Python define the [], len(), items() and keys() operators for a custom class

Question

I have some code to define a class in which the main entity is a dictionary. For example:

class MyDictionary:

    def __init__(self):
        self.__my_dict = {'a': 1,
                          'b': 2,
                          'c': 3}

    def __getitem__(self, item):
        return self.__my_dict[item]

    def __len__(self):
        return len(self.__my_dict)

    def keys(self):
        # ?

    def items(self):
        # ?

my_dict = MyDictionary
for key, value in my_dict.items():
    print([key, value])
for key in my_dict.keys():
    print(my_dict[key])

How should I define the member functions such that I can use the [], len(), keys(), items() operators on the class instance my_dict as if it were a dictionary?

I don't understand the problem. You clearly know how to define the necessary methods, seeing how you've already successfully implemented `__getitem__` and `__len__`. I don't see what's preventing you from implementing `keys` and `items` in a similar fashion? — Aran-Fey, Mar 06 '19 at 13:07
The following link maybe help you: https://stackoverflow.com/questions/4014621/a-python-class-that-acts-like-dict — s3n0, Mar 06 '19 at 13:10
@Aran-Fey You're totally right. And indeed, I already tried this before. For some reason it wasn't working back then but now it is. I'm going to check if it works in my 'larger' programs. — Adriaan, Mar 06 '19 at 14:31

score 2 · Answer 1 · answered Mar 06 '19 at 13:13

#!/usr/bin/env python

class MyDictionary:
    def __init__(self, my_dict):
        self.my_dict = my_dict

    def __getitem__(self, item):
        return self.my_dict[item]

    def __len__(self):
        return len(self.my_dict)

    def items(self):
        return self.my_dict.items()

    def keys(self):
        return self.my_dict.keys()

    def values(self):
        return self.my_dict.values()


if __name__ == '__main__':
    d = {'a': 1,'b': 2,'c': 3}
    di = MyDictionary(d)

    print(di.__len__())
    print(di.items())
    print(di.keys())
    print(di.values())
    print(di.__getitem__('b'))

output:

3
dict_items([('a', 1), ('b', 2), ('c', 3)])
dict_keys(['a', 'b', 'c'])
dict_values([1, 2, 3])
2

score 1 · Answer 2 · answered Mar 06 '19 at 13:06

You can keep doing what you're doing with the __getitem__ and __len__ methods, and delegate the rest of the methods to the respective methods of __my_dict:

class MyDictionary:
    def __init__(self):
        self.__my_dict = {'a': 1,
                          'b': 2,
                          'c': 3}

    def __getitem__(self, item):
        return self.__my_dict[item]

    def __len__(self):
        return len(self.__my_dict)

    def keys(self):
        return self.__my_dict.keys()

    def items(self):
        return self.__my_dict.items()

my_dict = MyDictionary()
for key, value in my_dict.items():
    print([key, value])
for key in my_dict.keys():
    print(my_dict[key])

Note that you should instantiate an object of a class with parentheses.

score 1 · Answer 3 · answered Mar 06 '19 at 13:12

Try this

def keys(self):
    return self.__my_dict.keys()

def items(self):
    return self.__my_dict.iteritems() # python2.7

    return self.__my_dict.items() # python3

Also when you create an object of a class you should use parenthesis like

my_dict = MyDictionary()

Python define the [], len(), items() and keys() operators for a custom class

3 Answers3