Multiple stack code does not increment the stack number

Question

This is a SetOfStacks problem (CTCI 3.3) implementation by myself in Python. Simply, I want to limit the size of the stack by, say 10. So, when I try to push more than the capacity, I generate one more stack to let the user keep pushing new elements. I tried to represent this into collections.defaultdict(list) data structure. To track the stack number in use, I made a variable named current_stack. However, even in the over-capacity case (please see the design at ##### mark), it seems the current_stack does not increase as intended. What's wrong with my code? Wouldn't the self.current_stack work kind of a global variable within the object?

class SetOfStacks():
    def __init__(self, stacksize):
        self.size = stacksize  # const
        self.c = collections.defaultdict(list)
        self.current_stack = 0  # self.c[self.current_stack]

    def push(self, e):
        if len(c[self.current_stack]) + 1 > self.size: #####
            self.current_stack += 1
        self.c[self.current_stack].append(e)

    def pop(self):
        if self.current_stack > 0 and len(c[self.current_stack]) == 1 :
            popped = self.c[self.current_stack].pop()
            self.current_stack -= 1
            return popped
        elif self.current_stack == 0 and len(c[self.current_stack]) == 0:
            raise IndexError("No more elements to pop.")
        else:
            return self.c[self.current_stack].pop()

    def __repr__(self):
        return print(self.c)

st = SetOfStacks(10)
for i in range(21):
    st.push(i)
st

Result:

defaultdict(<class 'list'>, {0: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]})

Answer 1

You are missing self in the len() call in push():

def push(self, e):
    if len(c[self.current_stack]) + 1 > self.size: #####
           ^

You need:

def push(self, e):
    if len(self.c[self.current_stack]) + 1 > self.size: #####

You must have another c in the environment that you are referencing that is never being added to - so is constantly the same size.

And there are a couple of similar errors in pop(), e.g.:

def pop(self):
    if self.current_stack > 0 and len(c[self.current_stack]) == 1 :
                                      ^

Note: def __repr__(): should return a str not actually print(), e.g.:

def __repr__(self):
    return repr(self.c)

With those fixes you get the expected output:

defaultdict(<class 'list'>, {0: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
                             1: [10, 11, 12, 13, 14, 15, 16, 17, 18, 19],
                             2: [20]})