This is the stack difference for my previous experiment:

   * frame #0: 0x00000001001578ee python.exe`unicode_length(self=0x000000010140d220) at unicodeobject.c:12231:9
    frame #1: 0x000000010000b453 python.exe`PyObject_Size(o=0x000000010140d220) at abstract.c:57:26
    frame #2: 0x00000001001cea29 python.exe`builtin_len(module=0x00000001006ad280, obj=0x000000010140d220) at bltinmodule.c:1548:11
    frame #3: 0x0000000100044518 python.exe`_PyMethodDef_RawFastCallKeywords(method=0x00000001003e5540, self=0x00000001006ad280, args=0x00000001013f0e10, nargs=1, kwnames=0x0000000000000000) at call.c:656:18
    frame #4: 0x000000010004487a python.exe`_PyCFunction_FastCallKeywords(func=0x00000001006af210, args=0x00000001013f0e10, nargsf=9223372036854775809, kwnames=0x0000000000000000) at call.c:743:14
    frame #5: 0x00000001001edc09 python.exe`_PyObject_Vectorcall(callable=0x00000001006af210, args=0x00000001013f0e10, nargsf=9223372036854775809, kwnames=0x0000000000000000) at abstract.h:128:21
    frame #6: 0x00000001001eaa6a python.exe`call_function(tstate=0x0000000101103910, pp_stack=0x00007ffeefbfdc50, oparg=1, kwnames=0x0000000000000000) at ceval.c:4859:13
    frame #7: 0x00000001001e5436 python.exe`_PyEval_EvalFrameDefault(f=0x00000001013f0c90, throwflag=0) at ceval.c:3356:19


-------


  * frame #0: 0x0000000100153f7e python.exe`unicode_length(self=0x0000000101526580) at unicodeobject.c:12231:9
    frame #1: 0x000000010000a623 python.exe`PyObject_Size(o=0x0000000101526580) at abstract.c:57:26
    frame #2: 0x00000001001cab69 python.exe`builtin_len(module=0x00000001006a4280, obj=0x0000000101526580) at bltinmodule.c:1548:11
    frame #3: 0x0000000100042d28 python.exe`_PyMethodDef_RawFastCallKeywords(method=0x00000001003deff0, self=0x00000001006a4280, args=0x000000010150fe10, nargs=1, kwnames=0x0000000000000000) at call.c:650:18
    frame #4: 0x000000010004144d python.exe`_PyCFunction_FastCallKeywords(func=0x00000001006a6210, args=0x000000010150fe10, nargs=1, kwnames=0x0000000000000000) at call.c:736:14
    frame #5: 0x00000001001e6c5e python.exe`call_function(tstate=0x0000000101103910, pp_stack=0x00007ffeefbfdc40, oparg=1, kwnames=0x0000000000000000) at ceval.c:4825:9
    frame #6: 0x00000001001e1561 python.exe`_PyEval_EvalFrameDefault(f=0x000000010150fc90, throwflag=0) at ceval.c:3356:19