You will need to use `dis` to see what actually done by bytecode.


$ ./python -m dis tests.py

Disassembly of <code object error at 0x7fbb69da5340, file "tests.py", line 8>:
  9           0 SETUP_EXCEPT             4 (to 6)

 10           2 POP_BLOCK
              4 JUMP_FORWARD            12 (to 18)

 11     >>    6 POP_TOP
              8 POP_TOP
             10 POP_TOP

 12          12 POP_EXCEPT
             14 JUMP_FORWARD             2 (to 18)
             16 END_FINALLY

 14     >>   18 LOAD_CONST               0 (None)
             20 RETURN_VALUE


this is the actual bytecode in `error()`.

When the eval loop hit the line 9, it will perform bytecode `SETUP_EXCEPT`, thus push a block into the blockstack. 
Your code in trace wrote that `and frame.f_lineno > 12`, thus it will run line 12 and perform POP_EXCEPT to pop out the blockstack.

then goto line 13, catch by trace if statement, f_lineno changed to 12, and perform `POP_EXCEPT` again, there isn't any block inside the blockstack, thus it will get a "XXX block stack underflow".