Attribute error: Object has no attribute Python

The issue appears to be that your for row in cells loop is inside of your previous for x in range(0, self.sizex) loop. Here's what the step method should look like if you get it correctly indented:

def step (self):    cells = self.cells    for x in range (0,self.sizex):        if x==0: x_down = self.sizex-1        else: x_down = x-1        if x==self.sizex-1: x_up = 0        else: x_up = x+1        for y in range(0,self.sizey):            if y==0: y_down = self.sizey-1            else: Y_down = y-1            if y==self.sizey-1: y_up = 0            else: y_up = y+1            sum = cells[x_down][y].state + cells[x_up][y].state + cells[x][y_down].state + cells[x][y_up].state + cells[x_down][y_down].state +cells[x_up][y_up].state + cells[x_down][y_up].state + cells[x_up][y_down].state            cells[x][y].setNextState(sum)    for row in cells:                     # unindent these        for cell in row:                  # lines by one            cell.stepToNextState()        # level each

If all the indentation issues are taken care of (or were not in the original code), there's still an issue that may cause an issue in certain situations. The issue is that the Cell.setNextState method doesn't handle every situation. Specifically, it doesn't set nextState if a cell is alive and should stay so (it has two or three living neighbors). The lack of an else on your chain of if and elif statements should have raised this as a red flag for me, but I overlooked it the first time I examined the function.

Here's how it can be fixed:

def setNextState (self , Neighbours):    if self.state == Cell.Live and (Neighbours < 2 or Neighbours > 3):        self.nextState = Cell.Dead    elif self.state == Cell.Dead and Neighbours == 3:        self.nextState = Cell.Live    else: # remove the conditions on this block, all the state changes were handled above        self.nextState = self.state