insert-line.py

#https://stackoverflow.com/questions/10507230/insert-line-at-middle-of-file-with-python
def insert_file(filename, index, value):
    with open(filename, "r") as f:
        contents = f.readlines()

    contents.insert(index, value)

    with open(filename, "w") as f:
        contents = "".join(contents)
        f.write(contents)

# echo "test\nworld" > test.txt
insert_file('test.txt', 1, '# hello\n')
# cat test.txt
# test
# # hello
# world