1.2 Python Core Syntax & Variables

Python Core Syntax & Variables

Python has a simple and readable syntax, making it suitable for beginners as well as professional applications such as Data Science, Automation, Artificial Intelligence, Web Development, and Scientific Computing.

Before working with NumPy, pandas, automation scripts, or machine-learning libraries, you should understand Python's core syntax, variables, data types, input, output, and basic built-in functions.

1. Understanding Python Syntax

Syntax refers to the rules that determine how Python statements must be written.

Python is designed to be readable. Unlike some programming languages, Python uses indentation to define blocks of code.

Simple Python Statement

print("Hello, Python!")

The statement calls the built-in print() function and displays text on the screen.

Another Example

name = "Alex"

print(name)

Here, the string "Alex" is assigned to the variable name, which is then displayed.

2. Python Indentation

Python uses indentation to identify blocks of code. This is one of the most important syntax rules beginners must understand.

Correct Indentation

age = 20

if age >= 18:
    print("Adult")

The indented print() statement belongs to the if block.

Incorrect Indentation

age = 20

if age >= 18:
print("Adult")

This produces an indentation-related error because the body of the if statement is not properly indented.

3. Comments in Python

Comments are notes written inside source code for humans. Python ignores comments during normal execution.

Single-Line Comment

# Calculate the average score

score = 85

Comment After Code

score = 85  # Student score

Comments are useful for explaining complex logic, documenting assumptions, and making code easier to maintain.

4. What Is a Variable?

A variable is a name that refers to a value or object in a Python program.

Python variables are created when a value is assigned to a name.

name = "Alex"
age = 21
score = 87.5

In this example:

• name refers to a string.
• age refers to an integer.
• score refers to a floating-point number.

5. Variable Assignment

The assignment operator = assigns a value to a variable.

city = "London"

marks = 92

percentage = 92.5

Assignment is different from mathematical equality. The statement:

x = 10

means that the value 10 is assigned to x.

Updating a Variable

count = 10

count = 20

print(count)

Output:

20

6. Multiple Assignment

Python allows multiple variables to be assigned in a single statement.

name, age, score = "Alex", 21, 88

The equivalent individual assignments would be:

name = "Alex"
age = 21
score = 88

Assigning the Same Value

x = y = z = 0

All three variables refer to the assigned value.

7. Dynamic Typing in Python

Python is a dynamically typed language. You do not normally have to declare the data type of a variable before assigning a value to it.

value = 100

print(value)

The variable can later refer to an object of another type:

value = 100

value = "Python"

print(value)

The same variable name can therefore be rebound to an object of a different type.

8. Checking the Type of a Value

The built-in type() function can be used to inspect the type of an object.

age = 21

print(type(age))

Output will indicate that the value is an integer.

More Examples

name = "Alex"
price = 99.50
active = True

print(type(name))
print(type(price))
print(type(active))

Typical results are:

<class 'str'>
<class 'float'>
<class 'bool'>

9. Python Identifiers

An identifier is a name used to identify a variable, function, class, module, or other program entity.

Valid Identifiers

student_name
total_marks
score1
_age
data_2026

Invalid Identifiers

2students
student-name
total marks
class

These are invalid for different reasons, including starting with a digit, using an operator, containing spaces, or using a reserved keyword.

10. Rules for Naming Variables

1. A variable name may contain letters, digits, and underscores.
2. It cannot begin with a digit.
3. Spaces are not allowed.
4. Special characters such as - and @ should not be used in identifiers.
5. Python keywords cannot be used as ordinary variable names.
6. Python identifiers are case-sensitive.

Case Sensitivity

name = "Alex"

Name = "Jordan"

print(name)
print(Name)

name and Name are different identifiers.

11. Python Naming Conventions

Python programmers commonly follow the naming conventions described by PEP 8.

Good Naming

student_count = 50
average_score = 84.5
maximum_attempts = 3

Poor Naming

x = 50
a = 84.5
m = 3

Short names are sometimes appropriate for small local calculations, but meaningful names generally improve readability.

12. Python Built-in Functions

Python provides many functions that can be used without importing an external library.

13. Displaying Output with print()

The print() function displays values or text.

print("Hello, Python!")

print(100)

print(25 + 15)

Output:

Hello, Python!
100
40

Printing Multiple Values

name = "Alex"
score = 92

print(name, score)

By default, print() separates multiple arguments with a space.

14. sep and end in print()

The print() function provides optional parameters that allow you to control formatting.

Using sep

print("2026", "08", "23", sep="-")

Output:

2026-08-23

Using end

print("Hello", end=" ")
print("World")

Output:

Hello World

15. Taking Input with input()

The input() function allows a program to receive information entered by the user.

name = input("Enter your name: ")

print("Hello", name)

If the user enters:

Alex

the program can display:

Hello Alex

16. Type Conversion

When numeric input is required, the string returned by input() usually needs to be converted to the appropriate numeric type.

Convert to Integer

age = int(input("Enter your age: "))

print(age)

Convert to Float

price = float(input("Enter the price: "))

print(price)

Convert to String

number = 100

text = str(number)

print(text)

17. Formatted Output

Python provides several ways to format output. For modern Python programs, f-strings are usually the most convenient approach.

Basic f-string

name = "Alex"
score = 92

print(f"{name} scored {score} marks.")

Output:

Alex scored 92 marks.

Expression Inside an f-string

a = 10
b = 20

print(f"Total = {a + b}")

Output:

Total = 30

18. Formatting Numbers

f-strings can also control numeric formatting.

Two Decimal Places

price = 125.6789

print(f"Price: {price:.2f}")

Output:

Price: 125.68

Percentage Formatting

accuracy = 0.956

print(f"Accuracy: {accuracy:.1%}")

Output:

Accuracy: 95.6%

19. Combining Strings

Strings can be combined using the + operator.

first_name = "Alex"
last_name = "Morgan"

full_name = first_name + " " + last_name

print(full_name)

Output:

Alex Morgan

20. Variables in Calculations

Variables can participate in arithmetic expressions.

length = 10
width = 5

area = length * width

print(f"Area = {area}")

Output:

Area = 50

Data Science Example

total_sales = 125000
number_of_orders = 250

average_order_value = total_sales / number_of_orders

print(f"Average order value: {average_order_value:.2f}")

This type of calculation is common in Data Science and business analytics.

21. Swapping Variables

Python provides a concise way to swap two variable values.

a = 10
b = 20

a, b = b, a

print(a)
print(b)

Output:

20
10

No temporary variable is required.

22. Constants in Python

Python does not enforce immutable constants through a special variable declaration keyword. Instead, programmers commonly use uppercase names to indicate that a value should be treated as a constant.

PI = 3.141592653589793
MAX_RETRIES = 3
DEFAULT_TIMEOUT = 30

These names communicate intent to other developers.

23. Useful Built-in Function Examples

len()

course = "Python"

print(len(course))

Output:

6

sum()

scores = [80, 90, 75, 95]

print(sum(scores))

Output:

340

max() and min()

scores = [80, 90, 75, 95]

print(max(scores))
print(min(scores))

Output:

95
75

round()

value = 87.45678

print(round(value, 2))

Output:

87.46

24. Python Keywords

Python has reserved words with predefined meanings. They cannot normally be used as ordinary identifiers.

Examples

if
else
for
while
def
class
return
import
try
except
True
False
None

For example, this is invalid:

class = "Python"

because class is a Python keyword.

25. Finding Python Keywords Programmatically

Python provides the keyword module for inspecting reserved keywords.

import keyword

print(keyword.kwlist)

This displays the keywords recognised by the Python interpreter.

26. Common Beginner Syntax Errors

27. Mini Project — Student Score Calculator

Create a small Python program that accepts three subject scores and calculates the total and average.

name = input("Enter student name: ")

maths = float(input("Enter Mathematics score: "))
science = float(input("Enter Science score: "))
english = float(input("Enter English score: "))

total = maths + science + english
average = total / 3

print()
print(f"Student: {name}")
print(f"Total: {total:.2f}")
print(f"Average: {average:.2f}")

Skills Practised

• Variables
• Input
• Type conversion
• Arithmetic operations
• f-string formatting
• Basic program structure

28. Why These Basics Matter in Data Science

Data Science libraries automate many complex operations, but Python fundamentals remain essential.

29. Python Interview Questions

30. Examination Questions

Multiple Choice Questions

Short Answer Questions

1. Explain dynamic typing in Python.
2. State four rules for naming Python identifiers.
3. Differentiate between int() and float().
4. What is the purpose of f-strings?
5. Explain the importance of indentation in Python.

31. Practical Challenge

32. Python Core Syntax Quick Reference