Getting Started with Python

A program is a set of instructions written for a computer to carry out a task. A programming language is the set of words and rules used to write those instructions. Python is a high-level language, which means it is written in words close to English rather than in the 0s and 1s the machine actually understands.

The computer cannot run English. A translator program converts your code into machine language. Python uses an interpreter — a translator that takes your program one statement at a time, converts it and runs it immediately. A compiler, used by languages such as C++, translates the whole program at once before running any of it. Python reports an error together with the line number where it was found, which is very useful while learning. A grammar mistake is reported before the program starts running; other errors stop the program at the line where they occur.

You should be able to list the features of Python. Learn these five:

• Simple and easy to read. The syntax uses ordinary words like if, else, while and print.
• Free and open source. It can be downloaded and used without paying, and its source code may be studied and modified.
• Interpreted. Statements are translated and executed one by one.
• Portable (platform independent). The same .py file runs on Windows, Linux or macOS, provided Python is installed.
• Rich library support. Ready-made modules exist for mathematics, graphics, files and much more, so you do not have to write everything yourself.

One more feature is worth stating because it is tested: Python is case sensitive. Marks, marks and MARKS are three different names, and Print is not the same as print.

Where you type your code. Most schools use IDLE, the editor that is installed along with Python itself. IDLE stands for Integrated Development and Learning Environment. Other editors exist, and different labs set up different ones, so describe what your school uses rather than assuming everyone has the same screen.

Interactive mode. When you open IDLE you see the Python shell with the prompt >>>. Type one statement, press Enter, and the result appears at once.

>>> 12 + 8
20
>>> print("Namaste")
Namaste

Notice that in interactive mode, typing 12 + 8 alone displays the answer. That happens only at the >>> prompt. Interactive mode is useful for testing one line, checking an expression or trying an operator — but nothing you type is saved.

Script mode. Here you open a new file, type the complete program, save it with a .py extension, and then run the whole file. The output appears in the shell window. Script mode is what you use for any real program and for practical examinations, because the program is stored and can be corrected and run again.

# area.py — a program in script mode
length = 12
breadth = 5
print("Area =", length * breadth)

What the examiner expects. In a "differentiate" question, give two clean contrasts: interactive mode executes one statement at a time and does not store the program, while script mode executes a saved .py file as a whole and the program can be reused. Do not merely say "one is fast and one is slow".

A function is a named block of code that performs a particular job. print() is a built-in function — it is already available in Python — and its job is to display output on the screen. Whatever you want displayed goes inside the round brackets, called parentheses.

Text to be displayed exactly as typed is written inside quotes and is called a string. Python accepts single quotes or double quotes, as long as the same kind opens and closes the string.

print("Welcome to Class 9")
print('Welcome to Class 9')

Both lines produce the same output: Welcome to Class 9. The quotes themselves are never printed.

Printing values and expressions. If there are no quotes, Python treats what you wrote as a value or a calculation and prints the result.

print(45)
print(15 * 4)

This prints 45 and then 60. The difference between print("15 * 4") and print(15 * 4) is a commonly asked question: the first prints the text 15 * 4, the second prints 60.

Printing several items. Items separated by commas are all printed on the same line, and Python automatically inserts one space between them.

total = 275
print("Total marks =", total)

Output: Total marks = 275. Note that you did not have to type a space after the =; the comma supplied it. In a dry-run question you must show that single space.

Two extra settings are sometimes shown in textbooks. sep changes the separator between items, and end changes what is printed at the end instead of moving to a new line.

print("20", "08", "2026", sep="-")
print("A", end=" ")
print("B")

This prints 20-08-2026 on one line, then A B on the next. By default sep is a single space and end is a newline, which is why each print() normally starts a fresh line. An empty print() with nothing inside simply leaves a blank line.

Comments. A comment is a note written inside a program for human readers. The interpreter ignores it completely, so it never affects the output. In Python a comment begins with a hash sign # and continues to the end of that line.

# Program to find the area of a rectangle
length = 12      # length in centimetres
breadth = 5      # breadth in centimetres
print("Area =", length * breadth)

A comment may occupy a whole line or sit at the end of a statement. For several lines of explanation, put a # at the start of each line. Some books also show a triple-quoted block of text used as a note; strictly that is a string that Python creates and then discards, not a true comment, so if a question asks for "the symbol used for a comment", the answer is #.

Why comments matter. In the practical file and in long programming questions, a comment at the top saying what the program does is expected. It costs one line and shows the purpose of the code. Comments also let you switch a statement off temporarily while testing, without deleting it.

What the examiner expects. Define a comment as a non-executable statement used to explain code, state that it is ignored by the interpreter, and give the # example. "Non-executable" is the phrase to use.

A variable is a named location in memory used to store a value that can change while the program runs. You create a variable simply by assigning a value to it with the assignment operator =. The name goes on the left, the value on the right.

marks = 78
name = "Ananya"
price = 249.50

Read marks = 78 as "store 78 in marks", not as "marks equals 78". Python does not need you to declare the type in advance; it decides the type from the value you store. Storing a new value replaces the old one.

Rules for naming variables. A variable name is also called an identifier. The rules are frequently asked:

• It may contain letters (A–Z, a–z), digits (0–9) and the underscore _.
• It must not begin with a digit. marks1 is valid; 1marks is not.
• It must not contain a space or any special character such as @, # or -. Use total_marks, not total marks.
• It must not be a keyword — a word reserved by Python for its own use, such as if, else, while, for, True, None or import.
• Names are case sensitive, so Total and total are two separate variables.

A good name also describes what it holds. si for simple interest is acceptable in an exam; x1 for a student's name is not helpful.

Data types. A data type tells Python what kind of value a variable holds and what may be done with it. Four are in the Class 9 syllabus.

• int — whole numbers, positive or negative, with no decimal point: 78, -15, 0.
• float — numbers with a decimal point: 249.50, -3.5, 98.6.
• str — a string, that is, a sequence of characters inside quotes: "Ananya", "9-A", "2026". Note that "2026" in quotes is a string, not a number.
• bool — Boolean, holding only True or False, always with a capital first letter.

The built-in function type() reports the data type, which is handy in interactive mode.

>>> type(78)
<class 'int'>
>>> type(9 > 4)
<class 'bool'>

input() and the mistake almost everyone makes. The input() function reads what the user types from the keyboard. The text inside its brackets is the message shown to the user, called the prompt. The crucial rule: input() always returns a string, even when the user types digits.

a = input("Enter first number: ")
b = input("Enter second number: ")
print(a + b)

If the user types 12 and 8, this prints 128, not 20 — because + between two strings joins them end to end. This joining is called concatenation.

To do arithmetic you must convert, using int() for whole numbers or float() for decimals. This conversion is called type casting.

a = int(input("Enter first number: "))
b = int(input("Enter second number: "))
print("Sum =", a + b)

Now 12 and 8 give Sum = 20. Use float() when the value can have a decimal part, such as a price in rupees or a percentage. Note the two closing brackets — one for input and one for int. Forgetting one is a common syntax error.

What the examiner expects. Whenever a question says "accept from the user", write int(input(...)) or float(input(...)). A program that reads marks with a bare input() and then tries to add them joins the digits as text instead of adding them, and the answer is wrong.

An operator is a symbol that performs an operation on values. The values it works on are called operands. In 15 + 4, the + is the operator and 15 and 4 are the operands.

Arithmetic operators perform calculations. Seven are used at this level.

• + addition — 15 + 4 gives 19
• - subtraction — 15 - 4 gives 11
• * multiplication — 15 * 4 gives 60
• / division — 15 / 4 gives 3.75
• // floor division, the quotient with the decimal part discarded — 15 // 4 gives 3
• % modulus, the remainder — 15 % 4 gives 3
• ** exponent, or power — 15 ** 2 gives 225

Two points here are commonly tested. First, in Python 3 the ordinary division operator / always produces a float: 10 / 2 gives 5.0, not 5. Write the .0 in your answer. Second, % gives the remainder, which is the standard way to test divisibility — a number n is even when n % 2 == 0, and divisible by 5 when n % 5 == 0.

Relational (comparison) operators compare two values and always give a bool result, True or False. There are six: == equal to, != not equal to, > greater than, < less than, >= greater than or equal to, and <= less than or equal to.

>>> 45 > 33
True
>>> 45 == 45.0
True
>>> "abc" != "ABC"
True

The double equal sign == asks a question; the single = stores a value. Mixing them up is dealt with in the errors section, but fix the idea now: marks = 90 puts 90 into marks, while marks == 90 checks whether marks is 90.

Logical operators combine conditions. There are three, written as words.

• and — True only when both conditions are True.
• or — True when at least one condition is True.
• not — reverses the result: not True is False.

marks = 82
attendance = 90
print(marks >= 75 and attendance >= 75)

Both conditions are True, so this prints True. Had attendance been 60, and would give False while or would still give True.

Operator precedence is the order in which Python evaluates operators when several occur in one expression. From highest to lowest:

1. Parentheses ( )
2. Exponent **
3. Unary minus, as in -5
4. *, /, //, % — evaluated left to right
5. +, - — evaluated left to right
6. Relational operators < <= > >= == !=
7. not, then and, then or

Work an example the way the examiner wants it — one step per line.

10 + 20 * 2 // 5 - 3
= 10 + 40 // 5 - 3        # * first
= 10 + 8 - 3              # // next
= 18 - 3                  # + and - left to right
= 15

Parentheses override everything, so (10 + 20) * 2 gives 60 while 10 + 20 * 2 gives 50. One special case: ** groups from the right, so 2 ** 3 ** 2 is 2 ** 9, which is 512, not 64.

What the examiner expects. In "evaluate the expression" questions, show the intermediate steps. The working makes your method visible and a slip in one step is far easier to find than in a single bare answer.

Up to now every statement ran in order, top to bottom. That is called sequence. A conditional statement lets the program choose: run these lines only if a condition is True. This is called selection, and in Python it is written with if, elif and else.

The simple form has a condition, a colon, and an indented block:

marks = int(input("Enter marks: "))
if marks >= 33:
    print("Pass")

Two pieces of punctuation are compulsory. The colon : ends the if line. The lines that belong to the if are then pushed in from the left margin — that is indentation, and in Python it is not decoration, it is syntax. The indented lines form a block or suite. Most textbooks and IDLE use four spaces; whatever you choose, keep it identical for every line of the same block. If you mix spaces and tabs, or line the statements up unevenly, Python reports an IndentationError.

if…else supplies the alternative path. Exactly one of the two blocks runs.

marks = int(input("Enter marks: "))
if marks >= 33:
    print("Pass")
else:
    print("Fail")

Note that else is written at the same level as if, has a colon, and never carries a condition of its own.

if…elif…else handles more than two possibilities. elif is short for "else if". Python tests the conditions from the top; the moment one is True it runs that block and skips all the rest.

# Grade calculator
marks = int(input("Enter marks out of 100: "))
if marks >= 90:
    print("Grade A")
elif marks >= 75:
    print("Grade B")
elif marks >= 60:
    print("Grade C")
elif marks >= 33:
    print("Grade D")
else:
    print("Needs improvement")

The order matters. If you tested marks >= 33 first, then 95 would also satisfy it and print Grade D, and the later branches would never be reached. Arrange the conditions from the strictest to the loosest.

You may write as many elif branches as you need, but at most one else, and it must come last. The else is optional — leave it out and, if nothing matches, nothing is printed.

Two more standard programs. Even or odd, using the remainder:

n = int(input("Enter a number: "))
if n % 2 == 0:
    print(n, "is even")
else:
    print(n, "is odd")

Largest of three numbers, using a logical operator:

a = int(input("Enter first number: "))
b = int(input("Enter second number: "))
c = int(input("Enter third number: "))
if a >= b and a >= c:
    print("Largest =", a)
elif b >= a and b >= c:
    print("Largest =", b)
else:
    print("Largest =", c)

Trace it once with 45, 90 and 60. The first condition is False because 45 is not at least 90. The second is True, so it prints Largest = 90 and stops.

Nested if. An if placed inside another if block is a nested if; the inner block is indented one level further. It is useful when a second check only makes sense after the first has passed, for example checking a discount only after confirming the bill is above a limit. Keep the levels obvious, because deeply nested code is where indentation mistakes appear.

What the examiner expects. When you write a conditional program in the answer sheet, indent visibly — leave a clear gap of about four spaces — and put the colon at the end of every if, elif and else line. Missing colons and flat, unindented code count as errors even when the logic is right.

A loop repeats a block of statements. Repetition is also called iteration. Without a loop, printing a multiplication table would need ten almost identical print() lines; with a loop it needs two.

The while loop repeats as long as a condition stays True. It is the right choice when you do not know in advance how many repetitions are needed.

i = 1
total = 0
while i <= 10:
    total = total + i
    i = i + 1
print("Sum of first 10 natural numbers =", total)

This prints Sum of first 10 natural numbers = 55. Every correct while loop has three parts, and you should be able to point them out:

1. Initialisation — i = 1, before the loop, sets the starting value.
2. Condition — i <= 10, tested before each repetition. When it becomes False the loop ends.
3. Update — i = i + 1, inside the loop, moves the variable towards the condition becoming False.

Leave out the update and the condition never becomes False. The loop then runs for ever; this is an infinite loop. It is a logical error, not a syntax error, so Python reports nothing — the program simply never stops. In IDLE and in most terminals you interrupt it with Ctrl+C; some other editors give you a stop button instead. Being able to explain an infinite loop and give i = i + 1 as the missing statement is a common short answer.

Notice also that i = i + 1 is not algebra. It means "take the current value of i, add 1, and store the result back in i". The short form i += 1 does the same thing.

The for loop repeats a fixed number of times. It is normally used with the built-in range() function, which generates a sequence of whole numbers.

• range(stop) — starts at 0 and stops before stop. range(5) gives 0, 1, 2, 3, 4.
• range(start, stop) — range(1, 6) gives 1, 2, 3, 4, 5.
• range(start, stop, step) — range(2, 11, 2) gives 2, 4, 6, 8, 10.

The single rule to memorise: the stop value is never included. To count 1 to 10 you must write range(1, 11). A negative step counts downwards, so range(5, 0, -1) gives 5, 4, 3, 2, 1.

The multiplication table is the classic for program:

n = int(input("Enter a number: "))
for i in range(1, 11):
    print(n, "x", i, "=", n * i)

For n = 7 the first line printed is 7 x 1 = 7 and the last is 7 x 10 = 70. The loop variable i takes each value from the range in turn; you do not initialise or update it yourself, which is the main convenience of a for loop over a while loop.

Factorial is another program worth knowing by heart:

n = int(input("Enter a number: "))
f = 1
for i in range(1, n + 1):
    f = f * i
print("Factorial of", n, "is", f)

Start f at 1, not 0 — multiplying by 0 would make every answer 0. That single line is worth checking in every product-based program you write.

A for loop can also step through the characters of a string:

for ch in "INDIA":
    print(ch)

This prints I, N, D, I and A on five separate lines.

What the examiner expects. Choose for when the number of repetitions is known (print a table, sum ten numbers) and while when it depends on a condition (keep asking until the user types 0). For a "how many times does this loop run" question, write out the values of the loop variable in a small trace table rather than guessing.

Errors are called bugs, and removing them is debugging. Examiners often print a short program with a mistake and ask you to name the error or rewrite the code correctly, so learn the exact error names. Learn the name — IndentationError, NameError, TypeError, ValueError, SyntaxError — rather than the sentence printed after it, because the wording of that message is chosen by the interpreter and changes a little from one Python version to another.

There are three broad categories. A syntax error breaks a grammar rule of the language and the program does not run at all. A run-time error appears while the program is running, when it meets something it cannot do. A logical error lets the program run and finish, but the answer is wrong — the hardest kind, because Python prints no message. Now the four specific errors in the syllabus.

1. IndentationError. Raised when the indentation is missing or uneven.

marks = 80
if marks >= 33:
print("Pass")          # IndentationError: expected an indented block

The print belongs to the if, so it must be pushed in by four spaces. The same error appears when two statements in one block start at different columns, or when tabs and spaces are mixed. Fix: indent every line of the block by the same amount.

2. NameError. Raised when you use a name that has not been given a value yet — usually a spelling or capitalisation slip, since Python is case sensitive.

Marks = 80
print(marks)           # NameError: name 'marks' is not defined

Fix: use the same spelling and the same capital letters everywhere. Forgetting the quotes around a piece of text causes the same error, because print(Pass) makes Python look for a variable called Pass.

3. TypeError from adding a string and a number. Raised when an operator is used on types it cannot combine.

age = "15"
print(age + 1)         # TypeError: can only concatenate str (not "int") to str

Python will not guess whether you meant addition or joining. This is exactly what happens after input(), since input always returns a string. Fix: convert first with int(age) + 1, or, if you wanted text, convert the other way with age + str(1).

4. Using = instead of ==. The single = stores a value and cannot be used as a condition.

if marks = 90:         # SyntaxError: invalid syntax
    print("Excellent")

Fix: write if marks == 90:. Say clearly in your answer that = is the assignment operator and == is the relational operator that compares.

Two others worth recognising: a ValueError when int("hello") is attempted, and a plain SyntaxError when a colon or a closing bracket is missing.

Program 1 — simple interest.

# Simple interest calculator
p = float(input("Enter principal in rupees: "))
r = float(input("Enter rate of interest: "))
t = float(input("Enter time in years: "))
si = (p * r * t) / 100
print("Simple interest = Rs", si)

Program 2 — average of three subject marks.

m1 = int(input("Enter marks in subject 1: "))
m2 = int(input("Enter marks in subject 2: "))
m3 = int(input("Enter marks in subject 3: "))
total = m1 + m2 + m3
print("Total =", total)
print("Average =", total / 3)

Program 3 — sum of even numbers from 1 to 20.

total = 0
for i in range(2, 21, 2):
    total = total + i
print("Sum of even numbers up to 20 =", total)

The total displayed is 110.

Program 4 — count down using a while loop.

n = int(input("Enter starting number: "))
while n > 0:
    print(n, end=" ")
    n = n - 1
print("Liftoff")

What the examiner expects. In "find the error" questions, do three things: name the error, point to the exact line, and rewrite that line correctly. Naming the error on its own is only part of the answer.