Python List

The list type is a container, that implements the sequence protocol. List holds the number of objects in a given order and it allows add and remove objects from the sequence. List literals are written with in square brackets [].
To creare a list, put a number of expressions in square brackets.
Syntax:
L = [ ]

L = [expression, .....]

Example:

>>> colors=['red','green','blue','red']

>>> colors

['red', 'green', 'blue', 'red']

>>>

>>> numbers=[45,67,23,87]

>>> numbers

[45, 67, 23, 87]

This construct is known as a “list display”. Python also supports computed lists, called “list comprehensions”. I will discuss about it in later posts.
We can also use the built-in list type object to create list.
L = list() #empty list
Example:

>>> numbers=[45,67,23,87]

>>> numbers

[45, 67, 23, 87]

>>>

>>> num=list(numbers)

>>> num

[45, 67, 23, 87]

>>>

Index values are used to access the elements from the list.

>>> num[1]

67

>>> num[3]

87

Python never creates a new list if you assign a list to a variable. Assignment with an = on lists does not make a copy. Instead, assignment makes the two variables point to the one list in memory.
A = []
b = A # Both will point to the same list.
Example:

>>> a=['alice','bob']

>>> b=a # list 'a' aasign to 'b'

>>> b

['alice', 'bob']

>>> a.remove('alice') # Remove an item from list 'a'

>>> b # Item is also removes from list 'b'

['bob']

This shows that the two variables 'a' and 'b' pointing to the same memory location.

The '+' works to append two lists,

>>> [1,2]+[5,6]

[1, 2, 5, 6] 

Looping over list

The for construct 'for var in list ' is an easy way to look at each element in a list.

>>> names = ['alice','bob']

>>> for name in names :

... print name

...

alice

bob

If you need both the index and the item, use the enumerate function:

>>> for index,name in enumerate(names):

... print index, name

...

0 alice

1 bob

If you need only the index, use range and len:

>>> for index in range(len(names)):

... print index

...

0

1

The list object supports the iterator protocol. To explicitly create an iterator, use the built-in iter function:

>>> i=iter(names)

>>> item=i.next()

>>> item

'alice'

>>> item=i.next()

>>> item

'bob'

If a list contains numbers, the built-in sum function gives you the sum:

>>> num=[34,56,23,80]

>>> total=sum(num)

>>> total

193

If a list contains strings, you can combine the string into a single long string using the join string method:

>>> colors

['red', 'green', 'blue', 'red']

 >>> names=''.join(colors)
>>> names
'redgreenbluered'
Modifying Lists
Index values are used to assign values to list. 
>>> names[1]='tom'  # assign new value to list 'names'
>>> names
['alice', 'tom']

If multiple variables that point to the same list, all variables will be updated at the same time.

>>> names=['alice','bob']

>>> name=names

>>> names.append('tom')

>>> name

['alice', 'bob', 'tom']

>>> names

['alice', 'bob', 'tom']

Here both name and names are updated at the same time.

To create a separate list, you can use slicing or the list function to quickly create a copy:

>>> name=names[:]

>>> name

['alice', 'bob', 'tom']

>>> names.append('Rose')

>>> names

['alice', 'bob', 'tom', 'Rose']

>>> name

['alice', 'bob', 'tom']

Other list functions are:

• list.append(elem) -- adds a single element to the end of the list.
• list.insert(index, elem) -- inserts the element at the given index
• list.extend(list2) adds the elements in list2 to the end of the list.
• list.index(elem) -- searches for the given element from the start of the list and returns its index.
• list.remove(elem) -- searches for the first instance of the given element and removes it.
• list.sort() -- sorts the list in place (does not return it).
• list.reverse() -- reverses the list in place (does not return it)
• list.pop(index) -- removes and returns the element at the given index.

Searching List

The in operator can be used to check if an item is present in the list:

>>> if 'alice' in names:

... print 'Name exist'

...

Name exist

To get the index of the first matching item, use index:

>>> i=names.index('tom')

>>> print i

2

The index method does a linear search, and stops at the first matching item. If no matching item is found, it raises a ValueError exception.