How do I iterate through lists with nested lists in Python / why doesn't my code work?

Question

Can someone explain why Python won't let me use i in this manner?

unit1 = [["cats allowed", True], ["bedrooms", 0], ["Balcony", False]]

userPref = []
for i in unit1:
   userPref = userPref.append(unit1[i][1])
   print(unit1[i][1])

I get this error message:

TypeError: list indices must be integers or slices, not list

If I want to iterate through the second item in each nested list, how would I go about doing that?

(FYI: the for loop in nested in an if statement. I omitted that for simplicity.)

Answer 1

Some options you have to iterate over a list:

1)

for item in unit1:
   userPref.append(item[1])
   print(item[1])

which item[1] is the second parameter of nested list

2)

for i in range(len(unit1)):
    userPref.append(unit1[i][1])
    print(unit1[i][1])

or if you need item and index together:

for i,item in enumerate(unit1):
    userPref.append(item[1])
    print(item[1])

Answer 2

for i in unit1:

When you iterate over a list in this way, i becomes each value in the list, not the list index .

So on the first iteration, i is the sub-list ["cats allowed", True] .

If you want to iterate over the indexes of a list, use range() :

for i in range(len(unit1)):

Answer 3

Python's for statement iterates over the items of any sequence (a list or a string), in the order that they appear in the sequence.

You can found that differ in python official tutorial , the iteration will traverse all the value in the sequence rather index of that, which is quite different from other languages.