Using frozenset as Dictionary Keys for Compound Immutable Keys

# Scenario: Storing preferences for a pair of features, order-independent
feature_preferences = {}

# Define some features
feature_a = 'notifications'
feature_b = 'dark_mode'
feature_c = 'email_reports'

# Create frozensets for compound keys
# The order of elements in a frozenset does not matter for equality
key1 = frozenset([feature_a, feature_b])
key2 = frozenset([feature_b, feature_a]) # Same as key1
key3 = frozenset([feature_a, feature_c])

# Store data using frozenset keys
feature_preferences[key1] = {'display_order': 1, 'enabled': True}
feature_preferences[key3] = {'display_order': 2, 'enabled': False}

print(f"Preferences for {key1}: {feature_preferences.get(key1)}")
print(f"Preferences for {key2} (should be same as key1): {feature_preferences.get(key2)}")
print(f"Preferences for {frozenset(['dark_mode', 'notifications'])}: {feature_preferences.get(frozenset(['dark_mode', 'notifications']))}")

# Attempt to use a regular set (mutable) as a key - will raise TypeError
try:
    my_set = {feature_a, feature_c}
    feature_preferences[my_set] = "This will fail"
except TypeError as e:
    print(f"Error using mutable set as key: {e}")

# Use case: Caching results based on multiple, order-independent input parameters
def get_combined_setting(feature_set: frozenset) -> dict:
    # Imagine this fetches from a database or performs a heavy computation
    print(f"Fetching setting for {feature_set}...")
    return feature_preferences.get(feature_set, {'display_order': 99, 'enabled': False, 'default': True})

print(get_combined_setting(frozenset(['notifications', 'dark_mode'])))
print(get_combined_setting(frozenset(['notifications', 'email_reports'])))
print(get_combined_setting(frozenset(['unknown', 'features'])))