如何在我的子类中重新定义 append 方法？

Question

Implement the LoggableList class, inheriting it from the list and Loggable classes, so that when you add an item to a list using the append method, a message is sent to the log consisting of just the added item. How can i redefine append method in class LoggableList to call log method of Loggable class inside append method in LoggableList class.

Here is my try to write a code:

import time

class Loggable:

    def log(self, msg):
        print(str(time.ctime()) + ": " + str(msg))
        return


class LoggableList(Loggable, list):

    def __init__(self, data):
        super().__init__()
        self.data = list(input('Please input your massage: '))

    def append(self, element):
        super(LoggableList, self).append(element)
        Loggable.log()
        return

Answer 1

0. You can use the shortened version of super :

    super().append(element)

1. self.log() , not Loggable.log() .

2. You forgot to provide the argument msg .

3. Instead of calling str so many times in Loggable.log , just use .format (or f-string if using Python >= 3.6):

    print('{}: {}'.format(time.ctime(), msg))

Answer 2

The recommended approach to subclass built in container types, is to use the abstract base classes provided in the collections module:

In the case of list, you should use collections.UserList ; it allows you to override special methods that may be difficult otherwise. (see override of __setitem__ illustrated below).

In this case, you do not need to provide a constructor; the append method is overridden to include a call to self.log(elt) , prior to calling the append method of the superclass.

import time
from collections import UserList

class Loggable:
    def log(self, msg):
        print(f'{str(time.ctime())}: {msg}')

        
class LoggableList(UserList, Loggable):
    """A list that documents its successive transformations 
    """

    def append(self, elt):
        self.log(f'append({elt})')
        super().append(elt)
    
    def __setitem__(self, idx, elt):
        self.log(f'set[{idx}] {elt}')
        self.data[idx] = elt
    
        
log_list = LoggableList([1, 2, 3])
log_list.append(4)
log_list[2] = 0
log_list[3] = 'abc'
log_list.append(None)
print(log_list)

output

Fri Nov 13 23:33:33 2020: append(4)
Fri Nov 13 23:33:33 2020: set[2] 0
Fri Nov 13 23:33:33 2020: set[3] abc
Fri Nov 13 23:33:33 2020: append(None)
[1, 2, 0, 'abc', None]