dis module

Dis

Output interpretation

This stackoverflow post has a great diagram: For a sample code

from asyncio import sleep
async def f():
    await sleep(0)

We have below disassembled bytecode.

(1)|(2)|(3)|(4)|          (5)               |(6)|  (7)
---|---|---|---|----------------------------|---|-------
  2|   |   |  0| RETURN_GENERATOR           |   |
   |   |   |  2| POP_TOP                    |   |
   |   |   |  4| RESUME                     | 0 |
  3|   |   |  6| LOAD_GLOBAL                | 1 | (NULL + sleep)
   |   |   | 18| LOAD_CONST                 | 1 | (0)
   |   |   | 20| PRECALL                    | 1 |
   |   |   | 24| CALL                       | 1 |
   |-->|   | 34| GET_AWAITABLE              | 0 |
   |   |   | 36| LOAD_CONST                 | 0 | (None)
   |   |>> | 38| SEND                       | 3 | (to 46)
   |   |   | 40| YIELD_VALUE                |   |
   |   |   | 42| RESUME                     | 3 |
   |   |   | 44| JUMP_BACKWARD_NO_INTERRUPT | 4 | (to 38)
   |   |>> | 46| POP_TOP                    |   |
   |   |   | 48| LOAD_CONST                 | 0 | (None)
   |   |   | 50| RETURN_VALUE               |   |


(1): line number
(2): current instruction executed
(3): possible jump target
(4): bytecode address
(5): opname
(6): argument
(7): human-fridenly argument interpretation

As always, I would ask where the code is? Here is code.

Let’s talk about column 3 and 7. Column 3 means that this line may have a jump. And column 7 tells the argument, most likely the jump offset. In the bracket, it interprets this jump to a bytecode address in the form (to xxx). The question is how does it know whether a opcode has jump code or not. Here is the relevant code. Basically, Cpython maintains two dictionaries of opcodes with potential jumps: hasjrel and hasjabs; namely Has Jump Relative and Has Jump Absolute.

Note, I mean potential jumps. A line like

|   |>> | 38| SEND                       | 3 | (to 46)

means SEND command may jump to address 46. If does not jump, then it will execute the next opcode. you can read relevant code in ceval.c to figure out when SEND jumps.