Issue #19235 has been updated by zverok (Victor Shepelev). The `StringIO` doesn't have a proper concept of two encodings implemented :( Also reported here #16497 but still not fixed completely. I'll try to look into it this weekend. ---------------------------------------- Bug #19235: StringIO.new(encoding: "ext_enc:int_enc") doesn't set external encoding https://bugs.ruby-lang.org/issues/19235#change-100643 * Author: andrykonchin (Andrew Konchin) * Status: Open * Priority: Normal * ruby -v: 3.1.2 * Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN ---------------------------------------- I noticed a divergence between IO `:encoding` option documentation and StringIO's behaviour: ```ruby io = StringIO.new('', 'w', encoding: 'utf-8:ISO-8859-1') io.external_encoding # => #<Encoding:ISO-8859-1> ``` The IO class behaves in expected way: ```ruby f = File.open("test.txt", 'w', encoding: 'UTF-8:ISO-8859-1') f.external_encoding # => #<Encoding:UTF-8> ``` -- https://bugs.ruby-lang.org/