def create_dt_url(dt, root, fname, subdirs=[]):
    """
    Create URLs from root + fname and a list of subdirectories.
    fname and subdirs will be put through dt.strftime.

    Parameters
    ----------
    dt: datetime.datetime
        date as basis for the URL
    root: string
        root of the url
    fname: string
        filename to use
    subdirs: list, optional
        list of strings.
        Each element represents a subdirectory.
        For example the list ['%Y', '%m'] would lead to a URL of
        ``root/YYYY/MM/fname`` or for a dt of datetime(2000,12,31)
        ``root/2000/12/fname``

    Returns
    -------
    url: string
    """
    dt_subdirs = []
    for subdir in subdirs:
        dt_subdirs.append(dt.strftime(subdir))
    dt_fname = dt.strftime(fname)
    url = '/'.join([root] + dt_subdirs + [dt_fname])
    return url
