How do I build a __add__ method in a specific class in python?

Question

I defined a class named DictList . The DictList class represents a list of dict, with some keys appearing in more than one dict.It has one parameter: it matches one or more arguments( must be dictionary).

class DictList:
    def __init__(self,*args):
        self.n = args

Like :

A = DictList({'a':1, 'b':2, 'c':3}, {'c':5, 'd':7, 'e':16}, {'e':25, 'f':29)

Now I want to define a add method in this class. It will add two DictList together or add one DictList and one dictionary.

Example:

>>> A1 = DictList(dict(a=1,b=2), dict(b=12,c=13)) 
>>> A2 = DictList(dict(a='hi',b='hello'), dict(b='good',c='nice'))

>>> A1+A2 
DictList({'a': 1, 'b': 2}, {'b': 12, 'c': 13}, {'a': 'hi', 'b': 'hello'}, {'b': 'good', 'c': 'nice'})

>>> A2 + A1
DictList( {'a': 'hi', 'b': 'hello'}, {'b': good', 'c': 'nice'},{'a': 1, 'b': 2}, {'b': 12, 'c': 13})

My idea is to create a new DictList with a list of dictionaries that contains all the dicts in the two DictLists or one DictList and one dict.

def __add__(self,new):
        if type(new) == DictList:
            print(DictList( all dicts from self and new ))
        if type(new) == dict:
            print(DictList(all dicts from self and new))        

But I don't know how to get every dict in the two DictList and put all of them into the new DictList.How can I do?

Answer 1

Return a new DictList with the joined self.n argument lists. If you want to add regular dictionaries, too, check the type of the instance. You may want to define __radd__ to handle dict + DictList instead of only DictList + dict .

class DictList:
    def __init__(self,*args):
        self.n = args

    def __add__(self,new):
        L = list(self.n)
        if isinstance(new,DictList):
            L.extend(new.n)
        elif isinstance(new,dict):
            L.append(new)
        else:
            raise TypeError('Must be instance of DictList or dict')
        return DictList(*L)

    def __radd__(self,new):
        return self.__add__(new)

    def __repr__(self):
        return 'DictList'+repr(self.n)

Demo:

>>> d = DictList(dict(a=1,b=2,c=3),dict(a=2,b=3,e=4))
>>> e = DictList(dict(a=4,c=2,e=3))
>>> d+e
DictList({'b': 2, 'c': 3, 'a': 1}, {'b': 3, 'a': 2, 'e': 4}, {'c': 2, 'a': 4, 'e': 3})
>>> d+dict(x=1,y=2)
DictList({'b': 2, 'c': 3, 'a': 1}, {'b': 3, 'a': 2, 'e': 4}, {'x': 1, 'y': 2})
>>> dict(x=1,y=2)+d
DictList({'b': 2, 'c': 3, 'a': 1}, {'b': 3, 'a': 2, 'e': 4}, {'x': 1, 'y': 2})
>>> d+5
Traceback (most recent call last):
  File "<interactive input>", line 1, in <module>
  File "C:\Users\metolone\Desktop\x.py", line 12, in __add__
    raise TypeError('Must be instance of DictList or dict')
TypeError: Must be instance of DictList or dict