<｜begin▁of▁sentence｜>### Instruction:
Given a graph with vertices having labels assigned to them, how can you define an AttributeSpecification in igraph to retrieve these labels, convert them to uppercase, and provide a default function to generate the label if it is not specified?
### Response:
import igraph as ig

# Create a Graph
edges = [(0, 1), (1, 2), (2, 0)]
g = ig.Graph(edges=edges)

g.vs['label'] = ['A', 'B', 'C']  # Adding attribute 'label' to vertices

# Define an AttributeSpecification for 'label' attribute
attr_spec = ig.AttributeSpecification(name='label',
                                      alt_name='alt_label',
                                      default='Unknown',
                                      transform=str.upper,
                                      func=lambda idx: f'Label_{idx}')

# Use 'func' to generate a value for a vertex
vertex_label = attr_spec.func(1)  # Should return 'Label_1'
print(vertex_label)

# Get the attribute value using the alternative name from a dict
attributes = {'alt_label': 'Custom Label'}
label_value = attributes.get(attr_spec.alt_name, attr_spec.default)
print(label_value)
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