[FIXED] What is the `ExceptionTable` in the output of `dis`?

Issue

In python3.13, when I try to disassemble [i for i in range(10)], the result is as below:

>>> import dis
>>> 
>>> dis.dis('[i for i in range(10)]')
   0           RESUME                   0

   1           LOAD_NAME                0 (range)
               PUSH_NULL
               LOAD_CONST               0 (10)
               CALL                     1
               GET_ITER
               LOAD_FAST_AND_CLEAR      0 (i)
               SWAP                     2
       L1:     BUILD_LIST               0
               SWAP                     2
       L2:     FOR_ITER                 4 (to L3)
               STORE_FAST_LOAD_FAST     0 (i, i)
               LIST_APPEND              2
               JUMP_BACKWARD            6 (to L2)
       L3:     END_FOR
       L4:     SWAP                     2
               STORE_FAST               0 (i)
               RETURN_VALUE

  --   L5:     SWAP                     2
               POP_TOP

   1           SWAP                     2
               STORE_FAST               0 (i)
               RERAISE                  0
ExceptionTable:
  L1 to L4 -> L5 [2]

At the end of the output, there's something ExceptionTable. It does not exist in the previous versions of python.

Python 3.10.0b1 (default, May  4 2021, 00:00:00) [GCC 10.2.1 20201125 (Red Hat 10.2.1-9)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import dis
>>> 
>>> dis.dis('[i for i in range(10)]')
  1           0 LOAD_CONST               0 (<code object <listcomp> at 0x7f3d412503a0, file "<dis>", line 1>)
              2 LOAD_CONST               1 ('<listcomp>')
              4 MAKE_FUNCTION            0
              6 LOAD_NAME                0 (range)
              8 LOAD_CONST               2 (10)
             10 CALL_FUNCTION            1
             12 GET_ITER
             14 CALL_FUNCTION            1
             16 RETURN_VALUE

Disassembly of <code object <listcomp> at 0x7f3d412503a0, file "<dis>", line 1>:
  1           0 BUILD_LIST               0
              2 LOAD_FAST                0 (.0)
        >>    4 FOR_ITER                 4 (to 14)
              6 STORE_FAST               1 (i)
              8 LOAD_FAST                1 (i)
             10 LIST_APPEND              2
             12 JUMP_ABSOLUTE            2 (to 4)
        >>   14 RETURN_VALUE

I can't understand what that means, also I couldn't find any document for this.

Solution

ExceptionTable determines where to jump to when an exception is raised(it was implemented in python-3.11). Prior version uses separate opcodes to handle this. The advantage of this approach is that entering and leaving a try block normally does not execute any code, making execution faster.

To access this table, you can do so by accessing the co_exceptiontable attribute of the code object. The ExceptionTable representation returned by dis is the result of parsing this table(code->co_exceptiontable).

>>> def foo():
...     c = 1 + 2
...     return c
... 
>>> 
>>> foo.__code__.co_exceptiontable
b''
>>> def foo():
...     try:
...             1/0
...     except:
...             pass
... 
>>> foo.__code__.co_exceptiontable
b'\x82\x05\x08\x00\x88\x02\x0c\x03'
>>>
>>> from dis import _parse_exception_table
>>>
>>> _parse_exception_table(foo.__code__)
[_ExceptionTableEntry(start=4, end=14, target=16, depth=0, lasti=False), _ExceptionTableEntry(start=16, end=20, target=24, depth=1, lasti=True)]

A detailed explanation about ExceptionTable is available here. Quoting from the same link:

python-3.11 uses what is known as "zero-cost" exception handling. Prior to python-3.11, exceptions were handled by a runtime stack of "blocks".

In zero-cost exception handling, the cost of supporting exceptions is minimized. In the common case (where no exception is raised) the cost is reduced to zero (or close to zero). The cost of raising an exception is increased, but not by much.

The following code:

def f():
    try:
        g(0)
    except:
        return "fail"

compiles as follows in 3.10:

  2           0 SETUP_FINALLY            7 (to 16)

  3           2 LOAD_GLOBAL              0 (g)
              4 LOAD_CONST               1 (0)
              6 CALL_NO_KW               1
              8 POP_TOP
             10 POP_BLOCK
             12 LOAD_CONST               0 (None)
             14 RETURN_VALUE

  4     >>   16 POP_TOP
             18 POP_TOP
             20 POP_TOP

  5          22 POP_EXCEPT
             24 LOAD_CONST               3 ('fail')
             26 RETURN_VALUE

Note the explicit instructions to push and pop from the "block" stack: SETUP_FINALLY and POP_BLOCK.

In 3.11, the SETUP_FINALLY and POP_BLOCK are eliminated, replaced with a table to determine where to jump to when an exception is raised.

  1           0 RESUME                   0

  2           2 NOP

  3           4 LOAD_GLOBAL              1 (g + NULL)
             16 LOAD_CONST               1 (0)
             18 PRECALL                  1
             22 CALL                     1
             32 POP_TOP
             34 LOAD_CONST               0 (None)
             36 RETURN_VALUE
        >>   38 PUSH_EXC_INFO

  4          40 POP_TOP

  5          42 POP_EXCEPT
             44 LOAD_CONST               2 ('fail')
             46 RETURN_VALUE
        >>   48 COPY                     3
             50 POP_EXCEPT
             52 RERAISE                  1
ExceptionTable:
  4 to 32 -> 38 [0]
  38 to 40 -> 48 [1] lasti

(Note this code is from python-3.11, later versions may have slightly different bytecode.)

If an instruction raises an exception then its offset is used to find the target to jump to. For example, the CALL at offset 22, falls into the range 4 to 32. So, if g() raises an exception, then control jumps to offset 38.

Answered By - Abdul Niyas P M