Python - List And Tuple

List and Tuple are built-in container types defined in Python. Objects of both these types can store different other objects that are accessible by index. List as well as tuple is a sequence data type, just as string. List as well as tuple can store objects which need not be of same type.

List : A List is an ordered collection of items (which may be of same or different types) separated by comma and enclosed in square brackets.

In [1]:
L1=[10,25.5,3+2j,"Hello"]
L1
Out[1]:
[10, 25.5, (3+2j), 'Hello']

In above list, each item is of different type. Further, each item is accessible by positional index starting from 0. Hence L1[2] will return 25.5

In [2]:
L1[1]
Out[2]:
25.5

Tuple: Tuple looks similar to list. The only difference is that comma separated items of same or different type are enclosed in parentheses. Individual items follow zero based index, as in list or string.

In [3]:
T1=(10,25.5,3+2j,"Hello")
T1
Out[3]:
(10, 25.5, (3+2j), 'Hello')
In [4]:
T1[1]
Out[4]:
25.5

Difference between List and Tuple:

The obvious difference is the use of square brackets [] in List and parentheses () in tuple as enclosures. However, the important difference is that List as a mutable object and Tuple is an immutable object.

If contents of an object can be modified in place, after it has been instantiated, is a mutable object. On the other hand, any operation on immutable object that tries to modify its contents is prohibited.

In above example, any item in the list L1 can be assigned with different value. Let us change value of item at index=2 from 3+2j to 1.22E-5

In [5]:
L1[2]=1.22E-5
L1
Out[5]:
[10, 25.5, 1.22e-05, 'Hello']

The built-in List class has different methods that allow various operations on List object (such as insertion, deletion, sorting etc)

However, any such operation is not possible with Tuple object. If we try to modify T1 by changing value of item at index=2 to 1.22E-5, TypeError exception is raised.

In [6]:
T1[2]=1.22E-5
T1
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-6-18a8ef5d3740> in <module>()
----> 1 T1[2]=1.22E-5
      2 T1
TypeError: 'tuple' object does not support item assignment

Following built-in functions can be used along with List as well as Tuple.

In [7]:
L1=[10,30,50,20,40]
T1=(10,50,30,40,20)
print (len(L1))
print (len(T1))
print ('max of L1', max(L1))
print ('max of T1', max(T1))
print ('min of L1', min(L1))
print ('min of T1', min(T1))
print ('sum of L1', sum(L1))
print ('sum of T1', sum(T1))
print ('L1 in sorted order', sorted(L1))
print ('T1 in sorted order', sorted(T1))
Out[7]:
5
5
max of L1 50
max of T1 50
min of L1 10
min of T1 10
sum of L1 150
sum of T1 150
L1 in sorted order [10, 20, 30, 40, 50]
T1 in sorted order [10, 20, 30, 40, 50]

If items in list/tuple are strings, min() and max() functions returns string that comes first/last in alphabetical order. If list/tuple is made up of numeric and nonnumeric values, TypeError exception is raised as comparison of dissimilar objects is not possible.

In [8]:
L2=['pen', 'book','computer', 'table', 'file']
T2=('pen', 'book','computer', 'table', 'file')
print ('max of L2', max(L2))
print ('max of T2', max(T2))
print ('min of L2', min(L2))
print ('min of T2', min(T2))
max of L2 table
max of T2 table
min of L2 book
min of T2 book
Out [9]:
L3=[100, "hundred", 0.001]
print ('max of L3', max(L3))
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-9-9ea5851a5ca9> in <module>()
      1 L3=[100, "hundred", 0.001]
----> 2 print ('max of L3', max(L3))

TypeError: '>' not supported between instances of 'str' and 'int'

The built-in list class has following methods to perform various operations on list object. Following methods allow new items to be added to list.

In [10]:
L1=[10,30,50,20,40]
L1.append(100) #appends new item
print ("after append",L1)
L1.insert(2,30) #inserts new value at index
print ('after insert',L1)
c=L1.count(30)
print ('count of 30',c)
L1.extend([11,22,33])
print ('after extend', L1)        
Out [10]:
after append [10, 30, 50, 20, 40, 100]
after insert [10, 30, 30, 50, 20, 40, 100]
count of 30 2
after extend [10, 30, 30, 50, 20, 40, 100, 11, 22, 33]

Following methods are used to remove items from given list.

In [11]:
p=L1.pop()
print ('item popped:', p)
print ('list after popping', L1)
L1.remove(100)
print ('after removing value :',L1)
L1.clear()
print ('all cleared:', L1)       
Out [11]:
item popped: 33
list after popping [10, 30, 30, 50, 20, 40, 100, 11, 22]
after removing value : [10, 30, 30, 50, 20, 40, 11, 22]
all cleared: []

Following methods rearrange sequence of items in the list

In [12]:
L1=[10, 30, 30, 50, 20, 40, 11, 22]
print ('original list :', L1)
L1.reverse()
print ('after reversing:',L1)
L1.sort()
print ("sorted list: ", L1)
Out [12]:
original list : [10, 30, 30, 50, 20, 40, 11, 22]
after reversing: [22, 11, 40, 20, 50, 30, 30, 10]
sorted list:  [10, 11, 20, 22, 30, 30, 40, 50]

If you recall, tuple is an immutable object. Hence the tuple class doesn’t have similar methods perform insertion, deletion or rearrangement of items.

Conversion functions

All sequence type objects (string, list and tuple) are inter-convertible. Python’s built-in functions for this purpose are explained below:

In [13]:
L1=[10, 30, 30, 50, 20, 40, 11, 22]
T1=tuple(L1)
print (T1)
(10, 30, 30, 50, 20, 40, 11, 22)
In [14]:
T1=(10,50,30,40,20)
L1=list(T1)
print (L1)
[10, 50, 30, 40, 20]
In [15]:
s1="Hello"
L2=list(s1)
print ('string to list:', L2)
T2=tuple(s1)
print ('string to tuple', T2)
string to list: ['H', 'e', 'l', 'l', 'o']
string to tuple ('H', 'e', 'l', 'l', 'o')
In [16]:
s1=str(L1)
s2=str(T1)
print ('list to string',s1)
print ('tuple to string',s2)
list to string [10, 50, 30, 40, 20]
tuple to string (10, 50, 30, 40, 20)

When a list or tuple is converted to string by str() function, the string representation is not exactly similar to word, but list or tuple surrounded by single quote marks. To form a continuous sequence of characters in the list, use join() method of string object.

In [17]:
L2=['H', 'e', 'l', 'l', 'o']
s1=str(L2)
s1
Out[17]:
"['H', 'e', 'l', 'l', 'o']"
In [18]:
s2="".join(L2)
print ('string from list items:', s2)
string from list items: Hello

In this chapter, we discussed the list and tuple objects, their functions and methods. In next chapter we shall learn about dictionary data type.