Making a tree form a Dictionary of lists

Question

I'm a new programmer. I am trying to make a Tree from a dictionary that contains all the links between the nodes. Here is an example

{ 'A': ['B', 'C'],
'B': ['A', 'D'], 
'C': ['B', 'A']

}

Edit: I know it looks like a dictionary for a graph. If a 'son' is somewhere in the tree it should not be added.

I need to find, knowing the generation, all the 'sons' of that generation.

I have tried writing from scratch a class (first time I write a class), but it does not work.

class Tree:
def __init__(self, data):
    self.data = data
    self.children = []
    self.parent = None

def add_child(self, child):
    child.parent = self
    self.children.append(child)

def maketree(self, dictionary):
    for node in dictionary[self.data]:
        if node != self.parent:
            self.add_child(TreeNode(node))
    if self.children != []:
        for child in self.children:
            child.maketree(dictionary)

It says, referring to the last line of code

[Previous line repeated 993 more times] RecursionError: maximum recursion depth exceeded

If you have other questions or any doubt or clarification please ask