Informatics Practices: Class 11 — Chapter 3: Brief Overview of Python

To write and run Python programs, a Python interpreter must be installed, or an online interpreter can be used. The interpreter is also referred to as the Python shell . The symbol >>> is the Python prompt , indicating that the interpreter is ready to receive commands.

Python supports two primary execution modes:

• Interactive Mode: Commands are typed directly at the >>> prompt. Pressing Enter executes the statement immediately and displays the result. This mode is ideal for testing single-line code snippets but does not allow saving statements for future use.
• Script Mode: Programs are written in a \text file, saved with a .py extension, and executed using the interpreter. This mode is preferred for multi-line programs that need to be saved and run repeatedly.

Interactive Mode Example:

  
    >>> 1 + 2
    3
    >>> 4 - 2
    2
    >>> "Hello" + "World"
    'HelloWorld'
  

Keywords are reserved words with specific meanings for the Python interpreter. Because Python is case-sensitive, keywords must be written exactly as defined.

Identifiers are user-defined names used to identify variables, functions, classes, or other entities in a program.

Rules for naming identifiers in Python:

1. Must begin with an uppercase or lowercase letter (A-Z, a-z) or an underscore ( _ ).
2. Can be followed by any combination of letters, digits (0-9), or underscores. It cannot start with a digit.
3. Can be of any length, though short and meaningful names are recommended.
4. Must not be a reserved keyword.
5. Cannot contain special symbols such as ! , @ , # , $ , % , etc.

Stores numerical values. It is classified into three types:

• int (Integer): Whole numbers (e.g., -12, 0, 123). bool (Boolean) is a subtype of integer containing only True (non-zero) and False (zero).
• float (Floating Point): Numbers with fractional parts (e.g., -2.04, 4.0, 14.23).
• complex (Complex Numbers): Numbers with real and imaginary parts (e.g., 3 + 4j).

An ordered collection of items where each item is indexed by an integer:

• String: A group of characters enclosed in single or double quotes (e.g., 'Hello' , "452" ).
• List: A comma-separated sequence of items enclosed in square brackets [] . Items can be of different data types and are mutable (e.g., [5, 3.4, "New Delhi"] ).
• Tuple: A comma-separated sequence of items enclosed in parentheses () . Tuples are immutable (e.g., (10, 20, "Apple") ).

An unordered data type that stores data in key-value pairs enclosed in curly brackets {} . Keys must be unique and are typically strings, while values can be of any data type.

  
    >>> dict1 = {'Fruit':'Apple', 'Climate':'Cold', 'Price(kg)':120}
    >>> print(dict1['Price(kg)'])
    120
  

Used to perform mathematical calculations.

Compares values and returns either True or False .

Assignment Operators: Used to assign values to variables (e.g., = , += , -= , *= , /= ).

Logical Operators: Used to combine conditional statements. Written in lowercase: and , or , not .

Membership Operators: Used to test if a value is present in a sequence (string, list, tuple): in , not in .

Determines the order of evaluation in expressions containing multiple operators. Parentheses () have the highest precedence and can override default evaluation orders.

  
    # Precedence of * is higher than +
    20 + 30 * 40  # Evaluates to 20 + 1200 = 1220
    # Parentheses force addition first
    (20 + 30) * 40  # Evaluates to 50 * 40 = 2000
  

input(): Prompts the user for input and returns it as a string. Type conversion functions like int() or float() must be used to convert the input to numeric types.

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

print(): Evaluates and displays expressions on the screen.

  
    print("Hello")
    print(10 * 2.5) # Output: 25.0
  

The process of identifying and removing errors from a program is called debugging . Errors are categorized into three types:

1. Syntax Errors: Violations of the grammatical rules of the programming language. The interpreter stops execution immediately when a syntax error is encountered (e.g., missing parenthesis).
2. Logical Errors: The program runs successfully but produces incorrect or unexpected output. These are also called semantic errors (e.g., writing 10 + 12 / 2 instead of (10 + 12) / 2 ).
3. Runtime Errors: Occur during execution, causing abnormal termination of the program even though the syntax is correct (e.g., division by zero).

A function is a named, reusable block of statements that performs a specific task. Python provides built-in functions (e.g., print() , input() , type() , len() ) and allows importing functions from modules using the import statement.

Used to execute different blocks of code based on conditional outcomes:

• if statement: Executes code only if the condition is true.
• if..else statement: Executes one block if the condition is true, and another if it is false.
• if..elif..else statement: Checks multiple conditions sequentially.

  
    # Example of if..elif..else
    number = int(input("Enter a number: "))
    if number > 0:
    print("Positive")
    elif number < 0:
    print("Negative")
    else:
    print("Zero")
  

for Loop: Used to iterate over a sequence (range, list, tuple, string).

  
    # Print even numbers
    numbers = [1, 2, 3, 4, 5]
    for num in numbers:
    if num % 2 == 0:
    print(num, "is even")
  

range() Function: Generates a sequence of numbers. Syntax: range([start], stop[, step]) .

  
    >>> list(range(2, 10, 2))
    [2, 4, 6, 8]
  

Nested Loops: A loop inside another loop.

  
    for i in range(3):
    for j in range(2):
    print(i, j)