Python list and tuple Unpacking

Question

thanks for answering me at the first.. I have a List or a Tuple in Python; try to unpack instead of index like below:

list1 = [1,2,3,4]
first,second,third,fourth = list1

now its ok if I use it like this:

first ====> it will return 1

but when I use it inside [ ] its like below:

list1[first] ====> it will return 2

seams it wont count [0] index in this mod!! why? and how to fix?

Answer 1

first is set to 1, so normal to get the second element of the list. If first was set to 'spam' for instance, list1[first] would just fail:

>>> list1 = ["spam", "egg"]
>>> first, second = list1
>>> list1[first]

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: list indices must be integers or slices, not str

I suggest using a dict instead:

 >>> dict1 = {"first": 1, "second": 2}
 >>> dict1["first"]
 1

Answer 2

因为first持有整数1，所以list1[first]实际上就是list1[1]

Answer 3

如果您希望将list1的值用作基于 1 的索引但不想更改list1 ，则可以使用（例如）：

list1[first-1]