CodeHaven - Master Computer Science

Python Basics

Python is a high-level, interpreted programming language known for its simplicity and readability. This guide covers the fundamental concepts of Python programming.

Character Set

Python uses the following characters to form valid tokens:

1. Letters

Uppercase: A-Z
Lowercase: a-z

2. Digits

0-9

3. Special Characters

" ! @ # $ % ^ & * ( ) - _ + = [ ] { } | \ : ; " ' < > , . ? /

4. White Spaces

Space, tab, newline, and carriage return

Literals

Literals are notations representing a fixed value in the source code.

Example:

Types of Literals:

1. String Literal

'Python'

2. Numeric Literal

24.3 (Float)

3. Boolean Literal

False

4. Special Literal

Operators

1. Arithmetic Operators

Operator	Description	Example
+	Addition	5 + 3 = 8
-	Subtraction	10 - 3 = 7
*	Multiplication	4 * 5 = 20
/	Division (Float)	10 / 3 = 3.333...
//	Floor Division	10 // 3 = 3
**	Exponentiation	2 ** 3 = 8
%	Modulus (Remainder)	10 % 3 = 1

2. Assignment Operators

Operator	Description	Example
=	Assignment	x = 5
+=	Add and Assign	x += 3 (x = x + 3)
-=	Subtract and Assign	x -= 2 (x = x - 2)
*=	Multiply and Assign	x = 4 (x = x 4)
/=	Divide and Assign	x /= 2 (x = x / 2)
%=	Modulus and Assign	x %= 3 (x = x % 3)

3. Comparison Operators

Operator	Description	Example
<	Less than	5 < 10 → True
>	Greater than	10 > 5 → True
<=	Less than or equal	5 <= 5 → True
>=	Greater than or equal	10 >= 5 → True
==	Equal to	5 == 5 → True
!=	Not equal to	5 != 3 → True

4. Logical Operators

Operator	Description	Example
and	Logical AND	True and False → False
or	Logical OR	True or False → True
not	Logical NOT	not True → False

5. Identity Operators

Identity operators compare the memory locations of two objects.

Operator	Description	Example
is	Returns True if both variables point to same object	x is y
is not	Returns True if both variables point to different objects	x is not y

Example:

y = [1, 2, 3]

print(x is y) # False (different objects)

print(x == y) # True (same values)

6. Bitwise Operators

Bitwise operators perform operations on binary representations of numbers.

Operator	Description	Example (x=5, y=3)
&	Bitwise AND	5 & 3 = 1
\|	Bitwise OR	5 \| 3 = 7
^	Bitwise XOR	5 ^ 3 = 6
~	Bitwise NOT	~5 = -6
<<	Left Shift	5 << 1 = 10
>>	Right Shift	5 >> 1 = 2

7. Membership Operators

Membership operators test if a value is a member of a sequence (string, list, tuple, etc.).

Operator	Description	Example
in	Returns True if value is found in sequence	'a' in 'apple' → True
not in	Returns True if value is not found in sequence	'z' not in 'apple' → True

Example:

print('apple' in fruits) # True

print('grape' not in fruits) # True

Punctuators

Punctuators are special symbols used to organize code structure in Python.

Common Punctuators:

[ ] ( ) { } , : . ; @ = += -= *= /=

Usage:

[ ] - Lists, indexing
( ) - Tuples, function calls
{ } - Dictionaries, sets
: - Indentation, slicing

Keywords

Keywords are reserved words in Python that have special meanings and cannot be used as variable names, function names, or any other identifiers.

Keywords are case-sensitive
They cannot be used as identifiers

Python Keywords (35 keywords):

False

None

True

and

assert

async

await

break

class

continue

def

del

elif

else

except

finally

for

from

global

import

lambda

nonlocal

not

pass

raise

return

try

while

with

yield

Identifiers

Identifiers are names used to identify variables, functions, classes, modules, and other objects in Python.

Rules for Identifiers:

✓Must begin with an uppercase letter (A-Z), lowercase letter (a-z), or underscore (_)
✓Can contain letters, digits (0-9), and underscores
✓Only special character accepted is underscore (_)
✓Can be of any length
✗Cannot be a keyword
✗Cannot start with a digit
✗Cannot contain spaces or other special characters

✅ Valid Identifiers:

_count

total123

MyClass

❌ Invalid Identifiers:

student-name (has hyphen)

if (is a keyword)

my name (has space)

Comments

Comments are remarks or notes in the code that are ignored by the Python interpreter. They help document and explain code.

1. Single Line Comments

Use # for single-line comments

x = 10 # Assign value to x

2. Multi-Line Comments

Use triple quotes ''' or """

This is a

multi-line comment

'''

Uses of Comments:

Documenting code: Explain what the code does
Debugging: Temporarily disable code to identify errors
Code explanation: Clarify complex logic

Features of Python

1. High-Level Language

Easy to read and write, closer to human language than machine code

2. Interpreted Language

Code is executed line by line, no compilation needed

3. Easy to Understand

Simple syntax, readable code

4. Case-Sensitive

Python distinguishes between uppercase and lowercase

5. Predefined Libraries

Rich standard library with many built-in modules

6. Platform Independent

Runs on Windows, Mac, Linux, etc.

Applications of Python:

Machine Learning - Data science, AI
Web Development - Django, Flask
Data Analysis - Pandas, NumPy
Automation - Scripting, automation
Game Development - Pygame

Execution Modes in Python

1. Interactive Mode

Code is executed immediately after typing each line. Also known as Python Shell or REPL (Read-Eval-Print Loop).

Hello

>>> x = 10

>>> x + 5

Advantages: Quick testing, immediate feedback

2. Script Mode

Code is written in a file (with .py extension) and executed as a complete program.

print("Hello World")

x = 10

print(x + 5)

Advantages: Save and reuse code, better for larger programs

Datatypes in Python

1. Numeric/Number

int - Integer (e.g., 5, -10, 100)
float - Floating point (e.g., 3.14, -2.5)
complex - Complex numbers (e.g., 3+4j)

2. Boolean

True or False
Used for logical operations

3. Sequence

list - Ordered, mutable [1, 2, 3]
tuple - Ordered, immutable (1, 2, 3)
String - Text "Hello"

4. None

Represents absence of value

Mutable vs Immutable:

Mutable (Changes Allowed):

list - Can modify elements
dict - Can add/remove keys
set - Can add/remove elements

Immutable (Cannot Change):

int, float - Numbers
tuple - Cannot modify
String - Cannot modify

Type Conversion

1. Implicit Type Conversion

Python automatically converts one data type to another when needed (usually to a "larger" type).

Example:

y = 3.5 # float

result = x + y # float (5.0 + 3.5 = 8.5)

print(type(result)) # <class 'float'>

2. Explicit Type Conversion

Manually converting one data type to another using built-in functions.

Function	Description	Example
int()	Convert to integer	int(5.9) → 5
float()	Convert to float	float(10) → 10.0
str()	Convert to string	str(100) → "100"
bool()	Convert to boolean	bool(0) → False

Example:

num_int = int(num) # 100 (integer)

print(num_int + 5) # 105

Errors in Python

1. Syntax Error

Occurs when the code violates the rules of the Python programming language. Syntax means the rules of the language.

Example:

SyntaxError: expected ':'

Correction:

2. Logical Error

The program runs without crashing but produces incorrect output due to flawed logic.

Example:

return length + width # Should be length * width

print(area(5, 4)) # Output: 9 (incorrect, should be 20)

3. Runtime Error (Exception)

Errors that occur during program execution, such as division by zero, file not found, etc.

Example:

ZeroDivisionError: division by zero

PEP 8 - Python Style Guide

PEP 8 is the official style guide for Python code. Following PEP 8 makes your code more readable and maintainable.

1. Indentation

Use 4 spaces per indentation level (no tabs).

if True:

print("Indented correctly")

2. Maximum Line Length

Keep lines ≤ 79 characters. Use line continuation with \ or parentheses.

3. Naming Conventions

Type	Convention	Example
Variables	snake_case	user_name
Constants	UPPERCASE	MAX_LIMIT
Functions	snake_case	get_data()
Classes	PascalCase	DataProcessor

4. Comments

Use # for single-line comments. Write comments before the code, not inline.

def calculate_area(radius):

return 3.14 * radius ** 2