<｜begin▁of▁sentence｜>### Instruction:
Given a graph whose nodes are labeled 'A', 'B', 'C', and 'D', and edges 'AB' with a weight of 1.0, 'BC' with a weight of 2.0, 'CD' with a weight of 1.0, and 'AD' with a weight of 4.0, what is the length of the shortest path from node 'A' to node 'D' using A* search algorithm with a heuristic function that estimates the distance between any two nodes as the absolute difference of their ASCII character codes?
### Response:
import networkx as nx

# Create a weighted graph
G = nx.Graph()
G.add_edge('A', 'B', weight=1.0)
G.add_edge('B', 'C', weight=2.0)
G.add_edge('C', 'D', weight=1.0)
G.add_edge('A', 'D', weight=4.0)

# Define a heuristic function
def heuristic(u, v):
    return abs(ord(u) - ord(v))

# Calculate the length of the shortest path
try:
    length = nx.astar_path_length(G, 'A', 'D', heuristic=heuristic, weight='weight')
    print(f"A* path length from 'A' to 'D': {length}")
except nx.NetworkXNoPath:
    print("No path exists between 'A' and 'D'.")
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