Question

I have a nested list, named env, created in the constructor and another method to populate an element of the grid defined as below:

class Environment(object):
    def __init__(self,rowCount,columnCount):
        env = [[ None for i in range(columnCount)] for j in range(rowCount) ] 
        return env       

    def addElement(self, row, column):
        self[row][column] = 0

Later in the code I create an instance of Environment by running:

myEnv = createEnvironment(6,6)

Then I want to add an element to the environment by running:

myEnv.addElement(2,2)

So what I expected to happen was that I would receive a new Environment object as a 6×6 grid with a 0 in position 2,2 of the grid. But that did not work.

I have two errors:

1. I am unable to return anything other than None from the init method.

2. The main issue us when trying to execute addElement(2, 2) I get this error:

   "TypeError: 'Environment' object does not support indexing.

I looked at the __getitem__ and __setitem__ methods but was unable to get them working over a multidimensional list. Is there a better data structure I should be using to create a grid?

Answer 1

The problem here is that you can’t replace the object with __init__. You could subclass list and do something in __new__, probably, but that would be massive overkill, the better option is just to wrap the list:

class Environment(object):
    def __init__(self, rows, columns):
        self.env = [[None for column in range(columns)] for row in range(rows) ] 

    def addElement(self, row, column):
        self.env[row][column] = 0

Note that it’s a little odd you claim to be calling myEnv = createEnvironment(6,6) – using a function rather than the constructor is a little odd.

If you really want your object to act like the list, you can of course provide a load of extra wrapper functions like __getitem__/__setitem__. E.g:

def __getitem__(self, row, column):
    return self.env[row][column]

Which would allow you to do some_environment[5, 6], for example. (You may rather return the column, that depends on your system and what works best for you).