Like a string, a list is a sequence of values. In a string, the values are characters; in a list, they can be any type. The values in a list are called elements or sometimes itemsThe syntax for accessing the elements of a list is the same as for accessing the characters of a string—the bracket operator. The expression inside the brackets specifies the index. Remember that the indices start at 0:

       cheeses[0]

' Cheddar ' Unlike strings, lists are mutable. When the bracket operator appears on the left side of an assignment, it identifies the element of the list that will be assigned. numbers = [42, 123] numbers[1] = 5 numbers [42, 5] The most common way to traverse the elements of a list is with a for loop. The syntax is the same as for strings: for cheese in cheeses: print(cheese) This works well if you only need to read the elements of the list. But if you want to write or update the elements, you need the indices. A common way to do that is to combine the built-in functions range and len : for i in range(len(numbers)): numbers[i] = numbers[i] 2 This loop traverses the list and updates each element. len returns the number of elements in the list. range returns a list of indices from 0 to

n

1, where n is the length of the list. Each time through the loop i gets the index of the next element. The assignment statement in the body uses i to read the old value of the element and to assign the new value. The + operator concatenates lists: a = [1, 2, 3] b = [4, 5, 6] c = a + b c [1, 2, 3, 4, 5, 6] The operator repeats a list a given number of times: [0] 4 [0, 0, 0, 0] [1, 2, 3] 3 [1, 2, 3, 1, 2, 3, 1, 2, 3] The first example repeats [0] four times. The second example repeats the list [1, 2, 3] three times. *ython provides methods that operate on lists. For example, append adds a new element to the end of a list: t = [ ' a ' , ' b ' , ' c ' ] t.append( ' d ' ) t [ ' a ' , ' b ' , ' c ' , ' d ' ] extend takes a list as an argument and appends all of the elements: t1 = [ ' a ' , ' b ' , ' c ' ] t2 = [ ' d ' , ' e ' ] t1.extend(t2) t1 [ ' a ' , ' b ' , ' c ' , ' d ' , ' e ' ] This example leaves t2 unmodified. sort arranges the elements of the list from low to high: t = [ ' d ' , ' c ' , ' e ' , ' b ' , ' a ' ] t.sort() t [ ' a ' , ' b ' , ' c ' , ' d ' , ' e ' ] Most list methods are void; they modify the list and return None . If you accidentally write t = t.sort() , you will be disappointed with the result.

*A string is a sequence , which means it is an ordered collection of other values.

• You can access the characters one at a time with the bracket operator:

         fruit            =
  

  ' banana ' letter = fruit[1] The second statement selects character number 1 from fruit and assigns it to letter . The expression in brackets is called an index .A lot of computations involve processing a string one character at a time. Often they start at the beginning, select each character in turn, do something to it, and continue until the end. This pattern of processing is called a traversal A segment of a string is called a slice . Selecting a slice is similar to selecting a character: s = ' Monty Python ' s[0:5] ' Monty ' s[6:12] ' PythonIt is tempting to use the [] operator on the left side of an assignment, with the intention of changing a character in a string. For example: greeting = ' Hello, world! ' greeting[0] = ' J ' TypeError: ' str ' object does not support item assignment The “object” in this case is the string and the “item” is the character you tried to assign. For now, an object is the same thing as a value, but we will refine that definition later (Section 10.10). The reason for the error is that strings are immutable ,