<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def ensure_dir(f):
 """ Ensure a a file exists and if not make the relevant path """

    d = os.path.dirname(f)
    if not os.path.exists(d):
        os.makedirs(d)