>>> def sub(): a - b 
... 
>>> import dis 
>>> dis.dis(sub) 
 1      0 LOAD_GLOBAL       0 (a) 
       2 LOAD_GLOBAL       1 (b) 
       4 BINARY_SUBTRACT 
       6 POP_TOP 
       8 LOAD_CONST        0 (None) 
       10 RETURN_VALUE

case TARGET(BINARY_SUBTRACT): { 
  PyObject *right = POP(); 
  PyObject *left = TOP(); 
  PyObject *diff = PyNumber_Subtract(left, right); 
  Py_DECREF(right); 
  Py_DECREF(left); 
  SET_TOP(diff); 
  if (diff == NULL) 
  goto error; 
  DISPATCH(); 
}

# 通過調(diào)用__sub__()實現(xiàn)減法 
def sub(lhs: Any, rhs: Any, /) -> Any: 
  """Implement the binary operation `a - b`.""" 
  lhs_type = type(lhs) 
  try: 
    subtract = _mro_getattr(lhs_type, "__sub__") 
  except AttributeError: 
    msg = f"unsupported operand type(s) for -: {lhs_type!r} and {type(rhs)!r}" 
    raise TypeError(msg) 
  else: 
    return subtract(lhs, rhs)

# 減法的實現(xiàn)，其中表達(dá)式的左側(cè)和右側(cè)均可參與運(yùn)算 
_MISSING = object() 
 
def sub(lhs: Any, rhs: Any, /) -> Any: 
    # lhs.__sub__ 
    lhs_type = type(lhs) 
    try: 
      lhs_method = debuiltins._mro_getattr(lhs_type, "__sub__") 
    except AttributeError: 
      lhs_method = _MISSING 
 
    # lhs.__rsub__ (for knowing if rhs.__rub__ should be called first) 
    try: 
      lhs_rmethod = debuiltins._mro_getattr(lhs_type, "__rsub__") 
    except AttributeError: 
      lhs_rmethod = _MISSING 
 
    # rhs.__rsub__ 
    rhs_type = type(rhs) 
    try: 
      rhs_method = debuiltins._mro_getattr(rhs_type, "__rsub__") 
    except AttributeError: 
      rhs_method = _MISSING 
 
    call_lhs = lhs, lhs_method, rhs 
    call_rhs = rhs, rhs_method, lhs 
 
    if lhs_type is not rhs_type: 
      calls = call_lhs, call_rhs 
    else: 
      calls = (call_lhs,) 
 
    for first_obj, meth, second_obj in calls: 
      if meth is _MISSING: 
        continue 
      value = meth(first_obj, second_obj) 
      if value is not NotImplemented: 
        return value 
    else: 
      raise TypeError( 
        f"unsupported operand type(s) for -: {lhs_type!r} and {rhs_type!r}" 
      )

# Python中減法的完整實現(xiàn) 
_MISSING = object() 
 
def sub(lhs: Any, rhs: Any, /) -> Any: 
    # lhs.__sub__ 
    lhs_type = type(lhs) 
    try: 
      lhs_method = debuiltins._mro_getattr(lhs_type, "__sub__") 
    except AttributeError: 
      lhs_method = _MISSING 
 
    # lhs.__rsub__ (for knowing if rhs.__rub__ should be called first) 
    try: 
      lhs_rmethod = debuiltins._mro_getattr(lhs_type, "__rsub__") 
    except AttributeError: 
      lhs_rmethod = _MISSING 
 
    # rhs.__rsub__ 
    rhs_type = type(rhs) 
    try: 
      rhs_method = debuiltins._mro_getattr(rhs_type, "__rsub__") 
    except AttributeError: 
      rhs_method = _MISSING 
 
    call_lhs = lhs, lhs_method, rhs 
    call_rhs = rhs, rhs_method, lhs 
 
    if ( 
      rhs_type is not _MISSING # Do we care? 
      and rhs_type is not lhs_type # Could RHS be a subclass? 
      and issubclass(rhs_type, lhs_type) # RHS is a subclass! 
      and lhs_rmethod is not rhs_method # Is __r*__ actually different? 
    ): 
      calls = call_rhs, call_lhs 
    elif lhs_type is not rhs_type: 
      calls = call_lhs, call_rhs 
    else: 
      calls = (call_lhs,) 
 
    for first_obj, meth, second_obj in calls: 
      if meth is _MISSING: 
        continue 
      value = meth(first_obj, second_obj) 
      if value is not NotImplemented: 
        return value 
    else: 
      raise TypeError( 
        f"unsupported operand type(s) for -: {lhs_type!r} and {rhs_type!r}" 
      )

# 一個創(chuàng)建閉包的函數(shù)，實現(xiàn)了二元運(yùn)算的邏輯 
_MISSING = object() 
 
 
def _create_binary_op(name: str, operator: str) -> Any: 
  """Create a binary operation function. 
 
  The `name` parameter specifies the name of the special method used for the 
  binary operation (e.g. `sub` for `__sub__`). The `operator` name is the 
  token representing the binary operation (e.g. `-` for subtraction). 
 
  """ 
 
  lhs_method_name = f"__{name}__" 
 
  def binary_op(lhs: Any, rhs: Any, /) -> Any: 
    """A closure implementing a binary operation in Python.""" 
    rhs_method_name = f"__r{name}__" 
 
    # lhs.__*__ 
    lhs_type = type(lhs) 
    try: 
      lhs_method = debuiltins._mro_getattr(lhs_type, lhs_method_name) 
    except AttributeError: 
      lhs_method = _MISSING 
 
    # lhs.__r*__ (for knowing if rhs.__r*__ should be called first) 
    try: 
      lhs_rmethod = debuiltins._mro_getattr(lhs_type, rhs_method_name) 
    except AttributeError: 
      lhs_rmethod = _MISSING 
 
    # rhs.__r*__ 
    rhs_type = type(rhs) 
    try: 
      rhs_method = debuiltins._mro_getattr(rhs_type, rhs_method_name) 
    except AttributeError: 
      rhs_method = _MISSING 
 
    call_lhs = lhs, lhs_method, rhs 
    call_rhs = rhs, rhs_method, lhs 
 
    if ( 
      rhs_type is not _MISSING # Do we care? 
      and rhs_type is not lhs_type # Could RHS be a subclass? 
      and issubclass(rhs_type, lhs_type) # RHS is a subclass! 
      and lhs_rmethod is not rhs_method # Is __r*__ actually different? 
    ): 
      calls = call_rhs, call_lhs 
    elif lhs_type is not rhs_type: 
      calls = call_lhs, call_rhs 
    else: 
      calls = (call_lhs,) 
 
    for first_obj, meth, second_obj in calls: 
      if meth is _MISSING: 
        continue 
      value = meth(first_obj, second_obj) 
      if value is not NotImplemented: 
        return value 
    else: 
      exc = TypeError( 
        f"unsupported operand type(s) for {operator}: {lhs_type!r} and {rhs_type!r}" 
      ) 
      exc._binary_op = operator 
      raise exc