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# utr.py - functions for handling English UTRs
#
# Copyright (C) 2020 Holvi Payment Services
# Copyright (C) 2020 Arthur de Jong
#
# This library is free software; you can redistribute it and/or
# modify it under the terms of the GNU Lesser General Public
# License as published by the Free Software Foundation; either
# version 2.1 of the License, or (at your option) any later version.
#
# This library is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
# Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser General Public
# License along with this library; if not, write to the Free Software
# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
# 02110-1301 USA
"""UTR (United Kingdom Unique Taxpayer Reference).
A UTR (unique taxpayer reference) is a 10 digit number used to identify UK
taxpayers who have to submit a tax return.
More information:
* https://www.oecd.org/tax/automatic-exchange/crs-implementation-and-assistance/tax-identification-numbers/UK-TIN.pdf
>>> validate('1955839661')
'1955839661'
>>> validate('2955839661')
Traceback (most recent call last):
...
InvalidChecksum: ..
"""
from stdnum.exceptions import *
from stdnum.util import clean, isdigits
def compact(number):
"""Convert the number to the minimal representation. This strips the
number of any valid separators and removes surrounding whitespace."""
return clean(number, ' ').upper().strip().lstrip('K')
def calc_check_digit(number):
"""Calculate the check digit for the number. The passed number should not
have the check digit (the first one) included."""
weights = (6, 7, 8, 9, 10, 5, 4, 3, 2)
return '21987654321'[sum(int(n) * w for n, w in zip(number, weights)) % 11]
def validate(number):
"""Check if the number is a valid UTR."""
number = compact(number)
if not isdigits(number):
raise InvalidFormat()
if not len(number) == 10:
raise InvalidLength()
if number[0] != calc_check_digit(number[1:]):
raise InvalidChecksum()
return number
def is_valid(number):
"""Check if the number is a valid UTR."""
try:
return bool(validate(number))
except ValidationError:
return False
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