python write file from a precise line code example

Example: python write into file at position

mystring = 'some string'
pos = 10

with open('/path/to/file.txt', 'r+') as f:
    contents = f.read()
    contents = contents[:pos] + mystring + contents[pos + 1:]
    f.seek(0)
    f.truncate()
    f.write(contents)