Bash, Python - append string to the beginning specific line-number

Question

How can I append or add string or variable to the beginning of a specific line defined by its line-number?

FILE:

test1
test2
test3

variable

line-number=2

command

sed -i -e 'line-number/'...new-text...'/' FILE

desired update in FILE:

test1
...new-text...test2
test3

how can I write such a command using SED or AWK?

Thanks in adavcen for help, advice....