How to make a Python set without set()

tl;dr

via the data structures tutorial in the Python docs, this is a set comprehension in Python:

names = {p.name for p in [people]}

Sets can also be instantiated with curly braces:

names = {'Bob', 'Alice', 'Bob', 'Charlie', 'Alice'}
# {'Bob', 'Alice', 'Charlie'}

However, empty sets must be created with set(), as {} will create an empty dict.

The context

For some reason I’ve treated Python sets as just an ancillary structure, and I end up using them so infrequently that I have to google the actual syntax because I’m unsure if its constructor is Set() (that’s actually Ruby) or if I need to import it via the collections library. Turns out it’s as easy and basic as it is for lists and dicts.

Credit goes to the fantastic book Fluent Python, by Luciano Ramalho, a book I bought years ago but continues to pay dividends even as I casually skim chapters.

I noticed this basic Python syntax feature this morning when reading Chapter 6: Design Patterns with First-Class Functions, and actually taking time to read the code in Ramalho’s detailed examples:

class LargeOrderPromo(Promotion): # third Concrete Strategy 
  """7% discount for orders with 10 or more distinct items"""
    def discount(self, order):
        distinct_item = {item.product for item in order.cart}
        if len(distinct_items) >= 10:
            return order.total() * .07
        return 0

Ramalho, Luciano. Fluent Python: Clear, Concise, and Effective Programming (p. 177). O’Reilly Media. Kindle Edition.