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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def invalid_example_number(region_code):
 """Gets an invalid number for the specified region. This is useful for unit-testing purposes, where you want to test what will happen with an invalid number. Note that the number that is returned will always be able to be parsed and will have the correct country code. It may also be a valid *short* number/code for this region. Validity checking such numbers is handled with shortnumberinfo. Arguments: region_code -- The region for which an example number is needed. Returns an invalid number for the specified region. Returns None when an unsupported region or the region 001 (Earth) is passed in. """

    if not _is_valid_region_code(region_code):
        return None
    # We start off with a valid fixed-line number since every country
    # supports this. Alternatively we could start with a different number
    # type, since fixed-line numbers typically have a wide breadth of valid
    # number lengths and we may have to make it very short before we get an
    # invalid number.
    metadata = PhoneMetadata.metadata_for_region(region_code.upper())
    desc = _number_desc_by_type(metadata, PhoneNumberType.FIXED_LINE)
    if desc is None or desc.example_number is None:
        # This shouldn't happen; we have a test for this.
        return None  # pragma no cover
    example_number = desc.example_number
    # Try and make the number invalid. We do this by changing the length. We
    # try reducing the length of the number, since currently no region has a
    # number that is the same length as MIN_LENGTH_FOR_NSN. This is probably
    # quicker than making the number longer, which is another
    # alternative. We could also use the possible number pattern to extract
    # the possible lengths of the number to make this faster, but this
    # method is only for unit-testing so simplicity is preferred to
    # performance.  We don't want to return a number that can't be parsed,
    # so we check the number is long enough. We try all possible lengths
    # because phone number plans often have overlapping prefixes so the
    # number 123456 might be valid as a fixed-line number, and 12345 as a
    # mobile number. It would be faster to loop in a different order, but we
    # prefer numbers that look closer to real numbers (and it gives us a
    # variety of different lengths for the resulting phone numbers -
    # otherwise they would all be MIN_LENGTH_FOR_NSN digits long.)
    phone_number_length = len(example_number) - 1
    while phone_number_length >= _MIN_LENGTH_FOR_NSN:
        number_to_try = example_number[:phone_number_length]
        try:
            possibly_valid_number = parse(number_to_try, region_code)
            if not is_valid_number(possibly_valid_number):
                return possibly_valid_number
        except NumberParseException:  # pragma no cover
            # Shouldn't happen: we have already checked the length, we know
            # example numbers have only valid digits, and we know the region
            # code is fine.
            pass
        phone_number_length -= 1

    # We have a test to check that this doesn't happen for any of our
    # supported regions.
    return None