Louie Lu added the comment:
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".
----------
nosy: +louielu
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<http://bugs.python.org/issue30953>
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