How does a for loop work on a list?

What you're seeing is a syntax called generator expression . These types of expressions generate a series of values, for which they internally use a loop for, but the values ​​they generate can be computed with an arbitrary expression.

The general syntax of the generator expression is:

expresion for elemento in iterable

The forcauses an iteration over iterablewhatever you put at the end (which can be a list, or anything else that can be iterated over). On each iteration, the variable elementotakes a value from that iterable. Then the one expresionthat appears at the beginning is evaluated. Usually that expresioncontains some form of the elemento, like for example here:

(i+1)*2 for i in range(5)

In this case the iterable is range(5), which returns the numbers 0, 1, 2, 3and 4, which are the values ​​that the i. But the expression puts (i+1)*2, so that expression will take the values 2, 4, 6, 8,​​and 10. The generating expression will therefore create the latter.

When you enclose a generator expression in square brackets, you have what is called a list comprehension , which simply means that a list is created from the data generated by the generator expression. In the example above, the list would be created[2, 4, 6, 8, 10]

So your case is explained:

lista_2 = [i for i in lista_1]

The expression in this case is i, that is, each element is left as it was, without transforming. So in the end it lista_2will have the same elements as lista_1. In this case the assignment is equivalent to lista_2 = list(lista_1). Note that it is not the same as lista_2 = lista_1since in this second case we would be making both lists refer to the same object in memory, while in the first we are creating a new list, yes, with the same elements as the first.

additional details

• The expression does not have to contain i. For example the following would be valid:

  [ "hola" for i in lista_1 ]
  

  In this case the expression is "hola", so a new list would be created with the word "hola"repeated as many times as there are elements lista_1.

• Generator expressions also admit another form, with a condition at the end, like this:

  expresion for elemento in iterable if condicion
  

  In this case, if the condition is true, the expression is evaluated and added to the result, but if the condition is false, it is not.

  For example, the following assignment would copy lista_2only the uppercase letters in lista_1:

  lista_2 = [i for i in lista_1 if i.isupper() ]
  

• Generator expressions can be even more complicated, as they support fornesting.

• If the generator expression is enclosed in parentheses instead of brackets, then no list is created, but rather the entire expression is another iterator that can be iterated over, or passed as a parameter to functions that expect an iterator.

with instruction

for i in lista_1

you iterate through the list lista_1, plugging in ieach of its elements on each iteration.

By placing the iagain in front of the , you add each of the values ​​that it takes forto a new listlista_2i

Maybe with a list and a tuple you'll see it better:

lista_1 = ('a', 'b', 'c')
lista_2 = [i for i in lista_1]
print(lista_2)