Python Basics

 

Python is an interpreted, high-level, general-purpose programming language. Created by Guido van Rossum and first released in 1991. he started python as a hobby project to keep him occupied in the week around Christmas. Got its name from the name of British comedy troupe Monty Python. It is used in :

1. Lesser Libraries
   As compared to other programming languages like c++, java, .Net, etc. it has a lesser number of libraries, but libraries of other libraries can easily be implemented and imported
    
2. Slow Language
   Since being an interpreted language, the code is executed slowly
    
3. Weak on Type-Binding
   A variable that is initially declared int can be changed to string at any point in the program without any typecasting.

 

Python 2	Python 3
1. The output of the division operator is an integer	1. The output of the division operator is Float
2. print "hello" is the implementation of the print function here	2. print("hello") is the implementation of the print function here
3. Here Bytes and String is the same	3. Here Bytes and String is different
4. Implicit String type here is ASCII	4. Implicit String type here is Unicode
5. raise IOError, “your error message” is used to raise an exception	5. raise IOError(“your error message”) is used for raising an exception
6. The syntax here is comparatively difficult to understand.	6. The syntax is simpler and easily understandable.
7. The xRange() function used to perform iterations.	7. The new Range() function introduced to perform iterations.
8. The value of the global variable will change while using it inside for-loop.	8. The value of variables never changes.
9. Python 2 code can be converted to Python3 code	9. It is not backward compatible
10. It uses raw_input() for taking inputs from the command line	10. It uses input() for taking inputs from the command line

 

 

1. CPython

Written in C, the most common implementation of Python 

 

2. JPython

Written in Java, compiles to bytecode

 

3. IronPython

Implemented in C#, an extensibility layer to frameworks written in .NET

 

4. Brython

Browser Python runs in the browser 

 

5. RubyPython

A bridge between Python and Ruby interpreters

 

6. PyPy

Implementation in Python

 

7. MicroPython

Runs on a Microcontroller 

 

 

The following are then character set recognized by python. It uses the traditional ASCII character set.

 

The smallest individual unit in a program is known as a token.

1. Keywords
   There are in total of 31 reserved keywords. Ex: and, finally, not, etc.
    
2. Identifiers
   a python identifier is a name used to identify a variable, function, class, module, or other objects. Ex: MyBook, myBook, mybook, etc.
    
3. Literals
   Literals in Python can be defined as numbers, text, or other data that represent values to be stored in variables. Ex: age = 22, escape sequence, etc.
    
4. Operators
   It can be defined as symbols that are used to perform operations on operands. Ex: arithmetic operators, etc.
    
5. Punctuators
   Used to implement the grammatical and structure of a Syntax. Ex: &=, >>=, <<=, etc.

 

1. Expression
    
   Which is evaluated and hence produces results. Ex: (20=4)/4
    
2. Statements
    
   Instructs that we are doing something.
    
   Ex: a=20
    
   print("Calling in proper sequence")
    
3. Comments
    
   Which is readable for a programmer but ignored by the python interpreter.
    
   a. Single line comment (#)
    
   b. Multi-line comment (''' ''')
    
   Ex: #this is a single-line comment
    
   ''' this is
    
   multiple '''
    
4. Function
    
   a code that has some name and it can be reused. Ex: KetArgFunc()
    
5. Block & Indentation
    
   A group of statements in block indentation at the same level creates a block. Ex: all the statements that are under the function KetArgFunc()

 

A variable can be considered as a container that holds value. Python is a type-infer language i.e. one which can detect data types automatically or in other words it means you dont need to specify the data type of variable.

 

Note: although python supports unlimited variable length, following PEP-8, we keep the maximum length to 79 characters 

 

For example:

1. Number
   It is used to store numeric values. Python has 3 numeric types:
    
   a. Integers
   Ex: a = 10
    
   b. Floating Point numbers
   Ex: a = 10.0
    
   c. Complex numbers
   Ex: a = complex(10,5) # 10+5j
    
2. String
   A string is a sequence of characters. In python, we can create a string using a single ( ' ' ) or double quotes (" "). Both are same in python
   1. str = 'computer science'
   2. print('str- ', str) # print string
   3. print('str[0]-', str[0]) # print 1st char
   4. print('str[1:3]-', str[1:3]) # print string from position 1 to 3
   5. print('str[3:]-', str[3:]) # print string starting from 3rd char
   6. print('str*2-' , str*2) # print string two times
   7. print('str + yes-', str+'yes') # concatenated string
3. Boolean
   It is used to store two possible values either true or false
   1. str = "comp sc"
   2. b = str.isupper()
   3. print(b)
4. List
   List are collections of items and each item has its index value. Denoted by []
   1. list = [6,9]
   2. list[0] = 55
   3. print(list[0])
   4. print(list[1])
5. Tuple
   A tuple is an immutable Python Object. Immutable python objects mean you cannot modify the contents of a tuple once it is assigned. Denoted by ()
   1. tup = (66,99)
   2. Tup[0] = 3 #error will be displayed
   3. print(tup[0])
   4. print(tup[1])
6. Set
   It is an unordered collection of unique and immutable items. Denoted by {}
   1. set1 = {11, 22, 33, 22}
   2. print(set1)
7. Dictionary
   It is an unordered collection of items and each item consists of a key and a value.
   1. dict = {'subject' : 'comp sc", 'class' : '11'}
   2. print(dict)
   3. print( "Subject :", dict["subject"])
   4. print("class :", dict.get('class'))

 

The Precedence of Operators in Python can be found from the rule PEMDAS ( Parenthesis, Exponential, Multiply, Division, Addition, Subtraction). Operators with the same precedence are evaluated from left to right.

1. Python Arithmetic Operator
    
   It is used for mathematical operations
   1. x = 5
   2. y = 4
   3. print('x + y =',x+y) #Addition
   4. print('x - y =',x-y) #Substraction
   5. print('x * y =',x*y) #Multiplication
   6. print('x / y =',x/y) #Division
   7. print('x // y =',x//y) #Floor Division
   8. print('x ** y =',x**y) #Exponent
2. Python Comparison Operators
    
   used to compare values
   1. x = 101
   2. y = 121
   3. print('x > y is',x>y) #Greater than
   4. print('x < y is',x<y) #Less than
   5. print('x == y is',x==y) #Equal to
   6. print('x != y is',x!=y) #Not Equal to
   7. print('x >= y is',x>=y) #Greater than equal to
   8. print('x <= y is',x<=y) #Less than equal to
3. Python Logical Operators
   1. x = True
   2. y = False
   3. print('x and y is',x and y) #if both are true
   4. print('x or y is',x or y) #if either one is true
   5. print('not x is',not x) #returns the complement
4. Python Bitwise Operators
    
   Used to manipulate bit values
   1. a = 6
   2. b = 3
   3. print ('a=',a,':',bin(a),'b=',b,':',bin(b)) c = 0
   4. c = a & b
   5. print ("result of AND is ", c,':',bin(c))
   6. c = a | b
   7. print ("result of OR is ", c,':',bin(c))
   8. c = a ^ b
   9. print ("result of EXOR is ", c,':',bin(c))
   10. c = ~a
   11. print ("result of COMPLEMENT is ", c,':',bin(c))
   12. c = a << 2
   13. print ("result of LEFT SHIFT is ", c,':',bin(c))
   14. c = a>>2
   15. print ("result of RIGHT SHIFT is ", c,':',bin(c))
5. Python Membership Operators
    
   Test for membership in a sequence
   1. a = 5
   2. b = 10
   3. list = [1, 2, 3, 4, 5 ]
   4. if ( a in list ):
   5.     print ("Line 1 - a is available in the given list")
   6. else:
   7.     print ("Line 1 - a is not available in the given list")
   8. if ( b not in list ):
   9.     print ("Line 2 - b is not available in the given list")
   10. else:
   11.     print ("Line 2 - b is available in the given list")
6. Python Identity Operators
    
   They check if values on either side of equality operator point to the same object
   1. a = 10
   2. b = 10
   3. print ('Line 1','a=',a,':',id(a), 'b=',b,':',id(b))
   4. if ( a is b ):
   5.    print ("Line 2 - a and b have same identity")
   6. else:
   7.    print ("Line 2 - a and b do not have same identity")

In computer programming, operator overloading, sometimes termed operator ad hoc polymorphism, is a specific case of polymorphism, where different operators have different implementations depending on their arguments. Operator overloading is generally defined by a programming language, a programmer, or both.

Unary Operator	Method
-	__neg__(self)
+	__pos__(self)

Python Control Statements

 

Control statements are used to control the flow of execution depending upon the specified condition/logic.

1. If- Statement
    
   An if statement is a programming conditional statement that, if proved true, performs a function or displays information
    
   
   1. noofbooks = 2  
   2. if (noofbooks == 2) :
   3.    print(" You have")  
   4.    print("two books")  
   5. print("outside of if statement")  
   In the above code, we are checking if noofbooks is equal to 2, then execute the given code.
    
2. if-else statement
    
   the if-else statement executes some code if the test expression is true (non-zero) and some other code if the test expression is false
    
   
   1. a=10  
   2. if a<100:   
   3.       print('less than 100')  
   4. else:  
   5.       print('more than equal 100')  
   In the above code, we are checking if a is less than 100, else will print "more than equal 100'
    
3. Nested If-Else Statement
    
   The nested if ... else statement allows you to check for multiple test expressions and executes different codes for more than two conditions
    
   
   1. num = float(input("enter a number:"))  
   2. if num >=0:  
   3.      if num==0:  
   4.          print("zero")  
   5. else:  
   6.      print("Negative number")  
   In the above code, we are first checking if num is greater than or equal to 0 and then if it is zero if both the conditions are met, "zero" is printed, otherwise "Negative number"
    

1. While Loop
    
   It is used to execute a block of a statement as long as a given condition is true. And when the condition becomes false, the control will come out of the loop. The condition is checked every time at the beginning of the loop
    
   
   1. x=1  
   2. while x<=4 :  
   3.       print(x)  
   4.       x = x+1 
   In the above code, we are printing all the values from 1 to 4
    
2. For Loop
    
   It is used to iterate over items of any sequence, such as a list or a string.
    
   
   1. for i in range (3,5):  
   2.       print(i) 
In the above code, we are printing numbers from 3 to 5.
 
3. Nested For Loop
    
   For loop within for loop is what we mean by nested for loops
    
   
   1. for i in range (1,3):  
   2.     for j in range(1,11):  
   3.           k=i*j  
   4.           print(k, end = ' ')  
   5.     print() 
In the above code, we are running our loop firstly from 1 to 3 and then for each iteration running for another 10 times.
 

 

They are used to transfer the program's control from one location to another. This means these are used to alter the flow of a loop like-to skip a part of a loop or terminate a loop.

1. Break
    
   It is used to terminate the loop
    
   
   1. for val in "string":  
   2.       if val == "i":  
   3.              break  
   4.       print(val) 
In the above code, we break the loop as soon as "i" is encountered, hence printing "str"
 
2. continue
    
   It is used to skip all the remaining statements in the loop and move controls back to the top of the loop.
    
   
   1. for val in "init":  
   2.       if val == "i":  
   3.             continue  
   4.        print(val)  
   5. print(" The End") 
In the above code, we will be printing "nt", because for "i" it will skip.
 
3. pass
    
   This statement does nothing. It can be used when a statement is required syntactically but the program requires no action.
   1. for i in "initial":  
   2.       if(i == "i"):  
   3.             pass  
   4.       else:  
   5.            print(i) 
Continue forces the loop to start the next iteration while pass means "there is no code to execute here" and will continue through the remainder of the loop body.The output will be "ntal"

Python Errors & Exception 

 

 

A Syntax error is an error in the syntax of a sequence of characters or tokens that are intended to be written in a particular programming language.

 

>>> while True print 'Hello world'  In the above code, it is an Invalid Syntax error. It should be as :

>>> while True:

             print 'Hello world'

 

Standard Errors available in Python are:

IndentationError, SystemExit, ValueError, TypeError, RuntimeError

 

 

The other kind of error in Python are exceptions. Even if a statement or expression is syntactically correct, it may cause an error when an attempt is made to execute it. Errors detected during execution are called exceptions 

 

For Example: 

>> 10 * (1/10)

 

The above code is syntactically okay, but when we execute it, it will result in ZeroDivisionError: integer division or modulo by zero.

 

Standard Exceptions available in Python are:

SystenExit, OverflowError, FloatingPointError, ZeroDivisonError, EOFError, KeyboardInterrupt, IndexError, IOError 

 

 

If we see some suspicious code that may raise an exception, we can defend our program by placing the code in the try block 

 

 

 

Function	Description
list.append()	Add an item at end of a list
list.extend()	Add multiple items at the end of a list
list.insert()	insert an item at a defined index
list.remove()	remove an item from a lsit
del list[index]	delete an item from a list
list.clear()	empty all the list
list.pop()	remove an item at a defined index
list.index()	return index of the first matched item
list.sort()	sort the items of a list in ascending or descending order
list.reverse()	reverse the items of a list
len(list)	return total length of the list
max(list)	return item with maximum value in the list
min(list)	return item with the minimum value in the list
list(seq)	converts a tuple, string, set, dictionary into a list

 

 

Function	Description
dict.clear()	removes all elements of dictionary dict
dict.copy()	returns a shallow copy of dictionary dict
dict.items()	retuns a list of dict's (key, value) tuple pairs
dict.setdeafult(key, default=Nonre)	similiar to get(), but will set dict[key] = default
dict.update(dict2)	adds dictionary dict2's key-values pairs to dict
dict.keys()	returns list of dictionary dict's keys
dict.values()	returns list of dictionary dict's values

 

 

Function	Description
tuple(seq)	converts a list into a tuple
min(tuple)	returns item from the tuple with a minimum value
max(tuple)	returns item from the tuple with maximum value
len(tuple)	gives the total length of the tuple
cmp(tuple1,tuple2)	compares elements of both tuples

 

 

A class is a code template for creating objects. Objects have member variables and have behavior associated with them. In python, a class is created by the keyword class. An object is created using the constructor of the class. This object will then be called the instance of a class. In Python, we create instances in the following manner.

            statements 

 

In the above code, self is very important, as, without the presence of self, python will not allow us to use any of the class member functions. It is similar to "this" used in Java. With only one difference that in java using "this" is not compulsory

In the above code, we have created 3 methods namely set_location(), distance_from_origin() and distance(). 

 

 

A client can call the methods of an object in two ways:

1. object.method( parameter)
2. Class.method( object, parameters)

Python Class Constructors

 

It is equivalent to Java's toString which converts objects to a string. It is automatically involved when str or print is called.

 

 

Operator	Class Methods
-	__sub__(self, other)
+	__pos__(self, other)
*	__mul__(self, other)
/	__truediv__(self, other)
==	__eq__(self, other)
!=	__ne__(self, other)
<	__lt__(self, other)
>	__gt__(self, other)
<=	__le__(self, other)
>=	__ge__(self, other)

 

 

In the next chapter, we will learn some basics about Python Django.