The code that's in the inside of the sum() is a generator expression which is basically syntactic sugar for creating an iterator from another iterator.

In your example the cart variable is an object of type Dictionary which is also an iterator, and the prices[item] * cart[item] part is how each item from the cart (the dictionary keys) is transformed into a new item for the result iterator.

The snippet bill = sum(prices[item] * cart[item] for item in cart) is equivalent to

bill = 0
for item in cart:
  bill += prices[item] * cart[item]

you can even do it without a loop:

def sumAll(n):
    return  sum(range(1,n+1))

print(sumAll(10)) # -> 55

if you insist on using a loop:

def sumAll(n):
    s = 0
    for i in range(1,n+1):
        s += i
    return s

but the mathematical solution is simply:

def sumAll(n):
    return n * (n + 1) / 2  
    # in python 3:
    # return n * (n + 1) // 2

(there are n elements with an average value of (n+1)/2; this will always be an integer since either n or n+1 is even; the proof by induction is often the first example when math students learn about induction...).

You can add a list to the sum function so you can make something like this:

test = sum((1,23,5,6,100))
print("output:  ", test) 

And you get 135.

So with your "for loop" you get a list and put that list to the sum function and you get the sum of the list. The real sum function uses yield insight and use every value and sum them up.

This is the easiest solution that I can think of :

nop=int(input())
for _ in range(nop):
    fname,lname=input(),input()
    n=int(input())
    summ=sum(float(input()) for _ in range(n))
    print("For {0} {1} the sum is {2}".format(fname,lname,summ))

output:

$ python3 foo.py < abc
For Jane Doe the sum is 210.74
For William Jones the sum is 188.7

where abc contains:

2
Jane
Doe
4
38.4
29.3
33.3
109.74
William
Jones
2
88.8
99.9

Here's a simpler solution:

a = int(input('Enter a number plz '))
lst = []
for i in range(1, a+1):
    def func(root):
        print('Number', i, 'in square root is', root)
        lst.append(root)
    func(i*i)

print(f'The sum is {sum(lst)}')

You can create an empty list, then append all the values of root as you iterate through the for-loop. Now we have a list of all of the root values, so we can make use of the built-in sum function to achieve our final product.

Also note that it is not necessary to have your function in the for-loop:

a = int(input('Enter a number plz '))

def func(root):
        print('Number', i, 'in square root is', root)
        lst.append(root)
        
lst = []
for i in range(1, a+1):
    func(i*i)

print(f'The sum is {sum(lst)}')

There's a python function just for this - sum

print(sum([x for x in range(1, 101)]))

Edit: To address the concern about "struggling to understand"

[x for x in range(1, 101)] is called a list comprehension. They are used to create lists in a singular line, without using for loops - python can be a functional programming language after all. [x for x in range(1, 101)] returns a list from 1 to 100, that is-

[1, 2, 3, 4, 5, ......., 99, 100]

We skip 0 because it doesn't count in the sum anyway.

Now if you do sum() on this list, it'll return the sum of all elements in the list.

Hence sum([for x in range (1, 101)]) returns the sum of all numbers from 1 to 100 (inclusive) and print will print the final result.

Why use many lines when few do trick? :)

Remember, List Comprehension and sum() are 2 very important tools in the python toolbox, everyone should know about these 2.