Python object instantiation

Question

I am very new to python and need some help with instantiating an object. The python interpreter is giving me trouble when instantiating an object of a class I defined. There are two classes, BTNode and BST (which are stored in files bst_node.py and bst.py respectively):

# file: bst_node.py
class BTNode:
 """a binary search tree node implementation"""
 def ___init___(self, value):
 self.value = value
 self.left is None
 self.right is None
 self.parent is None
 def ___init___(self, value, left, right, parent):
 """set the parameters to corresponding class members"""
 self.value = value
 self.left = left
 self.right = right
 self.parent = parent
 def is_leaf(self):
 """check whether this node is a leaf"""
 if self.left.value is None and self.right.value is None:
 return True 
 return False

---

# file: bst.py
from bst_node import *
class BST:
 """a binary search tree implementation"""
 def ___init___(self, value):
 self.root = BTNode(value)
 def insert(self, curRoot, newValue):
 if curRoot.is_leaf():
 if newValue < curRoot.value:
 newNode = BTNode(newValue, None, None, curRoot)
 curRoot.left = newNode
 else:
 newNode = BTNode(newValue, None, None, curRoot)
 curRoot.right = newNode
 else:
 if newValue < curRoot.value:
 self.insert(curRoot.left, newValue)
 else:
 self.insert(curRoot.right, newValue)

So, in the interpreter I do:

import bst as b
t1 = b.BST(8)

and I get an error which says that this constructor takes no arguments

The constructor clearly takes an argument value so what is going wrong here? How can I fix this error?

Thanks, all help is greatly appreciated!

• askewchan: Not your question but unless I'm mistaken you'll run into a problem with the lines like self.left is none because is compares, and will either be True or False. Use self.left = None instead.

Answer 1

The first issue is that you called your functions ___init___ instead of __init__. All of the 'special methods' use two underscores.

A second issue in this code is that in BTNode you redefined __init__. You can't overload functions in python. When you reclare __init__ you effectively deleted the first constructor.

A third issue is your usage of is. is is an operator that checks whether two objects are exactly the same and returns True or False. In the constructor, you have a few self.left is None is examining the value of self.left (which wasn't declared yet), and examining whether or not it is None. To set it, use = as follows:self.left = None

To fix the second and third issue you should use default argument values. For example:

def __init__(self, value, left=None, right=None, parent=None):

Answer 2

In addition to the number of underscores problem, you should replace

def ___init___(self, value):
 self.value = value
 self.left is None
 self.right is None
 self.parent is None
def ___init___(self, value, left, right, parent):
 """set the parameters to corresponding class members"""
 self.value = value
 self.left = left
 self.right = right
 self.parent = parent

with

def __init__(self, value, left=None, right=None, parent=None):
 """set the parameters to corresponding class members"""
 self.value = value
 self.left = left
 self.right = right
 self.parent = parent

Because as @Moshe points out, you can't overload functions, you should use default arguments insted.

Answer 3

Changing ___init___ to __init__ should fix it. (2 underscores vs 3)