What is the meaning of a for loop in curly braces?

问题

what-is-the-meaning-of-curly-braces says that they are used for dictionaries, but what about this example?

resp = {
    requests.get('http://localhost:8000')
    for i in range(20)
}

Taken from here (actually it uses [], but it works with {} as well). What is the meaning of this, and why is this loop upside-down?

回答1:

Curly braces are also used for sets. This is a set comprehension. It creates the set of 20 requests.get objects, keeping only the unique ones.

If you use [] instead of {} it is a list comprehension. They are similar, but have two differences

• the list is ordered
• the list can have duplicate elements

Also, as you mention, this is a bad way to make requests. Comprehensions should be used for creating a list/set, not for calling a number of commands as a by-product. In that case, a simple for-loop is better, it makes the intent clearer.

来源：https://stackoverflow.com/questions/52641936/what-is-the-meaning-of-a-for-loop-in-curly-braces