Python Recipe #1

I like Python because it is a flexible and extensible programming language. I wrote this little and short article to remind myself of the advantages and power of Python.

How you know, we can’t subtract elements of two lists. We’ll get an error.

>>> [1,2,3] - [1,2,3]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for -: 'list' and 'list'

Suppose you need to subtract elements of two lists using symbol -.

[1,2,3] - [1,2,3] = [1-1,2-2,3-3] = [0,0,0]

Or, if you need to sum two lists using symbol +.

[1,2,3] + [1,2,3] = [1+1,2+2,3+3] = [2,4,6]

So, we are going to create a subclass of Python list object called List, and override the __sub__() and __add__() methods of list. I know… there are many libraries and modules that do that in a better and efficient way. This article aims to show the power of Python.

First of all, we are going to develop a subclass called List from the Python object list.

#!/usr/bin/env python3.8
class List(list):
    def append(self,e):
        if isinstance(e,list):
            super(List,self).append(List(e))
        else:
            super(List,self).append(e)
    def __sub__(self,other):
        r = List()
        for i,v in enumerate(self):
            r.append(v-other[i])
        return r
    def __add__(self,other):
        r = List()
        for i,v in enumerate(self):
            r.append(v+other[i])
        return r

Now, we can create two lists as follows.

list_1 = List([1,2,3])
list_2 = List([1,2,3])
list_result = list_1 - list_2
print(list_result)

The output will be.

[0,0,0]

if you want to sum list_1 plus list_2 you just do:

list_1 = List([1,2,3])
list_2 = List([1,2,3])
list_result = list_1 + list_2
print(list_result)

The output will be.

[2,4,6]

That’s all for now. I’ll try to write this kind of short articles every time I need a recipe or a Tip related to Python.