Why does comparing two equal strings with is sometimes fail?

is compares object identity; == compares values according to the type’s equality rules. Interpreter interning can make identity tests appear to work accidentally. Use a == b for text equality and reserve is for identity checks such as value is None or a private sentinel. Build test strings dynamically rather than relying only on identical literals that may be interned. Unicode strings can still look the same while containing different code points; if normalization is part of your contract, apply it deliberately. Do not use identity behavior observed in one interpreter build as a portable optimization.