Getting Started with Python Class 11 Computer Science Chapter 5 CBSE Notes 2025-26

Python is a high-level, open-source programming language known for its clear and straightforward syntax. Programs created with Python are easier to write, read, and understand, making it a popular choice for beginners and professionals alike. 

Computers only understand machine language (0s and 1s), but Python, being a high-level language, allows us to write instructions in a form more natural to humans. These instructions, or source code, are then translated into machine language using an interpreter, which executes instructions line by line.

Features of Python Python programs are portable and can run on different operating systems. The language is case-sensitive; for example, 'number' and 'NUMBER' are treated as two different identifiers. Python comes with a wide range of built-in functions, supports web development, and organizes code using indentation. All of these traits make it a powerful learning tool.

To use Python, you can work in interactive mode—typing instructions and getting instant results—or script mode—saving programs in .py files for later execution. The interactive mode is useful for testing single commands, whereas script mode is better when you want to reuse or run your code multiple times.

Python Keywords and Identifiers Keywords in Python are special words reserved for specific tasks, such as if, else, while, and def. These cannot be used as names for variables or functions. Identifiers are user-defined names given to elements such as variables and functions. They must begin with a letter or underscore (not a digit), can use letters, digits, and underscores, and must not include special symbols or be a keyword.

For example, 'marks1', 'average_score', and '_userName' are valid identifiers, but '1number' and 'for' (a keyword) are not.

Variables and Comments In Python, a variable acts as a label for a value stored in memory. Assigning a value to a variable declares it automatically; explicit declaration is not needed. For instance, score = 95 creates a variable 'score' holding the value 95. Variable names should be descriptive and not single letters.

Comments help explain what your code does and are ignored by the interpreter. In Python, comments start with the hash symbol (#). Using comments can clarify the purpose and logic of your code, making it more readable and easier to maintain.

Everything is an Object In Python, every value is treated as an object, which means each value has a unique identity (like a memory address). For example, after x = 10, both x and y = 10 will point to the same object in memory for value 10. The function id() returns an object's unique identity.

Data Types in Python Python supports several built-in data types, each with its own purpose:

• Numbers: int (integers), float (decimals), and complex (complex numbers).
• Boolean: True or False (a type of integer).
• Strings: Text data inside single or double quotes.
• List: Ordered, mutable collections inside square brackets, e.g., [1, 2, 'apple'].
• Tuple: Ordered, immutable collections inside parentheses, e.g., (5, 'hi').
• Set: Unordered, unique values inside curly braces, e.g., {1, 2, 3}.
• Dictionary: Unordered key-value pairs, e.g., {'name': 'Amit', 'age': 17}.
• None: Special type to represent the absence of value.

Lists can be changed after creation (mutable), while tuples and strings cannot (immutable). Sets enforce uniqueness, while dictionaries allow fast lookups using unique keys.

Operators in Python Operators are symbols that perform operations on variables and values. Python supports several types of operators:

• Arithmetic Operators: +, -, *, /, % (modulus), // (floor division), ** (power).
• Relational Operators: ==, !=, >, <, >=, <=.
• Assignment Operators: =, +=, -=, *=, /=, etc.
• Logical Operators: and, or, not.
• Identity Operators: is, is not.
• Membership Operators: in, not in.

The precedence of these operators determines the order in which parts of an expression are evaluated. For example, multiplication has higher precedence than addition, so in 2 + 3 * 4, multiplication happens first.

Expressions and Statements An expression is any combination of constants, variables, and operators that produces a value, like 3 + 4 or num * 2. A statement is a line of code that performs an action, such as variable assignment or a function call, like x = 5 or print(x).

User Input and Output Programs often interact with users using the input() function, which collects user input as a string. For example, name = input("Enter your name: ") asks the user to enter their name, storing it in the variable 'name'. All input from the user is string data by default, but you can convert it to integers or floats using int() or float(). Output is usually shown with the print() function, which displays values or messages on the screen.

Type Conversion Type conversion means changing the data type of a value. This can be explicit (using conversion functions like int(), float(), or str()) or implicit (Python automatically converts types in some mixed operations, like adding an integer and a float will result in a float). For example, converting a string '23' to an integer allows mathematical operations: int('23') + 7 results in 30.

Debugging Programs sometimes have errors, called bugs, which prevent them from running as expected. Debugging is the process of finding and fixing such errors. Types of errors include syntax errors (mistakes in code structure), logical errors (wrong output), and runtime errors (program crashes while running, such as dividing by zero).

Documentation Tips Good programs have clear objectives written at the top, functions have descriptive docstrings, variables and functions have meaningful names, and comments are used where helpful (but not overused). Consistency in naming and indentation makes code easy to follow and helps others understand your logic quickly.

Class 11 Computer Science Chapter 5 Notes – Getting Started with Python: Key Principles and Revision Points

These Class 11 Computer Science Chapter 5 notes provide a concise, step-by-step understanding of Python basics. Students can quickly review key features, data types, and operators in Python for last-minute revision and concept clarity. Main points are presented in simple language for easy recall.

With user-friendly explanations and examples, these notes make it easier to differentiate between Python keywords, identifiers, and variable types. Understanding input/output, expressions, comments, and debugging helps students excel in chapter-based assessments and practical exams.