如何在Python中使用生成器表达式合并多个集合？

Question

I want to join multiple sets that are obtained from instances of a class. Below is an example of what I am working with and what I tried. Changing the class is not an option.

class Salad:
   def __init__(self, dressing, veggies, others):
      self.dressing = dressing
      self.veggies = veggies
      self.others = others

SALADS = {
   'cesar'  : Salad('cesar',  {'lettuce', 'tomato'},  {'chicken', 'cheese'}),
   'taco'   : Salad('salsa',  {'lettuce'},            {'cheese', 'chili', 'nachos'})
}

I want OTHER_INGREDIENTS to be {'chicken', 'cheese', 'chili', 'nachos'} . So I tried:

OTHER_INGREDIENTS = sum((salad.others for salad in SALADS.values()), set())

That gives me an error "unsupported operand type(s) for +: 'set' and 'set' though. How do I do this?

I would prefer to use Python 2.7 without additional imports if possible.

Answer 1

You can use a set comprehension:

OTHER_INGREDIENTS = {
    element
    for salad in SALADS.values()
    for element in salad.others
}

Answer 2

You can use the function union from set:

OTHER_INGREDIENTS = set().union(*(salad.others for salad in SALADS.values()))

Output

{'chili', 'cheese', 'chicken', 'nachos'}