Issue #7837 has been updated by nobu (Nobuyoshi Nakada). Status changed from Open to Rejected It's common behavior to keyword arguments, $ ruby -e 'def foo(a, b, c:1);end' -e 'foo(2,3,42)' -e:1:in `foo': wrong number of arguments (3 for 2) (ArgumentError) from -e:2:in `<main>' The optional argument hash is stripped before arity check, so it's "invisible" as arity. ---------------------------------------- Bug #7837: IO.open with three arguments where third argument is bogus reports an arity problem instead of type problem https://bugs.ruby-lang.org/issues/7837#change-36196 Author: enebo (Thomas Enebo) Status: Rejected Priority: Normal Assignee: Category: core Target version: next minor ruby -v: ruby 2.0.0dev (2013-02-08 trunk 39161) [x86_64-darwin11.4.2] I am reporting this against 2.0.rc2 but it also applies to mri 1.9.3. If I run the following one-liner: mri20 -e 'f = File.open("tmp"); IO.open(f.fileno, "r", :heh)' I get the error: -e:1:in `initialize': wrong number of arguments (3 for 1..2) (ArgumentError) This seems wrong because if I change the third argument to an options hash it works fine. So having an arity of 3 is not a problem for open. It is that :heh is not a hash. I think this should generate a TypeError. -- http://bugs.ruby-lang.org/