Python List Functions

Python List Functions

• Length
• Append
• Extend
• Insert
• Index
• Count
• Remove
• Pop
• Sort
• Copy
• Reverse
• Clear

• Length of List: len() function can be used to get the number of items present in the input list.

  
  Syntax: len(list_object)
  

  • Example 1: Statement will return 5 as there are 5 items present in the list.

    
    a_list = [1, 2, 3, 6, 'A']
    len(a_list)
    
    Output: 5
    

  • Example 2: This will return Zero(0) as there is no item present in the list.

    
    a_list = []
    len(a_list)
    
    Output: 0
    

  
  
  
• Append to List: append() function appends the input value at the end of the list.

  
  Syntax: list_object.append(value)
  

  • Example 1: Statement will add value 'C' at the end of the list.

    
    a_list = [1, 2, 3, 6, 'A']
    a_list.append('C')
    print(a_list)
    
    Output: [1, 2, 3, 6, 'A', 'C']
    

  • Example 2: Statement will append input object "['C', 'D']" as single item at the end of the list.

    
    a_list = [1, 2, 3, 6, 'A']
    a_list.append(['C','D'])
    print(a_list)
    
    Output: [1, 2, 3, 6, 'A', ['C', 'D']]
    

    Visit this page to understand difference between extend and append functions
  
  
  
• Extend List: extend() function requires iterable object(such as string, list) as input otherwise it will throw error. Function will iterate through all the elements in the input object and add them as individual item in the existing list.

  
  Syntax: list_object.extend(object)
  

  • Example 1: Statement will fail as integer values are not iterable.

    
    a_list = [1, 2, 3, 6, 'A']
    a_list.extend(4)
    print(a_list)
    
    Output: TypeError: 'int' object is not iterable
    

  • Example 2: Statement will iterate thru input object list "['C', 'D']" and add each item in input list as individual item at the end of the main list.

    
    a_list = [1, 2, 3, 6, 'A']
    a_list.extend(['C','D'])
    print(a_list)
    
    Output: [1, 2, 3, 6, 'A', 'C', 'D']
    

  • Example 3: Statement will iterate thru input string 'DBMS' and add each character in the input string as individual item at the end of the list.

    
    a_list = [1, 2, 3, 6, 'A']
    a_list.extend('DBMS')
    print(a_list)
    
    Output: [1, 2, 3, 6, 'A', 'D', 'B', 'M', 'S']
    

    Visit this page to understand difference between extend and append functions
  
  
  
• Insert At in List: insert() function can be used to insert item at a specified index. -1 represent last index

  
  Syntax: list_object.insert(index_position, value)
  

  • Example 1: Index of value 'f' in list is 2, therefore item 'X' got inserted at 2nd index before value 'f'.

    
    a_list = ['a', 'b', 'f', 'h', 'j']
    a_list.insert(2,'X')
    print(a_list)
    
    Output: ['a', 'b', 'X', 'f', 'h', 'j']
    

  • Example 2: Item 'Y' got inserted at 2nd last position just before the last element.

    
    a_list = ['a', 'b', 'f', 'h', 'j']
    a_list.insert(-1,'Y')
    print(a_list)
    
    Output: ['a', 'b', 'f', 'h', 'Y', 'j']
    

  
  
  
• Index in List: index() function can be used to find position of item(value) in a list. By default it will return index of first occurence of value. Function will throw error if value is not present in the list.

  
  Syntax 1: list_object.index(value)
  Syntax 2: list_object.index(value, start=startingNumber)
  Syntax 3: list_object.index(value, start=startingNumber, stop=endingNumber)
  

  • Example 1: Statement will fail as 'c' character value is not present in the list.

    
    a_list = ['a', 'b', 'f', 'h', 'j']
    a_list.index('c')
    
    Output: ValueError: 'c' is not in list
    
    
    Solution: Check if value is present before running index function
    if 'c' in a_list:
        a_list.index('c')
    

  • Example 2: Statement will return 1 as the first occurence of value for 'b' is at index 1.

    
    a_list = ['a', 'b', 'f', 'b', 'j','k','l','b','g','b']
    a_list.index('b')
    
    Output: 1
    

  • Example 3: Statement will return 7 as the first occurence of value 'b' is starting from 4th item and is at index 7.

    
    a_list = ['a', 'b', 'f', 'b', 'j','k','l','b','g','b']
    a_list.index('b', 4)
    
    Output: 7
    

  • Example 4: This statement will only check the availability of 'b' in between 4th index and 6th index. It will result in error as there is no 'b' value in between 4th index and 6th index.

    
    a_list = ['a', 'b', 'f', 'b', 'j','k','l','b','g','b']
    a_list.index('b' , 4, 6)
    
    Output: ValueError: 'b' is not in list
    

  • Example 5: This statement will only check the availability of 'b' in between 8th index and 10th index. It will return 9 as it is first occurence of value 'b' is in between 8th index and 10th index.

    
    a_list = ['a', 'b', 'f', 'b', 'j','k','l','b','g','b']
    a_list.index('b' , 8, 10)
    
    Output: 9
    

  
  
  
• Count Occurences: count() function counts the occurence of value passed.

  
  Syntax: list_object.count(value_to_check_occurence)
  

  • Example 1: There is only one occurence of value 3

    
    a_list = [1, 2, 3, 6, 'A', 6]
    a_list.count(3)
    
    Output: 1
    

  • Example 2: There are two occurence of value 6.

    
    a_list = [1, 2, 3, 6, 'A', 6]
    a_list.count(6)
    
    Output: 2
    

  • Example 3: There is no occurence of value 5.

    
    a_list = [1, 2, 3, 6, 'A', 6]
    a_list.count(5)
    
    Output: 0
    

  
  
  
• Remove object from List: remove() function can be used to remove first occurence of value in the list. Function will throw error if value is not present in the list.

  
  Syntax 1: list_object.remove(value)
  

  • Example 1: Statement will fail as value 'd' is not present in the list.

    
    a_list = ['a', 'b', 'f', 'h', 'j']
    a_list.remove('d')
    
    Output: ValueError: list.remove(x): x not in list
    
    
    Solution: Check if value is present before running remove function
    if 'd' in a_list:
        a_list.remove('d')
    

  • Example 2: Statement will remove first occurence of value 'b' from the list.

    
    a_list = ['a', 'b', 'f', 'h', 'b', 'j']
    a_list.remove('b')
    print(a_list)
    
    Output: ['a', 'f', 'h', 'b', 'j']
    

    Visit this page to understand difference between remove and pop functions
  
  
  
• Pop object from List: pop() function can be used to remove & return item using index. By default it will remove and return value at last index in the list. Function will throw error if input index is not present in the list.

  
  Syntax 1: list_object.pop(value)
  Syntax 2: list_object.pop(value, indexToPop)
  

  • Example 1: Statement will fail as index '7' does not exists in the list. Solution is to Check if length of list is greater than referenced index in pop function before running pop function.

    
    a_list = ['a', 'b', 'f', 'h', 'j']
    a_list.pop(7)
    
    Output: IndexError: pop index out of range
    
    
    Solution: Check length of list before running pop function
    if len(a_list)>=7:
        a_list.pop(7)
    

  • Example 2: Statement will remove and return 'j' as 'j' is the last item in the list.

    
    a_list = ['a', 'b', 'f', 'h', 'j']
    a_list.pop()
    
    Output: 'j'
    

  • Example 3: Statement will return & remove 'b' from the list as 'b' is the item present at index 1.

    
    a_list = ['a', 'b', 'f', 'h', 'j']
    a_list.pop(1)
    
    Output: 'b'
    

    Visit this page to understand difference between remove and pop functions
  
  
  
• Sort List: sort() function can be used to sort a list in ascending or descending order. One requirement for using sort function is that elements in the list should be homogenous, meaning all elements should be of same type(either all numbers or all strings/characters) otherwise function will fail. By default sorting will be in ascending order.

  
  Syntax 1: list_object.sort()
  Syntax 2: list_object.sort(reverse=True) # Sort in descending order
  Syntax 3: list_object.sort(key=somefunction) # Sort using function
  Syntax 4: list_object.sort(reverse=True,key=somefunction) #Sort using function in descending Order
  

  • Example 1: Sort will fail as 'A' character value is present along with number elements.

    
    a_list = [1, 2, 3, 6, 'A', 6]
    a_list.sort()
    
    Output: TypeError: '<' not supported between instances of 'str' and 'int'
    

  • Example 2: Sort will rearrange the elements in ascending order.

    
    a_list = [1, 4, 3, 7, 6]
    a_list.sort()
    print(a_list)
    
    Output: [1, 3, 4, 6, 7]
    

  • Example 3: Sort will rearrange the elements in descending order as optional reverse parameter is passed as True.

    
    a_list = [1, 4, 3, 7, 6]
    a_list.sort(reverse=True)
    print(a_list)
    
    Output: [7, 6, 4, 3, 1]
    

  • Example 4: Each element of the list will be passed to function and sorting of elements will be based on output of function. By default it will ascending order of function output.

    
    a_list = [1, 4, 3, 7, 6]
    def remainder(num):
        return num%2
    
    a_list.sort(key=remainder)
    print(a_list)
    
    Output: [4, 6, 1, 3, 7]
    

  • Example 5: Sorting of elements will be based on output of function in the descending order as everse parameter is passed as True.

    
    a_list = [1, 4, 3, 7, 6]
    def remainder(num):
        return num%3
    
    a_list.sort(reverse=True, key=remainder)
    print(a_list)
    
    Output: [1, 4, 7, 3, 6]
    

  
  
  
• Copy List: copy() function returns a copy of the input list

  
  Syntax: list_object.copy()
  
  a_list = [1, 2, 3, 6, 'A']
  b_list = a_list.copy()
  print(a_list)
  Output 2: [1, 2, 3, 6, 'A']
  
  
  b_list.pop()   # Removing last item from b_list
  Output 3: 'A'
  
  
  print(b_list)   # 'A' value not present in b_list
  Output 4: [1, 2, 3, 6]
  
  
  print(a_list)   # 'A' value is still present in a_list
  Output 5: [1, 2, 3, 6, 'A']
  

  
  
  
• Reverse List: reverse() function reverses the input list(in place).

  
  Syntax: list_object.reverse()
  
  a_list = [1, 2, 3, 6, 'A']
  a_list.reverse()
  print(a_list)
  
  Output: ['A', 6, 3, 2, 1]
  

  
  
  
• Clear List: clear() function remove all items from list.

  
  Syntax: list_object.clear()
  
  a_list = [1, 2, 3, 6, 'A']
  a_list.clear()
  print(a_list)
  
  Output: []