1:
            new_list.append(f"{spam[i]},")
        else:
            new_list.append(f"and {spam[i]}")
        
    return " ".join(new_list)

And equal to this code:

def list_to_string(spam: list) -> str:

    new_list = []
    for i in range(len(spam)):
        new_list.append(f"{spam[i]}," if i < len(spam) - 1 else f"and {spam[i]}")
        
    return " ".join(new_list)

Edit: note that if the list include only one item the resulting string will be "and item" like your initial request state:

with and inserted before the last item.