human
I am working on the file "Python/Chapter-09/Huffman.ipynb".
The first task is:
This notebook presents <em style="color:blue;">coding trees</em>.  Given an alphabet $\Sigma$ of characters, 
we define the set $\mathcal{K}$ of coding trees by induction:

$\texttt{Leaf}(c,f) \in \mathcal{K} $ if $c \in \Sigma$ and $f \in \mathbb{N}$
An expression of the form $\texttt{Leaf}(c,f)$ represent a leaf in a coding tree.
 $c$ is a letter from the alphabet $\Sigma$ and 
* $f$ is the frequency* of the letter $c$ in the string $s$ that is to be encoded.

$\texttt{Node}(l,r) \in \mathcal{K}$ if $l \in\mathcal{K}$ and $r \in \mathcal{K}$
The expressions $\texttt{Node}(l,r)$ represent the inner nodes of the coding-tree.

The class CodingTree is a superclass for constructing coding trees.  It has one static variable sNodeCount.  This variable is used to equip all nodes with a unique identifier.  This identifier is used to draw the trees using graphviz.
Every object of class CodingTree has a uniques identifier mID that is stored as a member variable.  This identifier is only used by graphviz.
Can you write Python code for it?
assistant
#Here is the Python code you requested.

class CodingTree:
    sNodeCount = 0
    
    def __init__(self):
        CodingTree.sNodeCount += 1
        self.mID = CodingTree.sNodeCount
        
    def count(self):
        "compute the number of characters"
        pass
        
    def cost(self):
        "compute the number of bits used by this coding tree"
        pass
        
    def getID(self):
        return self.mID  # used only by graphviz