Why does for loop exit for "" (empty string) in python?

Question

To explain my query I have a simple code snippet below followed by my question.

def count_vowels(s):
    num_vowels = 0
    for char in s:
        if char in 'aeiouAEIOU':
             num_vowels = num_vowels + 1
    return num_vowels

print(count_vowels(""))
print("" in "aeiouAEIOU")

gives an output

0 
True

My doubt:

Why does an empty string "" returns True for the expression

"" in "aeiouAEIOU"

But it skips when it is present along with a for loop?

for char in s:  

My understanding is that empty strings are a subset of all strings then why it is ignored when the same expression is in the for loop? Feel free to correct me if there is something I am missing here.

Answer 1

Your understanding is correct: "empty strings are a subset of all strings"

But now let's see what happens when we use for for a sequence type such as string. Let's say we have:

lst = [1, 2, 3, 4, 5]

for i in lst:
    print(i ** 2)

You can just think that it turns into:

index = 0
while True:
    try:
        i = lst.__getitem__(index)
    except IndexError:
        break
    print(i ** 2)
    index += 1

In your Example, when it tries to get even the first item, it will raise an Exception and break out of the loop. So it doesn't even go inside For loop.

I said "just think" because in for-loop, iter() is get called on the object (here lst) and this built-in function will get an iterator out of the object. In order this to be happened the object should implement either the iterable protocol which is either __iter__ or it must support the sequence protocol (the __getitem__())).

lst = [1, 2, 3, 4, 5]

it = iter(lst)
while True:
    try:
        i = next(it)
    except StopIteration:
        break
    else:
        print(i ** 2)

Both str and list object have __iter__ so that is the method gets called rather than __getitem__. (__iter__ has precedence over __getitem__)