From 4e8d7e4bcb0eeb49f6df38b530eb39c6cf953874 Mon Sep 17 00:00:00 2001
From: Arthur de Jong <arthur@arthurdejong.org>
Date: Tue, 20 Sep 2011 21:14:48 +0000
Subject: implement a conversion function from ISBN13 to ISBN10

git-svn-id: http://arthurdejong.org/svn/python-stdnum/python-stdnum@80 9dea7c4f-944c-4273-ac1a-574ede026edc
---
 stdnum/isbn.py          | 24 ++++++++++++++++++++++++
 tests/test_isbn.doctest | 20 ++++++++++++++++++++
 2 files changed, 44 insertions(+)

diff --git a/stdnum/isbn.py b/stdnum/isbn.py
index 382b13d..59262ba 100644
--- a/stdnum/isbn.py
+++ b/stdnum/isbn.py
@@ -37,6 +37,8 @@ False
 'ISBN13'
 >>> to_isbn13('1-85798-218-5')
 '978-1-85798-218-3'
+>>> to_isbn10('978-1-85798-218-3')
+'1-85798-218-5'
 """
 
 from stdnum import ean
@@ -111,6 +113,28 @@ def to_isbn13(number):
         return '978' + number
 
 
+def to_isbn10(number):
+    """Convert the number to ISBN-13 format."""
+    number = number.strip()
+    min_number = compact(number)
+    if len(min_number) == 10:
+        return number  # nothing to do, already ISBN-13
+    elif isbn_type(min_number) != 'ISBN13':
+        raise ValueError('Not a valid ISBN13.')
+    elif not number.startswith('978'):
+        raise ValueError('Does not use 978 Bookland prefix.')
+    # strip EAN prefix
+    number = number[3:-1].strip().strip('-')
+    digit = _calc_isbn10_check_digit(min_number[3:-1])
+    # append the new check digit
+    if ' ' in number:
+        return number + ' ' + digit
+    elif '-' in number:
+        return number + '-' + digit
+    else:
+        return number + digit
+
+
 def split(number, convert=False):
     """Split the specified ISBN into an EAN.UCC prefix, a group prefix, a
     registrant, an item number and a check-digit. If the number is in ISBN-10
diff --git a/tests/test_isbn.doctest b/tests/test_isbn.doctest
index f84c3fc..e759114 100644
--- a/tests/test_isbn.doctest
+++ b/tests/test_isbn.doctest
@@ -51,6 +51,26 @@ True
 '9781857982183'
 
 
+See if ISBN13 to 10 conversion works.
+
+>>> isbn.to_isbn10('1-85798-218-5')  # ISBN10 should stay ISBN10
+'1-85798-218-5'
+>>> isbn.to_isbn10('978 1 85798218 3')
+'1 85798218 5'
+>>> isbn.to_isbn10('9781857982183')
+'1857982185'
+>>> isbn.to_isbn10('978-1-85798-218-3')
+'1-85798-218-5'
+>>> isbn.to_isbn10('979-20-1234567-8')  # incorrect check digit
+Traceback (most recent call last):
+    ...
+ValueError: Not a valid ISBN13.
+>>> isbn.to_isbn10('9791843123391')
+Traceback (most recent call last):
+    ...
+ValueError: Does not use 978 Bookland prefix.
+
+
 Regrouping tests.
 
 >>> isbn.split('9024538270')  # normal ISBN10
-- 
cgit v1.2.3