Python迭代器和可迭代列表

Question

I'm learning Python by watching a Pluralsight course and started to experiment with a iterator and iterable example. One issue I ran into that I can't explain is the following iterator and iterable example:

The following either returns the first item or raises a ValueError if empty:

def first(iterable):
    iterator = iter(iterable)
    try:
        return next(iterator)
    except StopIteration:
        raise ValueError("iterable is empty")

The example in the course worked:

first({"1st","2nd","3rd"})

by returning '1st' but when I changed the content of the list to:

first({"1","2","3"})

The value returned was '2' instead of '1'

Why is this occurring?

BTW, I also tried:

first({1,2,3})

and that returns the expected value of: 1

Answer 1

You're not working with list s at all. {"1st","2nd","3rd"} is a set literal, and set s are unordered (they iterate in some order, but not a useful order, or even necessarily repeatable order across different runs of Python or set s constructed in different ways within a single run of Python).

If you're trying to make a list literal, use [] , not {} . first(["1st","2nd","3rd"]) , first(["1","2","3"]) and first([1,2,3]) will all behave predictably (returning "1st" , "1" and 1 respectively), as list s have ordering.