Trying to write func("abcd") -> "abbcccdddd"

[ebolisa]

Although it's at the front, it's still "inside the loop". This is just a syntax for a generator comprehension, similar to a list comprehension.

letter_generator = (i*char for i, char in enumerate(s, start=1))

Is similar to:

def gen_expression(iterable):
    for i,char in enumerate(iterable, start=1):
        yield i*char
letter_generator = gen_expression(s)

And then the generator will hand out the sequence as requested.

Thanks. I came up with some similar which gets closer to my understanding of the function:

def tag(s):
    res = ''
    for i, char in enumerate(s, start=1):
        res += ''.join(i*char)
    return res

print(tag('tom'))