Python3: February 2018

iv)List :

List is the best of the data types in python3.

It can hold anything(Literally anything).

Syntax :

There are two ways you can declare an empty list:

list() or []

>>> type(list())

>>> type([])

>>> [1,2,"string", """ multiline \n comment """, 12.564]

[1, 2, 'string', ' multiline \n comment ', 12.564]

>>> a = [2, 5, 45.74, "python"]

>>> a[0]

>>> a[3]

'Python'

Python has a special provision for accessing elements from last in case the length of list is unknown.

You can access the last element by using negative indexing.

List[-n] returns nth element counting reverse , starting from the last in the List .

>>> a[-1]

'Python'

>>> a[-3]

Checking length(number of elements) of list :

len(list) is the method used to find.

>>> len(a)

Slicing in list :

Slicing is a process of selection of elements by using indexes.

List[a:b] will return list of elements in List from index a to b

Excluding element at bth position. b>=a

If a/b parameters are not provided then python takes extreme

Position.

For example :

>>> a[2:4]

[45.74, 'python']

>>> a[2:1]

[]

>>> a[3:]

['python']

>>> a[:3]

[2, 5, 45.74]

>>> a[:]

[2, 5, 45.74, 'python']

Slicing with negative index :

>>> a[:-1]

[2, 5, 45.74]

>>> a[:-2]

[2, 5]

a[-x ] is same as a[len(a) - x ]

>>> a[:-1]

[2, 5, 45.74]

>>> a[:-2]

[2, 5]

Reversing a list :

>>> a[::1]

[2, 5, 45.74, 'python']

reversed(x) can be used to reverse a list.

Rerversed function returns a reverseiterator . So, it has

to be type casted to list.

>>> list (reversed(a))

['python', 45.74, 5, 2]

Counting occurrences of certain element in a list.

Syntax : list.count(element_to_find_frequency_of)

Example :

>>> [1, 1, 2, 1, 2, 5, 8 ,6, ""].count(1)

Sorting a list :

sort attribute can be used to sort a list.

To use sort attribute, list cannot have any other type other than

that of int and float.

sort attribute does not return anything but performs sorting on

the list.

>>> a = [1, 2, 5, 4, 3]

>>> a.sort()

>>> a

[1, 2, 3, 4, 5]

Pop attribute of list :

Syntax : list.pop(index)

Where, index is index of element you want to remove

from the list.

This attribute returns the deleted element.

[1, 3.23, 4, 5]

>>> a.pop(1)

3.23

>>> a

[1, 4, 5]

Copying a list :

Suppose a=[1,2,3] is a list.

b = a will give reference of a to b, not copying the list.

Now, what does that mean??

That means, when you will modify elements of a, elements of

b will also be affected and vice versa.

For copying the elements of a list to another, copy attribute is

Used.

b = a.copy()

Now, elements in b will have elements same as a but

modifying a or b won’t affect the other list.

Append attribute :

Append takes exactly one argument as an input that will

include the argument passed inside the attribute in the list.

>>> a

[1, 45, 3, 4, 5]

>>> a.append([1,2,3])

>>> a

[1, 45, 3, 4, 5, [1, 2, 3]]

Here, i append a list [1, 2, 3] to the list a.

Removing all the elements :

clear attribute is used.

>>> a.clear()

>>> a

[]

List can be merged with another list using extend attribute of the list

list.extend(list1)

>>> a = [1, 2, 3, 5]

>>> a.extend([4, 6])

>>> a

[1, 2, 3, 5, 4, 6]

This is one of the way you can add a list to another list.

Another way is :

list = list + list1

>>> a = a + [9, 7, 8]

>>> a

[1, 2, 3, 5, 4, 6, 9, 7, 8]

These were some of the attributes and functionalities you can do using list.
More properties we will see in future posts.

Before starting with our next data Type, Let’s first know about variables in python:

Variables :

These are any names which can be assigned any data type:

Python has no restrictions on use of variables, We don’t need

any explicit declaration of type of variable. Python assigns

data type of the variable by the data it holds.

Confused, correct?? Let’s take an example:

>>> a = 3

Is a variable declaration where, a is variable name and 3 is an

int data type assigned to it.

>>> type(a)

You can overwrite same variable to hold value of same or different data type.

>>> a = 32

>>> a = 45

>>> a

>>> a = "string"

>>> a

'string'

>>> type(a)

Using variable to access the data it is storing.

>>> a = 23

>>> b = 3

>>> a+b

>>> a**b

12167

One of the best things what python provides is simultaneous

assignment of variables.

For example :

>>> a,b = 12,"string"

>>> a

>>> b

'string'

>>> a, b, c = 2,1, 9

>>> a+b+c

iii)String:

String can be defined in three ways :

using single quotes, double quotes, triple single quotes or Double single

quotes.

"String_name" or 'string_name' are the basic way you can define a string.

A special provision for writing a multiline string is provided by python

using three quotes:

For example : """ I am a multiline comment

Which can be extended for any number of lines

.............

String ends here """

Or, instead of """ comment """", ''' comment ''' can be used.

Note : Combination of quotes cannot be used. Either of the quotes

type has to be used.

By default, the string quotes used by python is ' '.

>>> "python3"

'python3'

>>> type(""" I am a

... multiline comment

... """)

>>> "hello "+"world"

'hello world'

Here, what interpreter has done is not addition of strings but

concatenation of strings.

Concatenation is appending of one string at the last of another

string.

String repetition:

>>> " I am repeating"*3

' I am repeating I am repeating I am repeating'

Saturday, 3 February 2018