import difflib


def first_diff(a, b, fromfile, tofile):
    """Returns the first few lines of a difference, if there is one.  Python
    diff can be very slow with large objects and the most interesting changes
    are the first ones. Truncate data before sending to difflib.  Returns None
    is there is no difference."""

    # Find first diff
    first_diff_idx = None
    for idx, val in enumerate(a):
        if val != b[idx]:
            first_diff_idx = idx
            break

    if first_diff_idx == None:
        # No difference
        return None

    # Diff to first line of diff plus some lines
    context = 3
    diff = difflib.unified_diff(a[:first_diff_idx+context],
                                b[:first_diff_idx+context],
                                fromfile,
                                tofile)
    difference = "\n".join(diff)
    if first_diff_idx + context < len(a):
        difference += "\n*** Diff truncated ***"
    return difference