Today's challenge was very easy but was pretty tricky to tackle. I found the solution to first part by using python sets and for the second part, I used help from Reddit.
Please share your solution too.
with open("day21.txt") as fp: lines = fp.read().splitlines() food_dict = {} safe_ingredient = set() all_ingredients = [] for food in lines: ingredients, allergens = food.split(" (contains ") allergens = allergens[:-1] ingredients = ingredients.split(" ") all_ingredients.extend(ingredients) unique_in = set(ingredients) safe_ingredient=safe_ingredient.union(unique_in) allergens = allergens.split(", ") for allergen in allergens: if food_dict.get(allergen) is None: food_dict[allergen] = unique_in else: food_dict[allergen] = food_dict[allergen].intersection(unique_in) safe_ingredient = safe_ingredient.difference(set(k for value in food_dict.values() for k in value)) safe_count = sum([ingredient in safe_ingredient for ingredient in all_ingredients]) print(f"Safe ingredients: {safe_count}") while any([len(i) > 1 for i in food_dict.values()]): for a, i in food_dict.items(): if len(i) == 1: alg = list(i)[0] for a2, i2 in food_dict.items(): if a2 == a: continue if len(i2) > 1: if alg in i2: food_dict[a2].remove(alg) names = list(food_dict.keys()) names.sort() print("My dangerous list: ", ",".join([",".join(food_dict[i]) for i in names])) 
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