Question

Python beginner running 2.7

I want to have a list which is constantly summed as new values are added to it. However, I keep getting Attribute Error: ‘int’ object has no ‘append’ function. I understand the basic problem – you can’t append to an integer – but would like to find a workaround. Do any of you have a solution?

Simplified version of my code, and then a possible solution I’d like to avoid.

my_list = sum([])

def myfunction (i):
    return i 

thing = myfunction (1)
my_list.append(thing)

thing2 = myfunction (2)
my_list.append(thing2)

def function_2 (a,b):
    #function which uses my_list

I suppose I could do the following solution, but I would like to avoid it (interferes with existing code).

my_list = []
summed_my_list = sum (mylist)

def myfunction (i):
    return i 

thing = myfunction (1)
my_list.append(thing)

thing2 = myfunction (2)
my_list.append(thing2)

Answer 1

If you don’t need the list the question is trivial, because you simply need

total += value 

at each step.

Class derived from list

If you do need both the list and the sum (which autoupdates) you could create a class derived from list such as – which should auto sum when you append to it.

class mylist(list):
    tot = 0
    def append(self, value):
        super(mylist, self).append(value)
        self.tot += value

Example usage

#!/usr/bin/python

class mylist(list):
  tot = 0
  def append(self, value):
    super(mylist, self).append(value)
    self.tot += value


a = mylist()
a.append(1)
a.append(20)
print a.tot
print a

output:

21
[1,20]