如何创建类的静态实例，当它会导致构造函数中的异常?

Question

我有以下代码:

class Position:
def __int__(self, x: int, y: int):
if out_of_range(x, 0, MAX_INT - 1):
raise ValueError("[error][{}.{}()] Failed to create position: {}n".format(__class__,
                 inspect.stack()[0].function,
                 "x must be in range [0, MAX_INT)"
                 ))
if out_of_range(y, 0, MAX_INT - 1):
raise ValueError("[error][{}.{}()] Failed to create position: {}n".format(__class__,
                 inspect.stack()[0].function,
                 "y must be in range [0, MAX_INT)"
                 ))
self.__x: int = x
self.__y: int = y

我想创建一个Position的静态实例，x和y都等于MAX_INT。
我该如何编写代码才能做到这一点:

if Position(2, 3) == Position.INVALID:  # INVALID <=> Position(MAX_INT, MAX_INT)
...

编辑:

我在Position类中有另一个方法:

def to(self, direction: Direction, steps: int = 1) -> 'Position':
"""
Creates a new position, to the given direction.
If one of the new coordinates goes below 0, then Position.INVALID is returned.
A negative 'steps' value inverses the direction. Ex: Position.to(RIGHT, 3) == Position.to(LEFT, -3)
"""
delta_x, delta_y = DIRECTION_DELTA[direction]
try:
return Position(self.__x + delta_x * steps, self.__y + delta_y * steps)
except ValueError:
return Position.INVALID

更具体地说，我的预期用法是

if Position(2, 3).to(LEFT, 3) == Position.INVALID:
do something
else:
do something else

第一种方法是添加另一个方法来检查一个位置是否有效。

def to(self, direction: Direction, steps: int = 1) -> 'Position':
"""
Creates a new position, to the given direction.
If one of the new coordinates goes below 0, then Position.INVALID is returned.
A negative 'steps' value inverses the direction. Ex: Position.to(RIGHT, 3) == Position.to(LEFT, -3)
"""
delta_x, delta_y = DIRECTION_DELTA[direction]
try:
return Position(self.__x + delta_x * steps, self.__y + delta_y * steps)
except ValueError:
position: Position = Position(0, 0)
position.__x = MAX_INT
position.__y = MAX_INT
return position

def is_valid() -> bool:
return not self.__x == self.__y == MAX_INT

但是现在，Position(MAX_INT, MAX_INT)不再是单例。

第二种方法是让方法to()从构造函数抛出错误，并在Position.to()之外捕获它。

Answer 1

不确定这是否是您需要的，但使用装饰器@classmethod和定义__eq__，您可以创建一个实例与x=MAX_INT并将其与您的实例进行比较:

class Position:
def __init__(self, x: int):
if x > MAX_INT:
raise ValueError
self.__x: int = x
def __eq__(self, other):
if self.__x == other.__x:
return True
return False
@classmethod
def INVALID(cls):
p = Position(0)
p.__x = MAX_INT
return p
def to(self, direction: Direction, steps: int = 1) -> 'Position': 
delta_x, delta_y = DIRECTION_DELTA[direction]
try:
return Position(self.__x + delta_x * steps, self.__y + delta_y * steps)
except ValueError:
return Position.INVALID()
if Position(MAX_INT-1) == Position.INVALID():
print('invalid')
else:
print('valid')