Question

I can’t figure out what I’m doing wrong. I have to create a nested list(of possible solutions to a problem), so I created a class that appends a solution to a list of existing solutions(since each solution is being calculated one at a time)

This function works fine:

def appendList(list):
    #print list
    list2.append(list)

x =0
while x < 10:
    x = x+1
    one = "one"
    two = "two"
    appendList([(one), (two)])

print list2

with a result of:

[['one', 'two'], ['one', 'two'], ['one', 'two'], ['one', 'two'], ['one', 'two'], ['one', 'two'], ['one', 'two'], ['one', 'two'], ['one', 'two'], ['one', 'two']]

but when I take this exact same function and put it in a class, I get an error saying:

TypeError: appendList() takes exactly 1 argument (2 given)

Here’s the class and how I call it:

class SolutionsAppendList:
    list2 = []
    def appendList(list):
    #print list
        list2.append(list)

appendList.appendList([('one'), ('two')])

I’m sure I’m making a very basic error but I can’t seem to figure out why, since I’m using the same function in the class.

Any suggestions/advice would be great.
Thanks

Answer 1

Since you’re dealing with classes here, it’s important to use instance variables. These can be accessed by using the self keyword. And if you’re dealing with class methods, be sure to make their first argument self. So, modified…

class SolutionsAppendList:
    def __init__(self):
        self.list = []

    def appendList(self, list):
    #print list
        self.list.append(list)

What’s cool about this is that you no longer need list2. Instance variables (self.list). Are different than local variables (the list you pass to the appendList() method). EDIT: I”ve also added an __init__ method, which is called once an instance of the SolutionsAppendList object is created. In my first reply, I overlooked the fact that self.* is not available outside of a method. This is the edited code (Thanks David).