According to this Python code visualizer, Python strings are allocated on the stack and not on the heap.
Why is this? I thought they would be similar to Java where Strings are allocated on the heap.
That visualizer is not showing the string data on the stack. It is showing the local references to the heap data as part of the call stack. This is very similar to Java where
The visualizer is free to make any kind of representation simplification it cares to. It does not imply that "Python strings are allocated on the stack" in any given implementation of Python.
Has Greg said in his answer, strings are allocated on the heap.
The visualizer has choosen to display some objects in the "frame" part, which seems wrong to me. Fortunately, it has little impact for someone learning python, since affected objects (int, string) are immutable. So reasoning about effects is still possible.
However it gives the false impression that strings are duplicated in memory when you do things like: