This line rang a bell to me. With no other changes to the class, it means that the arguments argument is now expected to be formatted differently, which breaks backward compatibility. Consider the following diff:

diff --git cssselect/parser.py cssselect/parser.py
index 4351f07..48e7604 100644
--- cssselect/parser.py
+++ cssselect/parser.py
@@ -160,6 +160,7 @@ class FunctionalPseudoElement(object):
 def __init__(self, name, arguments):
 self.name = ascii_lower(name)
 self.arguments = arguments
+ print("arguments:", arguments)
 
 def __repr__(self):
 return "%s[::%s(%r)]" % (

Then at the current master branch (9edc6c3):

In [1]: from cssselect import parse
In [2]: p = parse("a::asdf(arg1)")
arguments: [<IDENT 'arg1' at 8>]

While at the current add_support_for_of_s_part_in_nth_child branch (058b6b5):

In [1]: from cssselect import parse
In [2]: p = parse("a::asdf(arg1)")
arguments: ([<IDENT 'arg1' at 8>], None)

Could you explain the rationale of this change? Thanks!

You're right, the backward compatibility breaks, because I changed signature of method parse_arguments which is used both for functions and functional pseudo elements.
Now I splitted parsing arguments into separate methods, so nothing will break
Nice catch!