Understanding not in Python

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Python , , , . not



.

:

not True, False, False .

, ? , «» «», , , .

( , , « », , )

not

, , not



– UNARY_NOT. 

not a



:

>>> import dis
>>> def spam(): not a
... 
>>> dis.dis(spam)
  1           0 LOAD_GLOBAL              0 (a)
              2 UNARY_NOT
              4 POP_TOP
              6 LOAD_CONST               0 (None)
              8 RETURN_VALUE
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

UNARY_NOT , PyObject_IsTrue()



: True False, False True.

UNARY_NOT Python/ceval.c:

case TARGET(UNARY_NOT): {
            PyObject *value = TOP();
            int err = PyObject_IsTrue(value);
            Py_DECREF(value);
            if (err == 0) {
                Py_INCREF(Py_True);
                SET_TOP(Py_True);
                DISPATCH();
            }
            else if (err > 0) {
                Py_INCREF(Py_False);
                SET_TOP(Py_False);
                DISPATCH();
            }
            STACK_SHRINK(1);
            goto error;
        }
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

, True

not



, True. PyObject_IsTrue()



, , .

PyObject_IsTrue():

/* Test a value used as condition, e.g., in a for or if statement.
   Return -1 if an error occurred */


int
PyObject_IsTrue(PyObject *v)
{
    Py_ssize_t res;
    if (v == Py_True)
        return 1;
    if (v == Py_False)
        return 0;
    if (v == Py_None)
        return 0;
    else if (v->ob_type->tp_as_number != NULL &&
             v->ob_type->tp_as_number->nb_bool != NULL)
        res = (*v->ob_type->tp_as_number->nb_bool)(v);
    else if (v->ob_type->tp_as_mapping != NULL &&
             v->ob_type->tp_as_mapping->mp_length != NULL)
        res = (*v->ob_type->tp_as_mapping->mp_length)(v);
    else if (v->ob_type->tp_as_sequence != NULL &&
             v->ob_type->tp_as_sequence->sq_length != NULL)
        res = (*v->ob_type->tp_as_sequence->sq_length)(v);
    else
        return 1;
    /* if it is negative, it should be either -1 or -2 */
    return (res > 0) ? 1 : Py_SAFE_DOWNCAST(res, Py_ssize_t, int);
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

, :

1. True, True

   
   
   
   
   

2. False, False

   
   
   
   
   

3. None, False

   
   
   
   
   

4. , bool
   
   
   
   , , bool
   
   
   
   (, nb_bool
   
   
   
   )

   
   
   
   
   

5. len()
   
   
   
   (, mp_length



sq_length
   
   
   
   ):

   
   
   
   
   

   1. 0, True

      
      
      
      
      

   2. False

      
      
      
      
      

6. , True

   
   
   
   
   

1-3 6 , 4 5 .

bool

/ bool , « » True, False.

len()

len()



, . sq_length



( ) mp_length



( /).

, , .

len 

– / len



. , « , >= 0». , «» int



, , « »… « ». ?

index

« », / index



. , PyNumber_Index()



. , , :

1. int
   
   
   
   ,

   
   
   
   
   

2. index
   
   
   
   

   
   
   
   
   

3. index



int
   
   
   
   , ( , ).

   
   
   
   
   

4. TypeError
   
   
   
   .

   
   
   
   
   

Python operator.index()



. , PyNumber_Index()



, not



len



, . , :

PyNumber_Index()



Python:

def index(obj: Object, /) -> int:
    """Losslessly convert an object to an integer object.

    If obj is an instance of int, return it directly. Otherwise call __index__()
    and require it be a direct instance of int (raising TypeError if it isn't).
    """
    # https://github.com/python/cpython/blob/v3.8.3/Objects/abstract.c#L1260-L1302
    if isinstance(obj, int):
        return obj

    length_type = builtins.type(obj)
    try:
        __index__ = _mro_getattr(length_type, "__index__")
    except AttributeError:
        msg = (
            f"{length_type!r} cannot be interpreted as an integer "
            "(must be either a subclass of 'int' or have an __index__() method)"
        )
        raise TypeError(msg)
    index = __index__(obj)
    # Returning a subclass of int is deprecated in CPython.
    if index.__class__ is int:
        return index
    else:
        raise TypeError(
            f"the __index__() method of {length_type!r} returned an object of "
            f"type {builtins.type(index).__name__!r}, not 'int'"
        )
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

len()

len()



: int



. , , index()



len()



, PyLong_FromSsize_t()



, int



.

len()



, len()



index ()



, , int



, 0 .. , len()



:

def len(obj: Object, /) -> int:
    """Return the number of items in a container."""
    # https://github.com/python/cpython/blob/v3.8.3/Python/bltinmodule.c#L1536-L1557
    # https://github.com/python/cpython/blob/v3.8.3/Objects/abstract.c#L45-L63
    # https://github.com/python/cpython/blob/v3.8.3/Objects/typeobject.c#L6184-L6209
    type_ = builtins.type(obj)
    try:
        __len__ = _mro_getattr(type_, "__len__")
    except AttributeError:
        raise TypeError(f"type {type!r} does not have a __len__() method")
    length = __len__(obj)
    # Due to len() using PyObject_Size() (which returns Py_ssize_t),
    # the returned value is always a direct instance of int via
    # PyLong_FromSsize_t().
    index = int(_index(length))
    if index < 0:
        raise ValueError("__len__() should return >= 0")
    else:
        return index
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

operator.truth()

not, not



–  not not



. , , , , , .

Python . bool()



( bool.new()



), , , , operator.truth()



. , , PyObject_IsTrue()



. slot_nb_bool ()



, , , PyObject_IsTrue()



. , PyObject_IsTrue()



, , . 

, , operator.truth()



( bool



, , True False , 1 0 Python).

operator.truth()



:

def truth(obj: Any, /) -> bool:
    """Return True if the object is true, False otherwise.

    Analogous to calling bool().

    """
    if obj is True:
        return True
    elif obj is False:
        return False
    elif obj is None:
        return False
    obj_type = type(obj)
    try:
        __bool__ = debuiltins._mro_getattr(obj_type, "__bool__")
    except AttributeError:
        # Only try calling len() if it makes sense.
        try:
            __len__ = debuiltins._mro_getattr(obj_type, "__len__")
        except AttributeError:
            # If all else fails...
            return True
        else:
            return True if debuiltins.len(obj) > 0 else False
    else:
        boolean = __bool__(obj)
        if isinstance(boolean, bool):
            # Coerce into True or False.
            return truth(boolean)
        else:
            raise TypeError(
                f"expected a 'bool' from {obj_type.__name__}.__bool__(), "
                f"not {type(boolean).__name__!r}"
            )
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

not

operator.truth()



, operator.not_()



– :

lambda a, /: False if truth(a) else True
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

, , , .

, desugar. 

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