All the IPython Notebooks in this lecture series are available at https://github.com/rajathkumarmp/Python-Lectures

The Zen Of Python

In [1]:
import this
The Zen of Python, by Tim Peters

Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than *right* now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!

Variables

A name that is used to denote something or a value is called a variable. In python, variables can be declared and values can be assigned to it as follows,

In [2]:
x = 2
y = 5
xy = 'Hey'
In [3]:
print x+y, xy
7 Hey

Multiple variables can be assigned with the same value.

In [4]:
x = y = 1
In [5]:
print x,y
1 1

Operators

Arithmetic Operators

Symbol Task Performed
+ Addition
- Subtraction
/ division
% mod
* multiplication
// floor division
** to the power of
In [6]:
1+2
Out[6]:
3
In [7]:
2-1
Out[7]:
1
In [8]:
1*2
Out[8]:
2
In [9]:
1/2
Out[9]:
0

0? This is because both the numerator and denominator are integers but the result is a float value hence an integer value is returned. By changing either the numerator or the denominator to float, correct answer can be obtained.

In [10]:
1/2.0
Out[10]:
0.5
In [11]:
15%10
Out[11]:
5

Floor division is nothing but converting the result so obtained to the nearest integer.

In [12]:
2.8//2.0
Out[12]:
1.0

Relational Operators

Symbol Task Performed
== True, if it is equal
!= True, if not equal to
< less than
> greater than
<= less than or equal to
>= greater than or equal to
In [13]:
z = 1
In [14]:
z == 1
Out[14]:
True
In [15]:
z > 1
Out[15]:
False

Bitwise Operators

Symbol Task Performed
& Logical And
l Logical OR
^ XOR
~ Negate
>> Right shift
<< Left shift
In [16]:
a = 2 #10
b = 3 #11
In [17]:
print a & b
print bin(a&b)
2
0b10
In [18]:
5 >> 1
Out[18]:
2

0000 0101 -> 5

Shifting the digits by 1 to the right and zero padding

0000 0010 -> 2

In [19]:
5 << 1
Out[19]:
10

0000 0101 -> 5

Shifting the digits by 1 to the left and zero padding

0000 1010 -> 10

Built-in Functions

Python comes loaded with pre-built functions

Conversion from one system to another

Conversion from hexadecimal to decimal is done by adding prefix 0x to the hexadecimal value or vice versa by using built in hex( ), Octal to decimal by adding prefix 0 to the octal value or vice versa by using built in function oct( ).

In [20]:
hex(170)
Out[20]:
'0xaa'
In [21]:
0xAA
Out[21]:
170
In [22]:
oct(8)
Out[22]:
'010'
In [23]:
010
Out[23]:
8

int( ) accepts two values when used for conversion, one is the value in a different number system and the other is its base. Note that input number in the different number system should be of string type.

In [24]:
print int('010',8)
print int('0xaa',16)
print int('1010',2)
8
170
10

int( ) can also be used to get only the integer value of a float number or can be used to convert a number which is of type string to integer format. Similarly, the function str( ) can be used to convert the integer back to string format

In [25]:
print int(7.7)
print int('7')
7
7

Also note that function bin( ) is used for binary and float( ) for decimal/float values. chr( ) is used for converting ASCII to its alphabet equivalent, ord( ) is used for the other way round.

In [26]:
chr(98)
Out[26]:
'b'
In [27]:
ord('b')
Out[27]:
98

Simplifying Arithmetic Operations

round( ) function rounds the input value to a specified number of places or to the nearest integer.

In [28]:
print round(5.6231) 
print round(4.55892, 2)
6.0
4.56

complex( ) is used to define a complex number and abs( ) outputs the absolute value of the same.

In [29]:
c =complex('5+2j')
print abs(c)
5.38516480713

divmod(x,y) outputs the quotient and the remainder in a tuple(you will be learning about it in the further chapters) in the format (quotient, remainder).

In [30]:
divmod(9,2)
Out[30]:
(4, 1)

isinstance( ) returns True, if the first argument is an instance of that class. Multiple classes can also be checked at once.

In [31]:
print isinstance(1, int)
print isinstance(1.0,int)
print isinstance(1.0,(int,float))
True
False
True

cmp(x,y)

x ? y Output
x < y -1
x == y 0
x > y 1
In [32]:
print cmp(1,2)
print cmp(2,1)
print cmp(2,2)
-1
1
0

pow(x,y,z) can be used to find the power $x^y$ also the mod of the resulting value with the third specified number can be found i.e. : ($x^y$ % z).

In [33]:
print pow(3,3)
print pow(3,3,5)
27
2

range( ) function outputs the integers of the specified range. It can also be used to generate a series by specifying the difference between the two numbers within a particular range. The elements are returned in a list (will be discussing in detail later.)

In [34]:
print range(3)
print range(2,9)
print range(2,27,8)
[0, 1, 2]
[2, 3, 4, 5, 6, 7, 8]
[2, 10, 18, 26]

Accepting User Inputs

raw_input( ) accepts input and stores it as a string. Hence, if the user inputs a integer, the code should convert the string to an integer and then proceed.

In [35]:
abc = raw_input("Type something here and it will be stored in variable abc \t")
Type something here and it will be stored in variable abc 	Hey
In [36]:
type(abc)
Out[36]:
str

input( ), this is used only for accepting only integer inputs.

In [37]:
abc1 =  input("Only integer can be stored in variable abc \t")
Only integer can be stored in variable abc 	275
In [38]:
type(abc1)
Out[38]:
int

Note that type( ) returns the format or the type of a variable or a number