Unpacking a list in Python

Unpacking a list involves splitting the elements of a list into individual variables. This is an extremely useful technique in Python, especially when we want to work with multiple elements of a list at the same time. With unpacking, we can easily assign values to variables, swap the values of variables, and iterate over multiple variables in a loop. In this tutorial, we will explore how to unpack a list in Python.

Unpacking a list :

• Python provides a shortcut method of assigning all the elements of a list to individual variables at once.
• In conventional method to assign elements of the list to individual variables we will have to access all elements of list as follows :

var_1 = list_name[0]
var_2 = list_name[1]
var_3 = list_name[2] 

• The list unpacking method simplifies this cumbersome process to great extent.
• The syntax for list unpacking is :  var_1, var_2, var_3 = list_name 
• The unpacking process can be performed with tuples as well :  var_1, var_2, var_3 = tuple_name 
• The number of variables should be equal to the number of elements in the list.
• If the number of variables is less than the number of elements of the list, python interpreter will raise an error :  ValueError: too many values to unpack  
• If the number of variables is more than the number of elements of the list, python interpreter will raise an error :  ValueError: not enough values to unpack 
• Code Sample : 

my_list = [10, 20, 30]
print("my_list = ", my_list)

var_1, var_2, var_3 = my_list
print("var_1 = ", var_1)
print("var_2 = ", var_2)
print("var_3 = ", var_3)


# Output
# my_list =  [10, 20, 30]
# var_1 =  10
# var_2 =  20
# var_3 =  30

Unpacking - packing :

• There is another syntax in python in which we can 
  • assign the first few elements of the list to variables and 
  • save the rest as a slice of the list.
• We use an asterisk for this purpose.
• The syntax for list unpacking-packing is as follows :

var_1, var_2, *var_3 = list_name

# Here :
# the first element of the list is assigned to var_1
# the second element of the list is assigned to var_2
# the rest of the elements are packed as a new list and assigned to var_3

• In this process, the python interpreter :
  • first unpacks the original list
  • assigns the matching number of elements to variables.
  • packs the remaining elements to a new list and assign it to the variable with asterisk.
• We can have only one variable with asterisk in this syntax.
• If we try to put asterisk on more than one variable, python interpreter will raise an error :  SyntaxError: multiple starred expressions in assignment
• We can put asterisk on any of the variable.
• Code Sample : 

my_list = [10, 20, 30, 40, 50, 60]
print("my_list = ", my_list)

var_1, var_2, *var_3 = my_list
print("var_1 = ", var_1)
print("var_2 = ", var_2)
print("var_3 = ", var_3)


# Output
# my_list =  [10, 20, 30, 40, 50, 60]
# var_1 =  10
# var_2 =  20
# var_3 =  [30, 40, 50, 60]