Python for loop declaration and behaviour

Question

I am finding it difficult to understand how the following code works:

mylist = [1,2,3,4,5]
print(sum(i for i in mylist))

the code above gives a correct result of 15, but shouldn't "i" be used after it is declared and not before?

• L3viathan: What you see is — despite the name for — not a for loop, but a (generator) comprehension. Like some other special expressions (e.g. ternary if: x = 3 if foo=bar else 5) it has this "weird" order, because that's how you would say it in English.
• L3viathan: @RobJan Nit-picking here, but: No, that's not a list comprehension
• RobJan: @L3viathan I admit that this is way to create generator.

Answer 1

Read the documentation on generator expressions.

You will see that you are (effectively) creating a mini generator equivalent to:

def iter_list(lst):
 for i in lst:
 yield i
myList = [1,2,3,4,5]
print(sum(iter_list(myList)))

Answer 2

Python is an untyped language. That means you do not need to declare the variables such as i.

You can give a bit more clarity to compiler , and your understanding by doing the edit:

mylist = [1,2,3,4,5]
print(sum(int(i) for i in mylist ))

So, Python is just very smart in giving type to variables, and flexible too as compared to static C code.

Answer 3

In this particular case (list-compherensions) the syntax allows to do so

Answer 4

Because of the Syntax of sum() ... iterable and a start position

sum(i, start)

You can take an look with examples there --> https://www.programiz.com/python-programming/methods/built-in/sum