Index lookup is a way to access a specific element or slice of elements from a string or a list in Python. The syntax for index lookup varies slightly between strings and lists, but the general concept is the same.

For strings, you can access a specific character by specifying its index in square brackets []. The index starts at 0 for the first character, and continues incrementally for each subsequent character. Here's an example:

my_string = "Hello, world!"
first_char = my_string[0]
second_char = my_string[1]
print(first_char)   # Output: "H"
print(second_char)  # Output: "e"

You can also use negative indices to access characters from the end of the string. The last character has an index of -1, the second to last has an index of -2, and so on. Here's an example:

my_string = "Hello, world!"
last_char = my_string[-1]
second_last_char = my_string[-2]
print(last_char)         # Output: "!"
print(second_last_char)  # Output: "d"

For lists, the syntax for index lookup is the same as for strings. You can access a specific element by specifying its index in square brackets []. Here's an example:

my_list = [1, 2, 3, 4, 5]
first_element = my_list[0]
second_element = my_list[1]
print(first_element)   # Output: 1
print(second_element)  # Output: 2

You can also use negative indices to access elements from the end of the list. The last element has an index of -1, the second to last has an index of -2, and so on. Here's an example:

my_list = [1, 2, 3, 4, 5]
last_element = my_list[-1]
second_last_element = my_list[-2]
print(last_element)         # Output: 5
print(second_last_element)  # Output: 4

In addition to accessing single elements, you can use index lookup to access a slice of elements from a string or a list. This is done by specifying a range of indices separated by a colon : inside the square brackets []. Here's an example:

my_string = "Hello, world!"
first_five_chars = my_string[0:5]
print(first_five_chars)  # Output: "Hello"

In this example, the slice includes characters with indices from 0 to 4 (inclusive), but excludes the character with index 5. This is called a half-open interval. You can omit the starting or ending index to include all characters up to or from the beginning or end of the string or list, respectively.