Notes on Variable, Expression and Statement in Python Programming

Topic Learning Outcomes

  • Relational Operators: Understand the use of relational operators in Python.
  • Variables: Understand how to correctly use variables including naming conventions and reusability.
  • Boolean Operators: Learn how to employ Boolean logic and operators in programming.
  • Type Conversion: Grasp the concept of type conversion, including how to convert data types in Python.

Variables in Python

  • Definition: A variable is a named memory location used to store data which can be retrieved later by using the variable name.
  • Reusability: Once defined, the variable can store different values in different parts of the program (e.g., mileToKm = 1.60934).
  • In Python: Case Sensitivity is crucial. Variable names must be unique.

Creating Variables

  • Example of creating variables:
    • number = 10
    • number = 2.5 (Re-assignment of value to number)
    • a, b, c = 3, 3.5, "Python" (Multiple assignments)
    • a = b = c = "I love Python" (Same assignment to multiple variables)

Constants

  • Definition: Constants are variables whose values cannot change during execution. They serve as containers for fixed values.
  • Example of declaring a constant in Python:
    1. PI = 3.14
    2. Using constants in other modules: 
  # filename: week4.py
  import constant
  print(constant.PI)

Naming Variables and Constants

  • Rules:
    • Combine letters (lowercase & uppercase), digits, and underscores.
    • Avoid special symbols and starting names with digits.
    • Use meaningful names (e.g., first_name, age).
    • Constants should be declared in uppercase (e.g., PI).
  • Keywords (Reserved Words): Cannot be used as variable names (e.g., if, else, while, class).

Variable Types

  • Variables can be of different types: Integer, Float, String, etc.
  • Example of Type in Python: 
  ddd = 1 + 4  # ddd is of type integer
  eee = 'hello ' + 'there'  # eee is of type string
  • Checking Type: Use type(variable) to find out a variable's type.

Assignment Statements

  • Assignment involves equating a variable to an expression:
    • Example: x = 3.9 * x * (1 - x)
  • Explanation: This assigns the result of the expression on the right to x.

Arithmetic and Numeric Operations

  • Operations:
    • Multiplication: *
    • Addition: +
    • Exponentiation: ** (e.g., 2**3 = 8)
    • Integer Division: //
    • Modulus: %
  • Floating Point Behavior: If a float and integer are mixed, the result is a float (e.g., 1 / 2.0 = 0.5).

Expressions and Operator Precedence

  • Definition of Expression: A combination of values and operators that compute a single value (e.g., 1 + 4 * 3).
  • Precedence Rules:
    • Parentheses () > Exponentiation ** > Multiplication/Division/Remainder > Addition/Subtraction.
  • Example of Evaluation Order: 
  x = 1 + 2 * 3 - 4 / 5 ** 6  # evaluated according to precedence rules.

Boolean Values and Comparison Operators

  • Definition: Boolean values return True or False. Used in conditional statements.
  • Operators:
    • == (equal to)
    • != (not equal)
    • > (greater than)
    • and, or, not (logical operators).
  • Example: 
  x = 5  
  x == 5  # returns True
  x > 7  # returns False

Logic and Readability in Conditions

  • Logical combinations using operators like and enhance condition evaluation.
    • Example: xDivisible = (x % 2) == 0.
  • Always aim for readability by breaking down complex logical conditions across multiple lines.

Recap of Main Points

  • Understanding variables, their types, assignment rules, naming conventions, and type conversions is crucial in programming with Python.
  • Mastery of arithmetic operations and expressions, combined with logical operators, underpins effective conditional programming and decision-making.
  • Practice making variable names meaningful and maintaining clear, readable code to enhance maintainability and collaboration.