The most "pythonic" way would be to use collections.deque.

Deques support thread-safe, memory efficient appends and pops from either side of the deque with approximately the same O(1) performance in either direction.

Though list objects support similar operations, they are optimized for fast fixed-length operations and incur O(n) memory movement costs for pop(0) and insert(0, v) operations which change both the size and position of the underlying data representation.

Next code requires O(n) memory:

    def levelOrder(self,root):
        q = [ root ]
        #i = 0
        
        #while i < len( q ):
        #    current = q[i]
        #    i += 1
        
        for current in q:    
            if current:
                print(current.data, end=' ')

                q.append(current.left)
                q.append(current.right) 

Edit: based on AbhishekVermaIIT's suggestion