Python_Module_1.pdf

Python Programming 20CSI44
Dept. of School of Computing, NCET 2021-2022 Page 1
MODULE – 1
WHY SHOULD YOU LEARN TO WRITE PROGRAMS
Programs are generally written to solve the real-time arithmetic/logical problems. Nowadays,
computational devices like personal computer, laptop, and cell phones are embedded with operating
system, memory and processing unit. Using such devices one can write a program in the language
(which a computer can understand) of one’s choice to solve various types of problems. Humans are
tend get bored by doing computational tasks multiple times. Hence, the computer can act as a
personal assistant for people for doing their job!! To make a computer to solve the required problem,
one has to feed the proper program to it. Hence, one should know how to write a program!!
There are many programming languages that suit several situations. The programmer must be able to
choose the suitable programming language for solving the required problem based on the factors like
computational ability of the device, data structures that are supported in the language, complexity
involved in implementing the algorithm in that language etc.
Creativity and Motivation
When a person starts programming, he himself will be both the programmer and the end- user.
Because, he will be learning to solve the problems. But, later, he may become a proficient
programmer. A programmer should have logical thinking ability to solve a given problem. He/she
should be creative in analyzing the given problems, finding the possible solutions, optimizing the
resources available and delivering the best possible results to the end-user. Motivation behind
programming may be a job-requirement and such other prospects. But, the programmer should
follow certain ethics in delivering the best possible output to his/her clients. The responsibilities of a
programmer includes developing a feasible, user-friendly software with very less or no hassles. The
user is expected to have only the abstract knowledge about the working of software, but not the
implementation details. Hence, the programmer should strive hard towards developing most
effective software.
Computer Hardware Architecture
To understand the art programming, it is better to know the basic architecture of computer hardware.
The computer system involves some of the important parts as shown in Figure
These parts are as explained below:
 Central Processing Unit (CPU): It performs basic arithmetic, logical, control and I/O
operations specified by the program instructions. CPU will perform the given tasks with a
tremendous speed. Hence, the good programmer has to keep the CPU busy by providing
enough tasks to it.
 Main Memory: It is the storage area to which the CPU has a direct access. Usually, the
programs stored in the secondary storage are brought into main memory before the
execution. The processor (CPU) will pick a job from the main memory and performs the
tasks. Usually, information stored in the main memory will be vanished when the computer is
turned-off.

Figure 1.1 Computer Hardware Architecture
 Secondary Memory: The secondary memory is the permanent storage of computer.
Usually, the size of secondary memory will be considerably larger than that of main memory.
Hard disk, USB drive etc can be considered as secondary memory storage.
 I/O Devices: These are the medium of communication between the user and the computer.
Keyboard, mouse, monitor, printer etc. are the examples of I/O devices.
 Network Connection: Nowadays, most of the computers are connected to network
and hence they can communicate with other computers in a network. Retrieving the
information from other computers via network will be slower compared to accessing the
secondary memory. Moreover, network is not reliable always due to problem in connection.
The programmer has to use above resources sensibly to solve the problem. Usually, a programmer
will be communicating with CPU by telling it „what to do next‟. The usage of main memory,
secondary memory, I/O devices also can be controlled by the programmer.
To communicate with the CPU for solving a specific problem, one has to write a set of instructions.
Such a set of instructions is called as a program.
Understanding Programming
A programmer must have skills to look at the data/information available about a problem, analyze it
and then to build a program to solve the problem. The skills to be possessed by a good programmer
includes –
 Thorough knowledge of programming language: One needs to know the vocabulary and
grammar (technically known as syntax) of the programming language. This will help in
constructing proper instructions in the program.
 Skill of implementing an idea: A programmer should be like a „story teller‟. That is, he
must be capable of conveying something effectively. He/she must be able to solve the
problem by designing suitable algorithm and implementing it. And, the program must
provide appropriate output as expected.
Software
What
Next?
Secondary
Memory
Main
Memory
Central
Processing Unit
Input and Output
Devices

Thus, the art of programming requires the knowledge about the problem‟s requirement and the
strength/weakness of the programming language chosen for the implementation. It is always
advisable to choose appropriate programming language that can cater the complexity of the problem
to be solved.
Words and Sentences
Every programming language has its own constructs to form syntax of the language. Basic
constructs of a programming language includes set of characters and keywords that it supports. The
keywords have special meaning in any language and they are intended for doing specific task.
Python has a finite set of keywords as given in Table 1.1.
Table 1.1 Keywords in Python
And as Assert break class continue
Def del Elif else except False
Finally for From global if import
In is lambda None nonlocal not
Or pass Raise return True try
While with Yield
A programmer may use variables to store the values in a program. Unlike many other programming
languages, a variable in Python need not be declared before its use.
Python Editors and Installing Python
Before getting into details of the programming language Python, it is better to learn how to install
the software. Python is freely downloadable from the internet. There are multiple IDEs (Integrated
Development Environment) available for working with Python. Some of them are PyCharm,
LiClipse, IDLE etc. When you install Python, the IDLE editor will be available automatically. Apart
from all these editors, Python program can be run on command prompt also. One has to install
suitable IDE depending on their need and the Operating System they are using. Because, there are
separate set of editors (IDE) available for different OS like Window, UNIX, Ubuntu, Soloaris, Mac,
etc. The basic Python can be downloaded from the link:
https://www.python.org/downloads/
Python has rich set of libraries for various purposes like large-scale data processing, predictive
analytics, scientific computing etc. Based on one‟s need, the required packages can be downloaded.
But, there is a free open source distribution Anaconda, which simplifies package management and
deployment. Hence, it is suggested for the readers to install Anaconda from the below given link,
rather than just installing a simple Python.
https://anaconda.org/anaconda/python
Successful installation of anaconda provides you Python in a command prompt, the default editor
IDLE and also a browser-based interactive computing environment known as jupyter notebook.

The jupyter notebook allows the programmer to create notebook documents including live code,
interactive widgets, plots, equations, images etc. To code in Python using jupyter notebook, search
for jupyter notebook in windows search (at Start menu). Now, a browser window will be opened
similar to the one shown in Figure 1.2.
Figure 1.2 Homepage of Jupyter Notebook
Figure 1.3 IDE of Jupyter Notebook

You can choose the working directory of your choice for storing your work. To open a notebook for
Python programming, click on New button at the right-side of the screen. Now select Python 3 from
the drop-down list. A new notebook (or workbook will be created as shown in Figure 1.3. Type a
command of your choice and press Ctrl+Enter to run that command. One can give
headings/subheadings etc for the commands being typed, store the entire workbook for future
reference etc. Readers are advised to try and experience various options/menu‟s available.
Conversing with Python
Once Python is installed, one can go ahead with working with Python. Use the IDE of your choice
for doing programs in Python. After installing Python (or Anaconda distribution), if you just type
„python‟ in the command prompt, you will get the message as shown in Figure
The prompt >>> (usually called as chevron) indicates the system is ready to take Python
instructions. If you would like to use the default IDE of Python, that is, the IDLE, then you
can just run IDLE and you will get the editor as shown in Figure 1.5.
Figure 1.4 Python initialization in command prompt
After understanding the basics of few editors of Python, let us start our communication with Python,
by saying Hello World. The Python uses print() function for displaying the contents. Consider the
following code –
>>> print(“Hello World”) #type this and press enter key Hello World
#output displayed
>>> #prompt returns again
Here, after typing the first line of code and pressing the enter key, we could able to get the output of
that line immediately. Then the prompt (>>>) is returned on the screen. This indicates, Python is
ready to take next instruction as input for processing.

Figure 1.5 Python IDLE editor
Once we are done with the program, we can close or terminate Python by giving quit()
command as shown –
>>> quit() #Python terminates
Terminology: Interpreter and Compiler
All digital computers can understand only the machine language written in terms of zeros and ones.
But, for the programmer, it is difficult to code in machine language. Hence, we generally use high
level programming languages like Java, C++, PHP, Perl, JavaScript etc. Python is also one of the
high level programming languages. The programs written in high level languages are then translated
to machine level instruction so as to be executed by CPU. How this translation behaves depending
on the type of translators viz. compilers and interpreters.
A compiler translates the source code of high-level programming language into machine level
language. For this purpose, the source code must be a complete program stored in a file (with
extension, say, .java, .c, .cpp etc). The compiler generates executable files (usually with extensions
.exe, .dll etc) that are in machine language. Later, these executable files are executed to give the
output of the program.
On the other hand, interpreter performs the instructions directly, without requiring them to be pre-
compiled. Interpreter parses (syntactic analysis) the source code ant interprets it immediately. Hence,
every line of code can generate the output immediately, and the source code as a complete set, need
not be stored in a file. That is why, in the previous section, the usage of single line print(“Hello
World”) could able to generate the output immediately.

Consider an example of adding two numbers –
>>> x=10
>>> y=20
>>> z= x+y
>>>print(z)
30
Here, x, y and z are variables storing respective values. As each line of code above is processed
immediately after the line, the variables are storing the given values. Observe that, though each line
is treated independently, the knowledge (or information) gained in the previous line will be retained
by Python and hence, the further lines can make use of previously used variables. Thus, each line
that we write at the Python prompt are logically related, though they look independent.
NOTE that, Python do not require variable declaration (unlike in C, C++, Java etc) before its use.
One can use any valid variable name for storing the values. Depending on the type (like number,
string etc) of the value being assigned, the type and behavior of the variable name is judged by
Python.
Writing a Program
As Python is interpreted language, one can keep typing every line of code one after the other (and
immediately getting the output of each line) as shown in previous section. But, in real-time scenario,
typing a big program is not a good idea. It is not easy to logically debug such lines. Hence, Python
programs can be stored in a file with extension .py and then can be run. Programs written within a
file are obviously reusable and can be run whenever we want. Also, they are transferrable from one
machine to other machine via pen-drive, CD etc.
What is a Program?
A program is a sequence of instructions intended to do some task. For example, if we need to count
the number of occurrences of each word in a text document, we can write a program to do so.
Writing a program will make the task easier compared to manually counting the words in a
document. Moreover, most of the times, the program is a generic solution. Hence, the same program
may be used to count the frequency of words in another file. The person who does not know
anything about the programming also can run this program to count the words.
Programming languages like Python will act as an intermediary between the computer and the
programmer. The end-user can request the programmer to write a program to solve one‟s problem.
The Building Blocks of Programs
There are certain low-level conceptual structures to construct a program in any programming
language. They are called as building-blocks of a program and listed below –

 Input: Every program may take some inputs from outside. The input may be through
keyboard, mouse, disk-file etc. or even through some sensors like microphone, GPS etc.
 Output: Purpose of a program itself is to find the solution to a problem. Hence, every
program must generate at least one output. Output may be displayed on a monitor or can be
stored in a file. Output of a program may even be a music/voice message.
 Sequential Execution: In general, the instructions in the program are sequentially executed
from the top.
 Conditional Execution: In some situations, a set of instructions have to be
executed based on the truth-value of a variable or expression. Then conditional constructs
(like if) have to be used. If the condition is true, one set of instructions will be executed and if
the condition is false, the true-block is skipped.
 Repeated Execution: Some of the problems require a set of instructions to be repeated
multiple times. Such statements can be written with the help of looping structures like for,
while etc.
 Reuse: When we write the programs for general-purpose utility tasks, it is better to write
them with a separate name, so that they can be used multiple times whenever/wherever
required. This is possible with the help of functions.
The art of programming involves thorough understanding of the above constructs and using them
legibly.
What Could Possibly Go Wrong?
It is obvious that one can do mistakes while writing a program. The possible mistakes are
categorized as below –
 Syntax Errors: The statements which are not following the grammar (or syntax) of the
programming language are tend to result in syntax errors. Python is a case- sensitive
language. Hence, there is a chance that a beginner may do some syntactical mistakes while
writing a program. The lines involving such mistakes are encountered by the Python when
you run the program and the errors are thrown by specifying possible reasons for the error.
The programmer has to correct them and then proceed further.
 Logical Errors: Logical error occurs due to poor understanding of the problem.
Syntactically, the program will be correct. But, it may not give the expected output. For
example, you are intended to find a%b, but, by mistake you have typed a/b. Then it is a
logical error.
 Semantic Errors: A semantic error may happen due to wrong use of variables, wrong
operations or in wrong order. For example, trying to modify un-initialized variable etc.
Note that, some of textbooks/authors refer logical and semantic error both as same, as the
distinction between these two is very small.

NOTE: There is one more type of error – runtime error, usually called as exceptions. It may
occur due to wrong input (like trying to divide a number by zero), problem in database
connectivity etc. When a run-time error occurs, the program throws some error, which may
not be understood by the normal user. And he/she may not understand how to overcome such
errors. Hence, suspicious lines of code have to be treated by the programmer himself by the
procedure known as exception handling. Python provides mechanism for handling various
possible exceptions like ArithmeticError, FloatingpointError, EOFError, MemoryError etc.
A brief idea about exception handling is there in Section 1.3.7 later in this Module. For more
details, interested readers can go through the links –
https://docs.python.org/3/tutorial/errors.html and
https://docs.python.org/2/library/exceptions.html
VARIABLES, EXPRESSIONS AND STATEMENTS
After understanding some important concepts about programming and programming languages, we
will now move on to learn Python as a programming language with its syntax and constructs.
Values and Types
A value is one of the basic things in a program. It may be like 2, 10.5, “Hello” etc. Each value in
Python has a type. Type of 2 is integer; type of 10.5 is floating point number; “Hello” is string etc.
The type of a value can be checked using type function as shown below –
>>> type("hello")
<class 'str'> #output
>>> type(3)
<class 'int'>
>>> type(10.5)
<class 'float'>
>>> type("15")
<class 'str'>
In the above four examples, one can make out various types str, int and float. Observe the 4th
example – it clearly indicates that whatever enclosed within a double quote is a string.
Variables
A variable is a named-literal which helps to store a value in the program. Variables may take value
that can be modified wherever required in the program. Note that, in Python, a variable need not be
declared with a specific type before its usage. Whenever you want a variable, just use it. The type of
it will be decided by the value assigned to it. A value can be assigned to a variable using assignment
operator (=). Consider the example given below–
>>> x=10
>>> print(x)
10 #output

>>> type(x)
<class 'int'> #type of x is integer
>>> y="hi"
>>> print(y)
hi #output
>>> type(y)
<class 'str'> #type of y is string
It is observed from above examples that the value assigned to variable determines the type of that
variable.
Variable Names and Keywords
It is a good programming practice to name the variable such that its name indicates its purpose in the
program. There are certain rules to be followed while naming a variable –
 Variable name must not be a keyword
 They can contain alphabets (lowercase and uppercase) and numbers, but should not start with
a number.
 It may contain a special character underscore(_), which is usually used to combine variables
with two words like my_salary, student_name etc. No other special characters like @, $ etc.
are allowed.
 As Python is case-sensitive, variable name sum is different from SUM, Sum etc.
Examples:
>>> 3a=5 #starting with a number
SyntaxError: invalid syntax
>>> a$=10 #contains $
>>> if=15 #if is a keyword
Statements
A statement is a small unit of code that can be executed by the Python interpreter. It indicates some
action to be carried out. In fact, a program is a sequence of such statements. Following are the
examples of statements –
>>> print("hello") #printing statement hello
>>> x=5 #assignment statement
>>> print(x) #printing statement

Operators and Operands
Special symbols used to indicate specific tasks are called as operators. An operator may work on
single operand (unary operator) or two operands (binary operator). There are several types of
operators like arithmetic operators, relational operators, logical operators etc. in Python.
Arithmetic Operators are used to perform basic operations as listed in Table 1.2.
Table 1.2 Arithmetic Operators
Operator Meaning Example
+ Addition Sum= a+b
- Subtraction Diff= a-b
* Multiplication Pro= a*b
/ Division Q = a/b
X = 5/3
(X will get a value 1.666666667)
// Floor Division – returns only
integral part after division
F = a//b
X= 5//3 (X will get a value 1)
% Modulus
division
– remainder after R = a %b
(Remainder after dividing a by b)
** Exponent E = x** y
(means x to the powder of y)
Relational or Comparison Operators are used to check the relationship (like less than, greater than
etc) between two operands. These operators return a Boolean value – either True or False.
Assignment Operators: Apart from simple assignment operator = which is used for assigning
values to variables, Python provides compound assignment operators. For example,
x=x+y
can be written as –
x+=y
Now, += is compound assignment operator. Similarly, one can use most of the arithmetic and
bitwise operators (only binary operators, but not unary) like *, /, %, //, &, ^ etc. as compound
assignment operators. For example,
>>> x=3
>>> y=5
>>> x+=y #x=x+y
>>> print(x) 8
>>> y//=2 #y=y//2
>>> print(y)
2 #only integer part will be printed

NOTE:
1. Python has a special feature – one can assign values of different types to multiple variables in
a single statement. For example,
>>> x, y, st=3, 4.2, "Hello"
>>> print("x= ", x, " y= ",y, " st= ", st) x=3y=4.2 st=Hello
2. Python supports bitwise operators like &(AND), | (OR), ~(NOT), ^(XOR), >>(right shift)
and <<(left shift). These operators will operate on every bit of the operands. Working
procedure of these operators is same as that in other languages like C and C++.
3. There are some special operators in Python viz. Identity operator (is and is not) and
membership operator (in and not in). These will be discussed in further Modules.
Expressions
A combination of values, variables and operators is known as expression. Following are few
examples of expression –
x=5
y=x+10
z= x-y*3
The Python interpreter evaluates simple expressions and gives results even without print(). For
example,
>>> 5
5 #displayed as it is
>>> 1+2
3 #displayed the sum
But, such expressions do not have any impact when written into Python script file.
Order of Operations
When an expression contains more than one operator, the evaluation of operators depends on the
precedence of operators. The Python operators follow the precedence rule (which can be
remembered as PEMDAS) as given below –
 Parenthesis have the highest precedence in any expression. The operations within
parenthesis will be evaluated first. For example, in the expression (a+b)*c, the addition has to
be done first and then the sum is multiplied with c.
 Exponentiation has the 2nd
precedence. But, it is right associative. That is, if there are two
exponentiation operations continuously, it will be evaluated from right to left (unlike most of
other operators which are evaluated from left to right). For example,
>>> print(2**3) #It is 23
8
>>> print(2**3**2) #It is 512 232
, so to be evaluated from right
 Multiplication and Division are the next priority. Out of these two operations, whichever
comes first in the expression is evaluated.
>>> print(5*2/4) #multiplication and then division 2.5
>>> print(5/4*2) #division and then multiplication 2.5

 Addition and Subtraction are the least priority. Out of these two operations, whichever
appears first in the expression is evaluated.
String Operations
String concatenation can be done using + operator as shown below –
>>> x="32"
>>> y="45"
>>> print(x+y) 3245
Observe the output: here, the value of y (a string “45”, but not a number 45) is placed just in front of
value of x( a string “32”). Hence the result would be “3245” and its type would be string.
NOTE: One can use single quotes to enclose a string value, instead of double quotes.
Asking the User for Input
Python uses the built-in function input() to read the data from the keyboard. When this function is
invoked, the user-input is expected. The input is read till the user presses enter- key. For example:
>>> str1=input()
Hello how are you? #user input
>>> print(“String is “,str1)
String is Hello how are you? #printing str1
When input() function is used, the curser will be blinking to receive the data. For a better
understanding, it is better to have a prompt message for the user informing what needs to be entered
as input. The input() function itself can be used to do so, as shown below –
>>> str1=input("Enter a string: ") Enter a string:
Hello
>>> print("You have entered: ",str1) You have
entered: Hello
One can use new-line character n in the function input() to make the cursor to appear in the next line
of prompt message –
>>> str1=input("Enter a string:n") Enter a string:
Hello #cursor is pushed here
The key-board input received using input() function is always treated as a string type. If you need an
integer, you need to convert it using the function int(). Observe the following example –
>>> x=input("Enter x:")
Enter x:10 #x takes the value “10”, but not 10
>>> type(x) #So, type of x would be str
<class 'str'>

>>> x=int(input("Enter x:")) #use int()
Enter x:10
>>> type(x) #Now, type of x is int
<class 'int'>
A function float() is used to convert a valid value enclosed within quotes into float number as shown
below –
>>> f=input("Enter a float value:") Enter a float
value: 3.5
>>> type(f)
<class 'str'> #f is actually a string “3.5”
>>> f=float(f) #converting “3.5” into float value 3.5
>>> type(f)
<class 'float'>
A function chr() is used to convert an integer input into equivalent ASCII character.
>>> a=int(input("Enter an integer:")) Enter an
integer:65
>>> ch=chr(a)
>>> print("Character Equivalent of ", a, "is ",ch) Character Equivalent of 65 is A
There are several such other utility functions in Python, which will be discussed later.
Comments
It is a good programming practice to add comments to the program wherever required. This will help
someone to understand the logic of the program. Comment may be in a single line or spread into
multiple lines. A single-line comment in Python starts with the symbol #. Multiline comments are
enclosed within a pair of 3-single quotes.
Ex1. #This is a single-line comment
Ex2. ''' This is a
multiline
comment '''
Python (and all programming languages) ignores the text written as comment lines. They are only
for the programmer‟s (or any reader‟s) reference.
Choosing Mnemonic Variable Names
Choosing an appropriate name for variables in the program is always at stake. Consider the
following examples –

Ex1.
a=10000
b=0.3*a
c=a+b
print(c) #output is 13000
Ex2.
basic=10000 da=0.3*basic
gross_sal=basic+da
print("Gross Sal = ",gross_sal) #output is 13000
P. T. O

One can observe that both of these two examples are performing same task. But, compared to Ex1,
the variables in Ex2 are indicating what is being calculated. That is, variable names in Ex2 are
indicating the purpose for which they are being used in the program. Such variable names are known
as mnemonic variable names. The word mnemonic means memory aid. The mnemonic variables are
created to help the programmer to remember the purpose for which they have been created.
Python can understand the set of reserved words (or keywords), and hence it flashes an error when
such words are used as variable names by the programmer. Moreover, most of the Python editors
have a mechanism to show keywords in a different color. Hence, programmer can easily make out
the keyword immediately when he/she types that word.
Debugging
Some of the common errors a beginner programmer may make are syntax errors. Though Python
flashes the error with a message, sometimes it may become hard to understand the cause of errors.
Some of the examples are given here –
Ex1. >>> avg sal=10000
Here, there is a space between the terms avg and sal, which is not allowed.
Ex2. >>>m=09
SyntaxError: invalid token
Python does not allow preceding zeros for numeric values.
Ex3. >>> basic=2000
>>> da=0.3*Basic
NameError: name 'Basic' is not defined
As Python is case sensitive, basicis different from Basic.
As shown in above examples, the syntax errors will be alerted by Python. But, programmer is
responsible for logical errors or semantic errors. Because, if the program does not yield into expected
output, it is due to mistake done by the programmer, about which Python is unaware of.
CONDITIONAL EXECUTION
In general, the statements in a program will be executed sequentially. But, sometimes we need a set
of statements to be executed based on some conditions. Such situations are discussed in this section.
Boolean Expressions
A Boolean Expression is an expression which results in True or False. The True and False
are special values that belong to class bool. Check the following –
>>> type(True)
Output: <class 'bool'>
>>> type(False)

Output: <class 'bool'>
Boolean expression may be as below –
>>> 10==12
Output: False
>>> x=10
>>> y=10
>>> x==y
Output: True
Various comparison operations are shown in Table 1.3.
Table 1.3 Relational (Comparison) Operators
> Greater than a>b
< Less than a<b
>= Greater than or equal to a>=b
<= Less than or equal to a<=b
== Comparison a==b
!= Not equal to a !=b
Is Is same as a is b
is not Is not same as a is not b
Few Examples:
>>> a=10
>>> b=20
>>> x= a>b
>>> print(x)
Output: False
>>> print(a==b)
Output: False
>>> print("a<b is ", a<b) a<b is True
>>> print("a!=b is", a!=b) a!=b is True
>>> 10 is 20
Output: False
>>> 10 is 10
Output: True
NOTE: For a first look, the operators ==and islook same. Similarly, the operators !=and is not look
the same. But, the operators == and != does the equality test. That is, they will compare the values
stored in the variables. Whereas, the operators is and is not does the identity test. That is, they will
compare whether two objects are same. Usually, two objects are same when their memory locations
are same. This concept will be more clear when we take up classes and objects in Python.

Logical Operators
There are 3 logical operators in Python as shown in Table 1.4. (NOTE that symbols like &&,
|| are not used in Python for representing logical operators)
Table 1.4 Logical Operators
NOTE:
1. Logical operators treat the operands as Boolean (True or False).
2. Python treats any non-zero number as True and zero as False.
3. While using and operator, if the first operand is False, then the second operand is not
evaluated by Python. Because False and’ed with anything is False.
4. In case of or operator, if the first operand is True, the second operand is not evaluated.
Because True or’ed with anything is True.
Example 1 (with Boolean Operands):
>>> x= True
>>> y= False
>>> print('x and y is', x and y) x and y is False
>>> print('x or y is', x or y) x or y is True
>>> print('Complement of x is ', not x)
Complement of x is False
Example 2 (With numeric Operands):
>>> a=-3
>>> b=10
>>> print(a and b) #and operation
10 #a is true, hence b is evaluated and printed
>>> print(a or b) #or operation
-3 #a is true, hence b is not evaluated
>>> print(0 and 5) #0 is false, so printed 0
And Returns true, if both operands are true a and b
Or Returns true, if any one of two operands is true a or b
not Return true, if the operand is false (it is a unary operator) not a

if condition:
Statement block
Conditional Execution
The basic level of conditional execution can be achieved in Python by using if statement. The syntax
and flowcharts are as below –
Observe the colon symbol after condition. When the
condition is true, the Statement block will be executed.
Otherwise, it is skipped. A set (block) of statements to be
executed under if is decided by the indentation (tab space)
given.
Consider an example –
>>> x=10
>>> if x<40:
print("Fail") #observe indentation after if
Fail #output
Usually, the if conditions have a statement block. In any case, the programmer feels to do nothing
when the condition is true, the statement block can be skipped by just typing pass statement as
shown below –
>>> if x<0:
pass #do nothing when x is negative
Entry
condition?
Exit
Statement Block
True
False

if condition1:
Statement Block-1
elif condition2:
Statement Block-2
|
|
|
|
elif condition_n:
Statement Block-n
else:
Statement Block-(n+1)
Statement
Block-(n+1)
Statement
Block-n
Statement
Block-2
Statement
Block-1
Alternative Execution
A second form of if statement is alternative execution. Here, when the condition is true, one set of
statements will be executed and when the condition is false, another set of statements will be
executed. The syntax and flowchart are as given below –
As the condition will be either true or false, only
one among Statement block-1 and Statement block-
2 will be get executed. These two alternatives are
known as branches.
Example:
x=int(input("Enter x:")) if x%2==0:
print("x is even") else:
print("x is odd")
Sample output:
Enter x: 13
x is odd
Chained Conditionals
Some of the programs require more than one possibility to be checked for executing a set of
statements. That means, we may have more than one branch. This is solved with the help of chained
conditionals. The syntax and flowchart is given below –
The conditions are checked one by one sequentially. If any condition is satisfied, the respective
statement block will be executed and further conditions are not checked. Note that, the last else
block is not necessary always.
if condition:
Statement block -1
else:
Statement block -2
True False
Condition?
Statement
block-1
Statement
block -2
F
Cond1 Cond2
F
Condn
F
T T T

Example:
marks=float(input("Enter marks:")) if marks >= 80:
print("First Class with Distinction") elif marks >= 60 and marks < 80:
print("First Class")
elif marks >= 50 and marks < 60: print("Second Class")
elif marks >= 35 and marks < 50: print("Third Class")
else:
print("Fail")
Sample Output:
Enter marks: 78
First Class
Nested Conditionals
The conditional statements can be nested. That is, one set of conditional statements can be nested
inside the other. It can be done in multiple ways depending on programmer‟s requirements.
Examples are given below –
Ex1. marks=float(input("Enter marks:")) ifmarks>=60:
if marks<70:
print("First Class") else:
print("Distinction")
Sample Output:
Enter marks:68
First Class
Here, the outer condition marks>=60 is checked first. If it is true, then there are two branches for the
inner conditional. If the outer condition is false, the above code does nothing.
Ex2. gender=input("Enter gender:")
age=int(input("Enter age:"))
if gender == "M" : if age
>= 21:
print("Boy, Eligible for Marriage") else:
print("Boy, Not Eligible for Marriage") elif gender =="F":
if age >= 18:
print("Girl, Eligible for Marriage") else:
print("Girl, Not Eligible for Marriage")

Sample Output:
Enter gender: F
Enter age: 17
Girl, Not Eligible for Marriage
NOTE: Nested conditionals make the code difficult to read, even though there are proper
indentations. Hence, it is advised to use logical operators like and to simplify the nested
conditionals. For example, the outer and inner conditions in Ex1 above can be joined as -
if marks>=60 and marks<70:
#do something
Short-Circuit Evaluation of Logical Expressions
When a logical expression (expression involving operands and, or, not) is being evaluated, it will be
processed from left to right. For example, consider the statements -
x=10
y=20
if x<10 and x+y>25:
#dosomething
Here, the expression x<10 and x+y>25 involves the logical operator and. Now, x<10 is evaluated
first, which results to be False. As there is an and operator, irrespective of the result of x+y>25, the
whole expression will be False. In such situations, Python ignores the remaining part of the
expression. This is known as short-circuiting the evaluation. When the first part of logical
expression results in True, then the second part has to be evaluated to know the overall result.
The short-circuiting not only saves the computational time, but it also leads to a technique known as
guardian pattern. Consider following sequence of statements –
>>> x=5
>>> y=0
>>> x>=10 and (x/y)>2 False
>>> x>=2 and (x/y)>2
Traceback (most recent call last):
File "<pyshell#3>", line 1, in <module> x>=2 and (x/y)>2
ZeroDivisionError: division by zero
Here, when we executed the statement x>=10 and (x/y)>2, the first half of logical expression itself
was False and hence by applying short-circuit rule, the remaining part was not executed at all.
Whereas, in the statement x>=2 and (x/y)>2, the first half is True and the second half is resulted in
runtime-error. Thus, in the expression x>=10 and (x/y)>2, short-circuit rule acted as a guardian by
preventing an error.

One can construct the logical expression to strategically place a guard evaluation just before the
evaluation that might cause an error as follows:
>>> x=5
>>> y=0
>>> x>=2 and y!=0 and(x/y)>2
False
Here, x>=2 results in True, but y!=0 evaluates to be False. Hence, the expression
(x/y)>2is never reached and possible error is being prevented from happening.
Debugging
One can observe from previous few examples that when a runtime error occurs, it displays a term
Traceback followed by few indications about errors. A traceback is a stack trace from the point of
error-occurrence down to the call-sequence till the point of call. This is helpful when we start using
functions and when there is a sequence of multiple function calls from one to other. Then, traceback
will help the programmer to identify the exact position where the error occurred. Most useful part of
error message in traceback are –
 What kind of error it is
 Where it occurred
Compared to runtime errors, syntax errors are easy to find, most of the times. But, whitespace errors
in syntax are quite tricky because spaces and tabs are invisible. For example –
>>> x=10
>>>y=15
SyntaxError: unexpected indent
The error here is because of additional space given before y. As Python has a different meaning
(separate block of code) for indentation, one cannot give extra spaces as shown above.
In general, error messages indicate where the problem has occurred. But, the actual error may be
before that point, or even in previous line of code.

FUNCTIONS
Functions are the building blocks of any programming language. A sequence of instructions intended
to perform a specific independent task is known as a function. In this section, we will discuss various
types of built-in functions, user-defined functions, applications/uses of functions etc.
Function Calls
A function is a named sequence of instructions for performing a task. When we define a function we
will give a valid name to it, and then specify the instructions for performing required task. Later,
whenever we want to do that task, a function is called by its name. Consider an example –
>>> type(15)
Output: <class 'int'>
Here type is a function name, 15 is the argument to a function and <class 'int'> is the result of the
function. Usually, a function takes zero or more arguments and returns the result.
Built-in Functions
Python provides a rich set of built-in functions for doing various tasks. The programmer/user need
not know the internal working of these functions; instead, they need to know only the purpose of
such functions. Some of the built in functions are given below –
 max(): This function is used to find maximum value among the arguments. It can be used for
numeric values or even to strings.
o max(10, 20, 14, 12) #maximum of 4 integers
Output: 20
o max("hello world")
Output: 'w'
#character having maximum ASCII code
o max(3.5, -2.1, 4.8, 15.3, 0.2)
Output: 15.3 #maximum of 5 floating point values
 min(): As the name suggests, it is used to find minimum ofarguments.
o min(10, 20, 14, 12) #minimum of 4 integers
Output: 10
o min("hello world")
Output: ' ' #space has least ASCII code here
o min(3.5, -2.1, 4.8, 15.3, 0.2)
Output: -2.1 #minimum of 5 floating point values
 len(): This function takes a single argument and finds its length. The argument can be a
string, list, tuple etc.
o len(“hello how are you?”)
Output: 18
There are many other built-in functions available in Python. They are discussed in further Modules,
wherever they are relevant.

Type Conversion Functions
As we have seen earlier (while discussing input() function), the type of the variable/value can be
converted using functions int(), float(), str(). Consider following few examples –
 int('20') #integer enclosed within single quotes
20 #converted to integer type
 int("20") #integer enclosed within double quotes 20
 int("hello") #actual string cannot be converted to int Traceback (most
recent call last):
File "<pyshell#23>", line 1, in <module> int("hello")
ValueError: invalid literal for int() with base 10: 'hello'
 int(3.8) #float value being converted to integer
Output: 3 #round-off will not happen, fraction is ignored
 int(-5.6)
Output: -5
 float('3.5') #float enclosed within single quotes
Output: 3.5 #converted to float type
 float(42) #integer is converted to float 42.0
 str(4.5) #float converted to string '4.5'
 str(21) #integer converted to string '21'
Random Numbers
Most of the programs that we write are deterministic. That is, the input (or range of inputs) to the
program is pre-defined and the output of the program is one of the expected values. But, for some of
the real-time applications in science and technology, we need randomly generated output. This will
help in simulating certain scenario. Radom number generation has important applications in games,
noise detection in electronic communication, statistical sampling theory, cryptography, political and
business prediction etc. These applications require the program to be nondeterministic. There are
several algorithms to generate random numbers. But, as making a program completely
nondeterministic is difficult and may lead to several other consequences, we generate pseudo-
random numbers. That is, the type (integer, float etc) and range (between 0 and 1, between 1 and
100 etc) of the random numbers are decided by the programmer, but the actual numbers are
unknown. Moreover, the algorithm to generate the random number is also known to the programmer.
Thus, the random numbers are generated using deterministic computation and hence, they are known
as pseudo-random numbers!!
Python has a module random for the generation of random numbers. One has to import this module
in the program. The function used is also random(). By default, this function generates a random
number between 0 and 1 (excluding 1). For example –
>>>import random
print(random.random())
#module random is
#random() function is invoked
imported
Output: 0.7430852580883088 #a random number generated
print(random.random())
Output: 0.5287778188896328 #one more random number

Importing a module creates an object. Using this object, one can access various functions and/or
variables defined in that module. Functions are invoked using a dot operator.
There are several other functions in the module random apart from the function random(). (Do not
get confused with module name and function name. Observe the parentheses while referring a
function name). Few are discussed hereunder:
 randint(): It takes two arguments low and high and returns a random integer between these
two arguments (both low and high are inclusive). For example,
>>>random.randint(2,20)
Output: 14 #integer between 2 and 20 generated
>>> random.randint(2,20)
Output: 10
 choice(): This function takes a sequence (a list type in Python) of numbers as an argument
and returns one of these numbers as a random number. For example,
>>> t=[1,2, -3, 45, 12, 7, 31, 22] #create a list t
>>> random.choice(t) #t is argument to choice()
Output: 12 #one of the elements in t
>>> random.choice(t)
Output: 1 #one of the elements in t
Various other functions available in random module can be used to generate random numbers
following several probability distributions like Gaussian, Triangular, Uniform, Exponential, Weibull,
Normal etc.
Math Functions
Python provides a rich set of mathematical functions through the module math. To use these
functions, the math module has to be imported in our code. Some of the important functions
available in math are given hereunder –
 sqrt(): This function takes one numeric argument and finds the square root of that argument.
>>> math.sqrt(34) #integer argument
Output: 5.830951894845301
>>> math.sqrt(21.5) #floating pointargument
Output: 4.636809247747852
 pi: The constant value pi can be used directly whenever we require.
>>>print(math.pi)
Output: 3.141592653589793
 log10(): This function is used to find logarithm of the given argument, to the base 10.
>>>math.log10(2)
Output: 0.3010299956639812

 log(): This is used to compute natural logarithm (base e) of a given number.
>>> math.log(2)
Output: 0.6931471805599453
 sin(): As the name suggests, it is used to find sine value of a given argument. Note that, the
argument must be in radians (not degrees). One can convert the number of degrees into
radians by multiplying pi/180 as shown below –
>>>math.sin(90*math.pi/180) #sin(90) is 1 1.0
 cos(): Used to find cosine value –
>>>math.cos(45*math.pi/180)
Output: 0.7071067811865476
 tan(): Function to find tangent of an angle, given as argument.
>>>math.tan(45*math.pi/180)
Output: 0.9999999999999999
 pow(): This function takes two arguments x and y, then finds x to the power of y.
>>> math.pow(3,4)
Output: 81.0
Adding New Functions (User-defined Functions)
Python facilitates programmer to define his/her own functions. The function written once can be
used wherever and whenever required. The syntax of user-defined function would be –
def fname(arg_list):
statement_1
statement_2
……………
Statement_n
return value
Here def is a keyword indicating it as a function definition.
fname is any valid name given to the function
arg_list is list of arguments taken by a function. These are treated as inputs to the
function from the position of function call. There may be zero or more
arguments to a function.
statements are the list of instructions to perform required task.
return is a keyword used to return the output value. This statement is optional
The first line in the function def fname(arg_list)is known as function header. The remaining lines
constitute function body. The function header is terminated by a colon and the function body must
be indented. To come out of the function, indentation must be terminated. Unlike few other
programming languages like C, C++ etc, there is no main()

function or specific location where a user-defined function has to be called. The programmer has to
invoke (call) the function wherever required.
Consider a simple example of user-defined function –
Observe indentation
Statements outside the
function without indentation.
myfun()is called here.
The output of above program would be –
Example of function Hello
Inside the function Example
over
The function definition creates an object of type function. In the above example, myfun is internally
an object. This can be verified by using the statement –
>>>print(myfun) # myfun without parenthesis
Output: <function myfun at 0x0219BFA8>
>>> type(myfun) # myfun without parenthesis
Output: <class 'function'>
Here, the first output indicates that myfun is an object which is being stored at the memory address
0x0219BFA8 (0x indicates octal number). The second output clearly shows myfun is of type
function.
(NOTE: In fact, in Python every type is in the form of class. Hence, when we apply type on any
variable/object, it displays respective class name. The detailed study of classes will be done in
Module 4.)
The flow of execution of every program is sequential from top to bottom, a function can be invoked
only after defining it. Usage of function name before its definition will generate error. Observe the
following code:
print("Example of function")
myfun() #function call before definition
print("Example over")
def myfun(): #function definition is here
print("Hello")
print("Inside the function")
def myfun():
print("Hello")
print("Inside the function")
myfun()

def myfun():
print("Inside myfun()")
(3)
def repeat():
myfun()
print(“Inside repeat()”)
myfun()
(4)
(5)
(6)
(2)
repeat()
Execution
starts here(1)
The above code would generate error saying
NameError: name 'myfun' is not defined
Functions are meant for code-reusability. That is, a set of instructions written as a function need not
be repeated. Instead, they can be called multiple times whenever required. Consider the enhanced
version of previous program as below –
The output is –
Example of function Inside
myfun() Inside repeat()
Inside myfun() Example
over
Observe the output of the program to understand the flow of execution of the program. Initially, we
have two function definitions myfun()and repeat()one after the other. But, functions are not executed
unless they are called (or invoked). Hence, the first line to execute in the above program is –
Then, there is a function call repeat(). So, the program control jumps to this function. Inside repeat(),
there is a call for myfun(). Now, program control jumps to myfun()and executes the statements
inside and returns back to repeat() function. The statement print(“Inside repeat()”) is executed. Once
again there is a call for myfun() function and hence, program control jumps there. The function
myfun() is executed and returns to repeat(). As there are no more statements in repeat(), the control
returns to the original position of its call. Now there is a statement print("Example over")to execute,
and program is terminated.
Parameters and Arguments
In the previous section, we have seen simple example of a user-defined function, where the function
was without any argument. But, a function may take arguments as an input from the calling function.
Consider an example of a function which takes a single argument as below –

def test(var):
print("Inside test()")
print("Argument is ",var)
print("Example of function with arguments")
x="hello"
test(x)
y=20
test(y)
print("Over!!")
def test(var):
print("Inside test()")
print("Argument is ",var)
print("Example of function with arguments")
x="hello"
test(x*3)
y=20
test(y*3)
print("Over!!")
The output would be –
Example of function with arguments Inside test()
Argument is hello Inside
test() Argument is 20
Over!!
In the above program, var is called as parameter and x and y are called as arguments. The argument
is being passed when a function test() is invoked. The parameter receives the argument as an input
and statements inside the function are executed. As Python variables are not of specific data types in
general, one can pass any type of value to the function as an argument.
Python has a special feature of applying multiplication operation on arguments while passing them
to a function. Consider the modified version of above program –
Example of function with arguments Inside test()
Argument is hellohellohello #observe repetition Inside
test()
Argument is 60 #observe multiplication Over!!

def sum(a,b):
return a+b
x=int(input("Enter a number:"))
y=int(input("Enter another number:"))
s=sum(x,y)
print("Sum of two numbers:",s)
One can observe that, when the argument is of type string, then multiplication indicates that string is
repeated 3 times. Whereas, when the argument is of numeric type (here, integer), then the value of
that argument is literally multiplied by 3.
Fruitful Functions and void Functions
A function that performs some task, but do not return any value to the calling function is known as
void function. The examples of user-defined functions considered till now are void functions. The
function which returns some result to the calling function after performing a task is known as
fruitful function. The built-in functions like mathematical functions, random number generating
functions etc. that have been considered earlier are examples for fruitful functions. One can write a
user-defined function so as to return a value to the calling function as shown in the following
example –
The sample output would be –
Enter a number:3
Enter another number:4 Sum of
two numbers: 7
In the above example, The function sum() take two arguments and returns their sum to the receiving
variable s.
When a function returns something and if it is not received using a LHS variable, then the return
value will not be available. For instance, in the above example if we just use the statement
sum(x,y)instead of s=sum(x,y), then the value returned from the function is of no use. On the other
hand, if we use a variable at LHS while calling void functions, it will receive None. For example,
p= test(var) #function used in previous example print(p)
Now, the value of p would be printed as None. Note that, None is not a string, instead it is of type
class 'NoneType'. This type of object indicates no value.

Why Functions?
Functions are essential part of programming because of following reasons –
 Creating a new function gives the programmer an opportunity to name a group of statements,
which makes the program easier to read, understand, and debug.
 Functions can make a program smaller by eliminating repetitive code. If any modification is
required, it can be done only at one place.
 Dividing a long program into functions allows the programmer to debug the independent
functions separately and then combine all functions to get the solution of originalproblem.
 Well-designed functions are often useful for many programs. The functions written once for
a specific purpose can be re-used in any other program.
For the Curious Minds (Something beyond the syllabus)
Special parameters of print() – sep and end :
Consider an example of printing two values using print() as below –
>>> x=10
>>> y=20
>>> print(x,y)
Output: 10 20 #space is added between two values
Observe that the two values are separated by a space without mentioning anything specific. This is
possible because of the existence of an argument sep in the print() function whose default value is
white space. This argument makes sure that various values to be printed are separated by a space for
a better representation of output.
The programmer has a liberty in Python to give any other character (or string) as a separator by
explicitly mentioning it in print() as shown below –
>>> print("18","2","2018",sep='-')
Output: 18-2-2018
We can observe that the values have been separated by hyphen, which is given as a value for the
argument sep. Consider one more example –
>>> college=”NCET"
>>> address="Bengaluru "
>>> print(college, address, sep='@')
Output: NCET@Bengaluru

If you want to deliberately suppress any separator, then the value of sep can be set with empty string
as shown below –
>>> print("Prof.","Suresh", sep='')
Output: Prof.Suresh
You might have observed that in Python program, the print() adds a new line after printing the data.
In a Python script file, if you have two statements like –
print(“Sun”) print(“Shine”)
then, the output would be
Sun
Shine
This may be quite unusual for those who have experienced programming languages like C, C++ etc.
In these languages, one has to specifically insert a new-line character (n) to get the output in
different lines. But, in Python without programmer‟s intervention, a new line will be inserted. This is
possible because, the print() function in Python has one more special argument end whose default
value itself is new-line. Again, the default value of this argument can be changed by the programmer
as shown below (Run these lines using a script file, but not in the terminal/command prompt) –
print(“Suresh”,end=”@‟)
print(“Ramesh”)
Suresh@Ramesh
In fact, when you just type print and open a parentheses in any Python IDE, the intelliSense (the
context-aware code completion feature of a programming language which helps the programmer
with certain suggestions using a pale-yellow box) of print() will show the existence of sep and end
arguments as below –
The above figure clearly indicates that the sep and end have the default values space and new-line
respectively.
(NOTE: You can see two more arguments file and flush here. The default value sys.stdout of the
argument file indicates that print() will send the data to standard output, which is usually keyboard.
When you are willing to print the data into a specific file, the file-object can be given as a value for
file argument. The flush argument with True value makes sure that operations are successfully
completed and the values are flushed into the memory from the buffer. The default value of flush is
False, because in most of the cases we need not check whether the data is really got flushed or not –
as it would be happening even otherwise. While printing the data into a file (that is, when a file is
open for write purpose), we may need to make sure whether the data got flushed or not. Because,
someone else in the network trying to read the same file (trying to open a file for read purpose) when
write operation is under progress may result in file corruption. In such situations, we need to set flush
argument as True. Indeed, this is just a basic vague explanation of flush argument and it has much
more meaning in real.)

Formatting the output:
There are various ways of formatting the output and displaying the variables with a required number of
space-width in Python. We will discuss few of them with the help of examples.
 Ex1: When multiple variables have to be displayed embedded within a string, the
format() function is useful as shown below –
>>> x=10
>>> y=20
>>> print("x={0}, y={1}".format(x,y)) x=10, y=20
While using format() the arguments of print() must be numbered as 0, 1, 2, 3, etc. and they must be
provided inside the format() in the same order.
 Ex2: The format() function can be used to specify the width of the variable (the number of
spaces that the variable should occupy in the output) as well. Consider below given example
which displays a number, its square and its cube.
for x in range(1,5):
print("{0:1d} {1:3d} {2:4d}".format(x,x**2, x**3))
Output:
1 1 1
2 4 8
3 9 27
4 16 64
Here, 1d, 3d and 4d indicates 1-digit space, 2-digit space etc. on the output screen.
 Ex3: One can use % symbol to have required number of spaces for a variable. This will be useful
in printing floating point numbers.
>>> x=19/3
>>> print(x)
Output: 6.333333333333333 #observe number of digits after dot
>>> print("%.3f"%(x)) #only 3 places after decimal point 6.333
>>> x=20/3
>>> y=13/7
>>> print("x= ",x, "y=",y) #observe actual digits
Output: x=6.666666666666667
y= 1.8571428571428572
>>> print("x=%0.4f, y=%0.2f"%(x,y))
Output: x=6.6667, y=1.86 #observe rounding off digits

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