从超类返回 str 时出错

Question

我的代码输出不准确。当它调用

super().__str__()

输出是

<__main__.Trip object at 0x00000251D30D5A30>

输出应该是

Driver Id: 1 Name: John Contact: 82121355

from datetime import datetime

class Driver:
    _nextId = 1
    def __init__(self,name,contact):
        self._name = name
        self._contact = contact
        self._driverId = Driver._nextId
        Driver._nextId += 1

    @property
    def driverId(self):
        return self._driverId
    @property 
    def name(self):
        return self._name
    @property
    def contact(self):
        return self._contact
    @contact.setter
    def contact(self, newContact):
        self._contact = newContact

    def __str__(self):
        return f'Driver Id: {self._driverId} Name: {self._name} Contact: {self._contact}'

class Trip:
    def __init__(self,tripDate,startPoint,destination,distance,driver):
        self._tripDate = tripDate
        self._startPoint = startPoint
        self._destination = destination
        self._distance = distance
        self._driver = driver

    @property
    def tripDate(self):
        return self._tripDate
    @property
    def destination(self):
        return self._destination
    @property
    def driver(self):
        return self._driver
    
    def __str__(self):
        return f'{self._tripDate}, From: {self._startPoint} To: {self._destination}\n Distance: {self._distance}km' + super().__str__()

if __name__ == '__main__':
    d1 = Driver('John','82121355')
    t1 = Trip(datetime(2021,5,30,17,45),'Empire State Building','Rockerfeller Centre','2.25',d1)
    print(t1)

Answer 1

您的 Trip 代码中的问题：

def __str__(self):
    return f'...' + super().__str__()

是 Trip 不是 Driver 的子 class 也没有从 Driver 继承任何东西。 super 调用将不会调用 Driver 的 __str__ 方法，但是 default/built-in object.__str__(self) 会调用所有 Python classes:

>>> class XYZ: pass
... 
>>> obj1 = XYZ()
>>> print(obj1)
<__main__.XYZ object at 0x10e28b040>

只有当您的 class 是另一个 class 的子class 时，super() 才有效：

>>> class Animal:
...   def __str__(self):
...     return 'Animal __str__'
... 

>>> class Dog(Animal):
...   def __str__(self):
...     return f'{super().__str__()} + Dog __str__'
... 

>>> d = Dog()
>>> print(d)
Animal __str__ + Dog __str__

我不知道你为什么期望 Trip 的 superclass 是 Driver，因为旅行不是 driver，而是 Trip 涉及 Driver，因此您当前使用 driver 实例化旅行的实现很有意义。

您唯一需要更改的是将 super() 替换为 self._driver，这是您传递给 Trip.[=38= 的 Driver 的一个实例]

# super().__str__() --> self._driver.__str__()

def __str__(self):
    return f'{self._tripDate}, From: {self._startPoint} To: {self._destination}\n Distance: {self._distance}km' + self._driver.__str__()

2021-05-30 17:45:00, From: Empire State Building To: Rockerfeller Centre
 Distance: 2.25kmDriver Id: 1 Name: John Contact: 82121355

或更简单地说：

# super().__str__() --> str(self._driver)
# And put it inside the f-string

def __str__(self):
    return f'{self._tripDate}, From: {self._startPoint} To: {self._destination}\n Distance: {self._distance}km {str(self._driver)}'

2021-05-30 17:45:00, From: Empire State Building To: Rockerfeller Centre
 Distance: 2.25km Driver Id: 1 Name: John Contact: 82121355

因为 Python 的 str(obj) calls that object's __str__ method.

从超类返回 str 时出错

error in returning str from superclass

python

oop

superclass

从超类返回 __str__ 时出错

error in returning __str__ from superclass

python

oop

superclass

从超类返回 str 时出错

error in returning str from superclass