Add An+B serialization alongside parse_nth

docs/api_reference.rst

@@ -74,6 +74,7 @@ Color Level 5
 
 .. module:: tinycss2.nth
 .. autofunction:: parse_nth
+.. autofunction:: serialize_nth
 
 
 AST nodes

tests/test_tinycss2.py

@@ -21,7 +21,7 @@
 from tinycss2.color4 import Color  # isort:skip
 from tinycss2.color4 import parse_color as parse_color4  # isort:skip
 from tinycss2.color5 import parse_color as parse_color5  # isort:skip
-from tinycss2.nth import parse_nth  # isort:skip
+from tinycss2.nth import parse_nth, serialize_nth  # isort:skip
 
 
 def generic(func):
@@ -153,6 +153,22 @@ def test_nth(input):
     return parse_nth(input)
 
 
+@pytest.mark.parametrize(('a', 'b', 'expected'), [
+    (0, 0, '0'), (0, 1, '1'), (0, -2, '-2'), (1, 0, 'n'), (1, 3, 'n+3'), (1, -3, 'n-3'),
+    (-1, 0, '-n'), (-1, 3, '-n+3'), (-1, -3, '-n-3'), (2, 0, '2n'), (2, 1, '2n+1'),
+    (2, -4, '2n-4'), (-2, 0, '-2n'), (-2, 1, '-2n+1'), (-2, -4, '-2n-4'),
+])
+def test_serialize_nth(a, b, expected):
+    assert serialize_nth(a, b) == expected
+    assert parse_nth(expected) == (a, b)
+
+
+@json_test(filename='An+B.json')
+def test_round_trip_nth(input):
+    if numbers := parse_nth(input):
+        return parse_nth(serialize_nth(*numbers))
+
+
 @pytest.mark.parametrize('invalid', ['+', '+/**/', 'n+', 'n +', '-n-', '2n +'])
 def test_nth_invalid_does_not_crash(invalid):
     # Truncated/invalid An+B fragments must return None per parse_nth's

tinycss2/nth.py

@@ -74,6 +74,20 @@ def parse_nth(input):
                     return parse_end(tokens, 1, int(match.group(1)))
 
 
+
+def serialize_nth(a, b):
+    """Serialize `<An+B> <https://drafts.csswg.org/css-syntax/#serializing-anb>`_.
+
+    :param int a: The step coefficient.
+    :param int b: The offset.
+    :returns: An ``an+b`` string.
+
+    """
+    an = 'n' if a == 1 else '-n' if a == -1 else f'{a}n' if a else ''
+    b = f'{b}' if not a else f'{b:+}' if b else ''
+    return an + b
+
+
 def parse_b(tokens, a):
     token = _next_significant(tokens)
     if token is None: