3-Python Python oho pytho hdiwefjhdsjhds

Unit I: Introduction to Python Visit to website: learnpython4cbse.com
By: Amjad Khan Page 1 of 23
Chapter- 3 Python Programming Fundamentals
"The only way to learn a new programming language is
by writing programs in it." - Dennis Ritchie
You will learn the following topics in this Tutorial
S. No. Topics
1 Python Character Set
2 Tokens
 Keywords
 Identifiers / Variable
 Literals / Values
 Delimiters
 Operators
 Variable
3 Concepts of L-Value and R-Value
4 Comments
5 Statement
6 Clarity and simplicity of expression
7 Blocks and Indentation
8 The input ( ) function
9 The print ( ) function
10 Program Testing and Debugging
11 Errors and Exceptions
12 A) Compile – Time errors
13 A) Syntax Error
14 B) Semantic Errors

Python Character Set:
A character can represents any letter, digit, or any other sign. Following
are some of the python character set.
LETTERS A to Z and a to z
DIGITS 0 -9
SPECIAL SYMBOLS Space, + -* ^ *+ ,- =! = < > . ‗ ‗ ; : & #, under score(_) etc.
WHITE SPACE
Blank space, horizontal tab ( - > ), carriage return , Newline,
Form feed.
OTHER
CHARACTERS
Python can process all ASCII and Unicode characters as part
of data or literals.
Tokens:
The smallest individual unit in a program is known as token or lexical unit.
1) Keywords:
Keywords are special identifiers with predefined meanings that cannot
change. As these words have specific meaning for interpreter, they
cannot be used for any other purpose.
TOKENS
Keywords
Identifier
or
Varuables
Literals Delimiters Operators

Some Commonly Used Keywords:
2) Identifiers / Variable:
Identifiers are names given to identify something. Identifiers are
fundamental building blocks of a program and are used as general
terminology for the names given to different part of the program that is
variables, objects, classes, functions, lists, dictionaries etc.
NOTE: An identifier must not be a keyword of Python
There are some rules you have to follow for naming identifiers:
 An identifier starts with a letter A to Z or a to z or an underscore (_)
followed by zero or more letters, underscores and digits (0 to 9).
 Python does not allow special characters
 Identifier must not be a keyword of Python.
 Python is a case sensitive programming language.
Thus, Sname and sname are two different identifiers in Python.
and exec not continue if return except else
as finally or def import try class lambda
assert for Pass del in while global yield
break from print elif is with raise class

Some valid identifiers: Mybook, file123, z2td, date_2, _no
Some invalid identifier:
Some invalid variable name which produces syntax errors are as follows
>>> 98Sep_sco SyntaxError: invalid syntax because variable start with a digit
>>> Your Age SyntaxError: invalid syntax because variable contains space
>>> while SyntaxError: invalid syntax because variable is a reserve word
>>> myname@ SyntaxError: invalid syntax because variable contains a special character
3) Literals / Values:
Literals in Python can be defined as number, text, or other data that
represent values to be stored in variables. The data items which never
change their value throughout the program run. There are several kinds
of literals:
 String Literals
 Numeric Literals
 Boolean Literals
 Special Literal None
 Literal Collections

4) Delimiters:
Used to implement the grammatical and structure of Syntax.
Delimiters are used in various areas of the Python language. They are
used to build expressions, string literals, tuples, dictionaries or lists.
Following are the python delimiters.
5) Operators:
Operators are special symbols that perform specific operations on one,
two, or three operands, and then return a result.

Variable:
 Variable is an identifier whose value can change. For example variable
age can have different value for different person.
 Variable name should be unique in a program.
 Variables must always be assigned values before they are used in the
program, otherwise it will lead to an error.
 Wherever a variable name occurs in the program, the interpreter
replaces it with the value of that particular variable.
 Value of a variable can be string (for example, ‘A’, ‘Excellent’), number
(for example 98.5) or any combination of alphanumeric (alphabets and
numbers for example ‘B10’) characters.
In Python, we can use an assignment statement to create new variables
and assign specific values to them.
The basic assignment statement has this form:
<variable> = <expr>
Here, <variable> is an identifier and <expr> is an expression.
Operators Operators type
( ) [ ] { } Grouping
+ - * / // % ** Arithmetic Operator
== != <> <= >= Is In Relational Operators
And Not Or Logical Operators
& | ~ ^ << >> Bit-wise Operators

For Example:
Class = 12 # Variable created of numeric (integer) type
Remark = "Keep it up!" # Variable created of string type
Percentage = 98.5 # Variable created of numeric (floating point) type
Multiple Assignments:
 Assigning same value to multiple variables:
x = y = z = 100
It will assign value 100 to all three variables x, y and z.
 Assigning multiple value to multiple variables
p, q, r = 10, 20, 30
It will assign the value order wise that is value 10 assign to variable p,
value 20 assign to variable q and value 30 assign to variable r.
Example: Write a Python program to find the sum of two numbers.
num1 = 100
num2 = 200
result = num1 + num2
print(result)
Output: 300
Example: Write a Python program to find the area of a triangle given that
its base is 20 units and height is 40 units.
base = 20
height = 40

area = 0.5 *base * height
print(area)
Output: 400
Concepts of L-Value and R-Value
 Lvalue refers to object to which you can assign value. It refers to
memory location. It can appear LHS or RHS of assignment
 Rvalue refers to the value we assign to any variable. It can appear on
RHS of assignment
For example:
Num1 = 10 # Num1 is an L-value
Num2 = 20 # Num2 is an L-value
sum = Num1 + Num2 # (Num1 +Num2) is an R-value
print(sum)
Output: 30
Comments: (Code tells you how, comments should tell you why)
 Comments are used to add a remark or a note in the source code.
 Comments are not executed by interpreter.
 In Python, a single line comment starts with # (hash sign).
Python supports 3 ways to enter comments:

 Full line comment
 Inline comment
 Multiline comment
 Full line comment : Physical lines between with #
Example:
#This is program of volume of cylinder
 Inline comment: It start in the middle of the physical line
Example
area = length*breadth # calculating area of rectangle
 Multiline comment: You can put a multiple line or a block commment
Example: A. adding in the beginning of every line (using #)
# Program name: area of circle
# Date: 20/07/18
#Language : Python
Example : B. Multiline comment (using “ “ “) triple quotes
‘ ’ ’
Created by: learnpython4cbse.com
Date : 07/11/2020
Topic : Use of triple – qouted multi line string
‘ ’ ’

Statement
 It is a programming instruction that does something i.e. some action
takes place.
 Instructions that a Python interpreter can execute are called
statements
Example:
print (“Welcome to python”)
The above statement call print function
Multi-Line/ Continuation Statements:
Statements in Python typically end with a new line. Python does,
however, allow the use of the line continuation character () at the end of
the line to continue the statement onto the next line. For example,
# Assigns more values to the variables
Mark1, Mark2, Mark3, Mark4, Mark5 = 70, 80, 87, 89, 78
Total = Mark1 +
Mark2 +
Mark3 +
Mark4 +
Mark5
print ("Total mark is:", Total)
Output: Total mark is: 404

NOTE: Statements contained within the [ ], { }, or ( ) brackets do not need
to use the line continuation character.
For example:
days = ['Monday',
'Tuesday',
'Wednesday',
‘Thursday',
'Friday']
Clarity and simplicity of expression
Parenthesis should be used to show the precedence of the operators and
to enclose a sub-expression. This removes the ambiguities and
misinterpretation leading to wrong outputs.
Example,
Z = (a * a) + (3 * a * b) – (c * c)
For example,
import math
Root = (–b + math. sqrt (b * b – 4 * a * c)) / (2 * a)
It is better to break this expression i.e.
import math
D = math. sqrt (b * b – 4 * a * c)
X = 2 * a
Root = (–b + D) / X
No two operators can
occur one after the other.
The operators should have
operands before and after
them. In case there are
many sub-expressions in
an expression, then we
break it up into individual
sub- expressions.

Blocks and Indentation
A group of statements which are part of another statement or a function
are called block or code – block or suite in Python.
It’s best to use four spaces of indent for each code block level. If you use
another number of spaces (2, 6, 8), that’s fine. The important thing is that
all the code in the code block must have the same number of spaces.
Note: You cannot unnecessarily indent a statement; Python will raise
error for that.
Consider the following Example:
if n1<n2:
Temp =n1
n1=n2
n2=Temp
print (“I Understand Block”)
Example: Write a program to input a number (N) and find the sum of all
numbers till N and its average.
Also, find the sum of all even and odd numbers till N. The program also
justifies the indentation for both looping and branching to find the
correct result.
Solution:
Block of if statement with all its statements
at same indentation level

# Program to find sum of natural numbers, average of natural numbers,
# sum of even numbers and odd numbers
c1 = c2 = avg1 = avg2 = sum1 = sum2 = 0
i = 1
n = int(input("Enter the value of n: "))
while (i <= n): # while loop body started
if (i % 2 == 0): # Conditional construct
# Two lines body in true part
sum1 = sum1 + i
c1 = c1 + 1
else:
# Two lines body in false part
sum2 = sum2 + i
c2 = c2 + 1;
i+=1 # while loop counter
# Calculation part
avg1 = sum1 / c1;
avg2 = sum2 / c2;
sum = sum1 + sum2;
avg = sum / n;
print ("*************************************")
print ("The sum of natural numbers is => ", sum)
print ("The avg of natural numbers is => ", avg)
print ("The sum of even numbers is => ", sum1)
print ("The sum of odd numbers is => ", sum2)
print ("The avg of even numbers is => ", avg1)
print ("The avg of odd numbers is => ", avg2)
print ("*************************************")
if part indentation
else part indentation
while part
indentation

Function indentation
Indent the function body when a function starts.
# Function to find sum of two numbers
def Sum ( ): # Multiline in a block using function
a = 20
b = 40
result = a + b
print("Sum of two numbers is:", result)
Sum ( )
The input ( ) function
 The input ( ) function prompts the user to enter data.
 It accepts all user input as string.
 The user may enter a number or a string but the input ( ) function
treats them as strings only.
The syntax for input ( ) is:
input ([Prompt])
 Prompt is the string we may like to display on the screen prior to taking
the input, and it is optional.
For example,
Function body statements

Inputting Numeric Values
Suppose we want to find sum of two numbers and the program is:
The value that you type in front of
displayed prompt will be assigned to given
variables name and marks respectively.

When we execute the above program, the program doesn’t add the two
numbers together; it just puts one right after the other as 1020
The solution to the above problem is:
Python offers two functions int ( ) and float ( ) to be used with input ( ) to
convert the values received through input ( ) into int and float types.
Accept an Integer input from User
We need to convert an input string value into an integer using a int( )
function.
num1 = int (input("Enter first number: "))
num2 = int(input("Enter second number: "))
sum = num1 + num2
print ('The sum of ', num1, ' and ', num2, ' is ',sum)
OUTPUT: Enter first number: 10
Enter second number: 30
The sum of 10 and 30 is 40
Accept an float input from User
You need to convert user input to the float number using the float()
function as we did for the integer value.
num1 = float (input("Enter first number: "))
num2 = float(input("Enter second number: "))
sum = num1 + num2
print ('The sum of ', num1, ' and ', num2, ' is ',sum)
OUTPUT: Enter first number: 10.5
Enter second number: 30.5
Both input values are
converted into integer type
Both input values are
converted into float type

The sum of 10.5 and 30.5 is 41.0
Get multiple input values from a user in one line
In Python, we can accept two or three values from the user in one input()
call.
The print ( ) function
 The print ( ) function allows you to print out the value of a variable and
strings in parenthesis.
The general form of print ( ) function is:
print ( )
print (value/expr)
print (value, ...., sep=' ', end='n', file=sys.stdout, flush=False)
 The print ( ) function prints a blank line. For example:
>>> print ( )

 The print ( ) function prints the value of variable or expression. For
example:
>>> N1 = 20
>>> N2 = 30
>>> Sum = N1 + N2
>>> print (Sum) # prints a value
50
>>> print (N1+N2) # prints a value of an expression i.e., N1+N2
50
>>> Ver = 3.6
>>> Prog = "Python"
>>> print (Prog, Ver) # prints two values separated with comma (,)
Python 3.6
 The print() function contains number of options. You can use either
one, two or all at once.
For example:
− Print a message.
>>> print ("The sum is") # prints a message
The sum is
>>> print ("My Python")

My Python
− Print a value, message or both message and values and vice versa.
>>> print ("The sum is", Sum) # prints a message with a value
The sum is 50
>>> print (Sum, "is the sum of", N1, "and", N2) # print is alternate way
50 is the sum of 20 and 30
 The sep separator is used between the values. It defaults into a space
character. For example:
>>> print (10,20,30,40,sep='*') # prints a separator *
10*20*30*40
 After all values are printed, end is printed. It defaults into a new line.
>>> print (10, 20, 30, 40, sep = '*', end = '@') # A new end is assigned @
10*20*30*40@
Let us take other examples, where a print() function can output multiple
things at once, each item separated by a comma.
# The following code prints: The answer to 10+10 is 20
>>>print (" The answer to 10+10 is" , 10+10)
# The following code prints: The answer to 10+10 is 10+10
>>>print (" The answer to 10+10 is", "10+10 ")

# The following code does not work and product Invalid Syntax because
the comma (,) is inside the quotation marks and not outside.
>>>print (" The answer to 10+10 is ," 10+10)
 print () with Concatenate Operator (+)
The print() function can print two strings either using comma (,) operator,
or a concatenate operator (+).
For example,
>>> print ("My name is Ahil", "Class-XI A.") # Print with space separator.
My name is Ahil Class-XI A.
Similarly, by using concatenate operator (+), the print() function is:
>>> print ("My name is Ashaz" + "Class-XI B.") # Print without separator.
My name is AshazClass-XI A.
Formatting Data with print()
The general Syntax:
% width.precision type-char
For example:
a=2.45567
print("%.2F" %a)

Program Testing and Debugging
A programmer can make mistakes while writing a program, and hence,
the program may not execute or may generate wrong output.
Program Testing:
Program Testing means running the program, executing all its instructions
or functions and testing the logic by entering sample data in order to
check the output.
Debugging:
Debugging is the process of finding and correcting the errors in the
program code.
Errors and Exceptions:
An errors and exceptions both disrupt a program and stop its execution.
But Errors and Exceptions are not the same.
Errors in a program:
An error, also termed as ‘a bug’, is anything in the code that prevents a
program from compiling and running correctly.
Type of errors:
There are two types of errors:
1) Compile – time Errors
2) Runtime Errors

1) Compile – Time errors:
Errors that occur when you violate the rules of writing syntax are known
as compile – time errors.
Compiled time errors are broadly classified into two categaries.
A) Syntax error:
Every programming language has its own rules and regulations (syntax).
If we overcome the particular language rules and regulations, the
syntax error will appear (i.e. an error of language resulting from code
that does not conform to the syntax of the programming language).
It can be recognized during compilation time.
Example
a = 0
while a < 10
a = a + 1
print (a)
In the above statement, the second line is not correct. Since the while
statement does not end with ‘:’. This will flash a syntax error.
B) Semantic Errors:
It refers the set of rules which give the meaning of statement.

For Example:
X + Y = A
will result in a semantically error as an expression cannot come on the
left side of an assignment statement.
Run time Errors:
Errors that occur during the execution of a program are known as run
time errors.
Logical error:
It is a type of runtime error that may simply produce the wrong output
or may cause a program to crash while running.
It is an error in a program's source code that results in incorrect or
unexpected result.
The logical error might only be noticed during runtime, because it is
often hidden in the source code and are typically harder to find and
debug.
a = 100
while a < 10:
a = a + 1
print (a)
In the above example, the while loop will not execute even a single time,
because the initial value of ‘a’ is 100.

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