Debug the CPython opcode stack

CPython 3.7 introduced the ability to step through individual opcodes in a debugger. However, I can't figure out how to read variables out of the bytecode stack.

For example, when debugging

def f(a, b, c):
    return a * b + c

f(2, 3, 4)

I want to find out that the inputs of the addition are 6 and 4. Note how 6 never touches locals().

So far I could only come up with the opcode information, but I don't know how to get the opcode inputs:

import dis
import sys


def tracefunc(frame, event, arg):
    frame.f_trace_opcodes = True
    print(event, frame.f_lineno, frame.f_lasti, frame, arg)
    if event == "call":
        dis.dis(frame.f_code)
    elif event == "opcode":
        instr = next(
            i for i in iter(dis.Bytecode(frame.f_code))
            if i.offset == frame.f_lasti
        )
        print(instr)
    print("-----------")
    return tracefunc


def f(a, b, c):
    return a * b + c


sys.settrace(tracefunc)
f(2, 3, 4)

Output:

call 19 -1 <frame at 0x7f97df618648, file 'test_trace.py', line 19, code f> None
 20           0 LOAD_FAST                0 (a)
              2 LOAD_FAST                1 (b)
              4 BINARY_MULTIPLY
              6 LOAD_FAST                2 (c)
              8 BINARY_ADD
             10 RETURN_VALUE
-----------
line 20 0 <frame at 0x7f97df618648, file 'test_trace.py', line 20, code f> None
-----------
opcode 20 0 <frame at 0x7f97df618648, file 'test_trace.py', line 20, code f> None
Instruction(opname='LOAD_FAST', opcode=124, arg=0, argval='a', argrepr='a', offset=0, starts_line=20, is_jump_target=False)
-----------
opcode 20 2 <frame at 0x7f97df618648, file 'test_trace.py', line 20, code f> None
Instruction(opname='LOAD_FAST', opcode=124, arg=1, argval='b', argrepr='b', offset=2, starts_line=None, is_jump_target=False)
-----------
opcode 20 4 <frame at 0x7f97df618648, file 'test_trace.py', line 20, code f> None
Instruction(opname='BINARY_MULTIPLY', opcode=20, arg=None, argval=None, argrepr='', offset=4, starts_line=None, is_jump_target=False)
-----------
opcode 20 6 <frame at 0x7f97df618648, file 'test_trace.py', line 20, code f> None
Instruction(opname='LOAD_FAST', opcode=124, arg=2, argval='c', argrepr='c', offset=6, starts_line=None, is_jump_target=False)
-----------
opcode 20 8 <frame at 0x7f97df618648, file 'test_trace.py', line 20, code f> None
Instruction(opname='BINARY_ADD', opcode=23, arg=None, argval=None, argrepr='', offset=8, starts_line=None, is_jump_target=False)
-----------
opcode 20 10 <frame at 0x7f97df618648, file 'test_trace.py', line 20, code f> None
Instruction(opname='RETURN_VALUE', opcode=83, arg=None, argval=None, argrepr='', offset=10, starts_line=None, is_jump_target=False)
-----------
return 20 10 <frame at 0x7f97df618648, file 'test_trace.py', line 20, code f> 10
-----------

from Debug the CPython opcode stack