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PYTHON - LISTS

The most basic data structure in Python is the sequence. Each element of a sequence is assigned a number - its position or index. The first index is zero, the second index is one, and so forth.

Python has six built-in types of sequences, but the most common ones are lists and tuples, which we would see in this tutorial.

There are certain things you can do with all sequence types. These operations include indexing, slicing, adding, multiplying, and checking for membership. In addition, Python has built-in functions for finding the length of a sequence and for finding its largest and smallest elements.

Python Lists

The list is a most versatile datatype available in Python which can be written as a list of comma-separated values (items) between square brackets. Important thing about a list is that items in a list need not be of the same type.

Creating a list is as simple as putting different comma-separated values between square brackets. For example:

list1 = ['physics', 'chemistry', 1997, 2000];
list2 = [1, 2, 3, 4, 5 ];
list3 = ["a", "b", "c", "d"];

Similar to string indices, list indices start at 0, and lists can be sliced, concatenated and so on.

Accessing Values in Lists

To access values in lists, use the square brackets for slicing along with the index or indices to obtain value available at that index. For example:

#!/usr/bin/python
list1 = ['physics', 'chemistry', 1997, 2000];
list2 = [1, 2, 3, 4, 5, 6, 7 ];
print "list1[0]: ", list1[0]
print "list2[1:5]: ", list2[1:5]

When the above code is executed, it produces the following result:

list1[0]: physics
list2[1:5]: [2, 3, 4, 5]

Updating Lists

You can update single or multiple elements of lists by giving the slice on the left-hand side of the assignment operator, and you can add to elements in a list with the append() method. For example:

#!/usr/bin/python
list = ['physics', 'chemistry', 1997, 2000];
print "Value available at index 2 : "
print list[2];
list[2] = 2001;
print "New value available at index 2 : "
print list[2];

Note: append() method is discussed in subsequent section.

When the above code is executed, it produces the following result:

Value available at index 2 :
1997
New value available at index 2 :
2001

Deleting List Elements

To remove a list element, you can use either the del statement if you know exactly which element(s) you are deleting or the remove() method if you do not know. For example:

#!/usr/bin/python
list1 = ['physics', 'chemistry', 1997, 2000];
print list1;
del list1[2];
print "After deleting value at index 2 : "
print list1;

When the above code is executed, it produces following result:

['physics', 'chemistry', 1997, 2000]
After deleting value at index 2 :
['physics', 'chemistry', 2000]

Note: remove() method is discussed in subsequent section.

Basic List Operations

Lists respond to the + and * operators much like strings; they mean concatenation and repetition here too, except that the result is a new list, not a string.

In fact, lists respond to all of the general sequence operations we used on strings in the prior chapter.

Python Expression	Results	Description
len([1, 2, 3])	3	Length
[1, 2, 3] + [4, 5, 6]	[1, 2, 3, 4, 5, 6]	Concatenation
['Hi!'] * 4	['Hi!', 'Hi!', 'Hi!', 'Hi!']	Repetition
3 in [1, 2, 3]	True	Membership
for x in [1, 2, 3]: print x,	1 2 3	Iteration

Indexing, Slicing, and Matrixes

Because lists are sequences, indexing and slicing work the same way for lists as they do for strings.

Assume the following input:

L = ['spam', 'Spam', 'SPAM!']

Python Expression	Results	Description
L[2]	'SPAM!'	Offsets start at zero
L[-2]	'Spam'	Negative: count from the right
L[1:]	['Spam', 'SPAM!']	Slicing fetches sections

Built-in List Functions and Methods

Python includes the following list functions:

Sr. No.	Function with Description
1	cmp(list1, list2) Compares elements of both lists.
2	len(list) Gives the total length of the list.
3	max(list) Returns item from the list with max value.
4	min(list) Returns item from the list with min value.
5	list(seq) Converts a tuple into list.

Let us go through the functions in detail:

Cmp(list1, list2)

Description

The method cmp() compares elements of two lists.

Syntax

Following is the syntax for cmp() method:

cmp(list1, list2)

Parameters

list1 -- This is the first list to be compared.
list2 -- This is the second list to be compared.

Return Value

If elements are of the same type, perform the compare and return the result. If elements are different types, check to see if they are numbers.

If numbers, perform numeric coercion if necessary and compare.
If either element is a number, then the other element is "larger" (numbers are "smallest").
Otherwise, types are sorted alphabetically by name.

If we reached the end of one of the lists, the longer list is "larger." If we exhaust both lists and share the same data, the result is a tie, meaning that 0 is returned.

Example

The following example shows the usage of cmp() method.

#!/usr/bin/python
list1, list2 = [123, 'xyz'], [456, 'abc']
print cmp(list1, list2);
print cmp(list2, list1);
list3 = list2 + [786];
print cmp(list2, list3)

When we run above program, it produces following result:

-1
1
-1

43. len(List)

Description

The method len() returns the number of elements in the list.

Syntax

Following is the syntax for len() method:

len(list)

Parameters

list -- This is a list for which number of elements to be counted.

Return Value

This method returns the number of elements in the list.

Example

The following example shows the usage of len() method.

#!/usr/bin/python
list1, list2 = [123, 'xyz', 'zara'], [456, 'abc']
print "First list length : ", len(list1);
print "Second list length : ", len(list2);

When we run above program, it produces following result:

First list length : 3
Second lsit length : 2

44. max(list)

Description

The method max returns the elements from the list with maximum value.

Syntax

Following is the syntax for max() method:

max(list)

Parameters

list -- This is a list from which max valued element to be returned.

Return Value

This method returns the elements from the list with maximum value.

Example

The following example shows the usage of max() method.

#!/usr/bin/python
list1, list2 = [123, 'xyz', 'zara', 'abc'], [456, 700, 200]
print "Max value element : ", max(list1);
print "Max value element : ", max(list2);

When we run above program, it produces following result:

Max value element : zara
Max value element : 700

45. min(list)

Description

The method min() returns the elements from the list with minimum value.

Syntax

Following is the syntax for min() method:

min(list)

Parameters

list -- This is a list from which min valued element to be returned.

Return Value

This method returns the elements from the list with minimum value.

Example

The following example shows the usage of min() method.

#!/usr/bin/python
list1, list2 = [123, 'xyz', 'zara', 'abc'], [456, 700, 200]
print "min value element : ", min(list1);
print "min value element : ", min(list2);

When we run above program, it produces following result:

min value element : 123
min value element : 200

Python includes following list methods:

Sr. No.	Methods with Description
1	list.append(obj) Appends object obj to list
2	list.count(obj) Returns count of how many times obj occurs in list
3	list.extend(seq) Appends the contents of seq to list
4	list.index(obj) Returns the lowest index in list that obj appears
5	list.insert(index, obj) Inserts object obj into list at offset index
6	list.pop(obj=list[-1]) Removes and returns last object or obj from list
7	list.remove(obj) Removes object obj from list
8	list.reverse() Reverses objects of list in place
9	list.sort([func]) Sorts objects of list, use compare func if given

Let us go through the methods in detail:

List.append(obj)

Description

The method append() appends a passed obj into the existing list.

Syntax

Following is the syntax for append() method:

list.append(obj)

Parameters

obj -- This is the object to be appended in the list.

Return Value

This method does not return any value but updates existing list.

Example

The following example shows the usage of append() method.

#!/usr/bin/python
aList = [123, 'xyz', 'zara', 'abc'];
aList.append( 2009 );
print "Updated List : ", aList;

When we run above program, it produces following result:

Updated List : [123, 'xyz', 'zara', 'abc', 2009]

46. list.count(obj)

Description

The method count() returns count of how many times obj occurs in list.

Syntax

Following is the syntax for count() method:

list.count(obj)

Parameters

obj -- This is the object to be counted in the list.

Return Value

This method returns count of how many times obj occurs in list.

Example

The following example shows the usage of count() method.

#!/usr/bin/python
aList = [123, 'xyz', 'zara', 'abc', 123];
print "Count for 123 : ", aList.count(123);
print "Count for zara : ", aList.count('zara');

When we run above program, it produces following result:

Count for 123 : 2
Count for zara : 1

47. list.extend(seq)

Description

The method extend() appends the contents of seq to list.

Syntax

Following is the syntax for extend() method:

list.extend(seq)

Parameters

seq -- This is the list of elements

Return Value

This method does not return any value but add the content to existing list.

Example

The following example shows the usage of extend() method.

#!/usr/bin/python
aList = [123, 'xyz', 'zara', 'abc', 123];
bList = [2009, 'manni'];
aList.extend(bList)
print "Extended List : ", aList ;

When we run above program, it produces following result:

Extended List : [123, 'xyz', 'zara', 'abc', 123, 2009, 'manni']

48. list.index(obj)

Description

The method index() returns the lowest index in list that obj appears.

Syntax

Following is the syntax for index() method:

list.index(obj)

Parameters

obj -- This is the object to be find out.

Return Value

This method returns index of the found object otherwise raise an exception indicating that value does not find.

Example

The following example shows the usage of index() method.

#!/usr/bin/python
aList = [123, 'xyz', 'zara', 'abc'];
print "Index for xyz : ", aList.index( 'xyz' ) ;
print "Index for zara : ", aList.index( 'zara' ) ;

When we run above program, it produces following result:

Index for xyz : 1
Index for zara : 2

49. list.insert(index,obj)

Description

The method insert() inserts object obj into list at offset index.

Syntax

Following is the syntax for insert() method:

list.insert(index, obj)

Parameters

index -- This is the Index where the object obj need to be inserted.
obj -- This is the Object to be inserted into the given list.

Return Value

This method does not return any value but it inserts the given element at the given index.

Example

The following example shows the usage of insert() method.

#!/usr/bin/python
aList = [123, 'xyz', 'zara', 'abc']
aList.insert( 3, 2009)
print "Final List : ", aList

When we run above program, it produces following result:

Final List : [123, 'xyz', 'zara', 2009, 'abc']

50. list.pop(obj=list[-1])

Description

The method pop() removes and returns last object or obj from the list.

Syntax

Following is the syntax for pop() method:

list.pop(obj=list[-1])

Parameters

obj -- This is an optional parameter, index of the object to be removed from the list.

Return Value

This method returns the removed object from the list.

Example

The following example shows the usage of pop() method.

#!/usr/bin/python
aList = [123, 'xyz', 'zara', 'abc'];
print "A List : ", aList.pop();
print "B List : ", aList.pop(2);

When we run above program, it produces following result:

A List : abc
B List : zara

51. List.remove(obj)

Parameters

obj -- This is the object to be removed from the list.

Return Value

This method does not return any value but removes the given object from the list.

Example

The following example shows the usage of remove() method.

#!/usr/bin/python
aList = [123, 'xyz', 'zara', 'abc', 'xyz'];
aList.remove('xyz');
print "List : ", aList;
aList.remove('abc');
print "List : ", aList;

When we run above program, it produces following result:

List : [123, 'zara', 'abc', 'xyz']
List : [123, 'zara', 'xyz']

52. List.reverse()

Description

The method reverse() reverses objects of list in place.

Syntax

Following is the syntax for reverse() method:

list.reverse()

Parameters

NA

Return Value

This method does not return any value but reverse the given object from the list.

Example

The following example shows the usage of reverse() method.

#!/usr/bin/python
aList = [123, 'xyz', 'zara', 'abc', 'xyz'];
aList.reverse();
print "List : ", aList;

When we run above program, it produces following result:

List : ['xyz', 'abc', 'zara', 'xyz', 123]

53. list.sort([func])

Description

The method reverse() reverses objects of list in place.

Syntax

Following is the syntax for reverse() method:

list.reverse()

Parameters

NA

Return Value

This method does not return any value but reverse the given object from the list.

Example

The following example shows the usage of reverse() method.

#!/usr/bin/python
aList = [123, 'xyz', 'zara', 'abc', 'xyz'];
aList.reverse();
print "List : ", aList;

When we run above program, it produces following result:

List : ['xyz', 'abc', 'zara', 'xyz', 123]

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PYTHON - LISTS