Python Introduction: Variables, Data Types, and Abstraction

Fundamentals of Python and Abstraction

  • Sequence of Definition:

    • A value must be defined before it can be used in a Python program.

  • Strings (str):

    • str represents text data and tells Python the type of data being printed.
    • String literals can be enclosed using single quotes ('), double quotes ("), or other quote formats.

  • Output Statements and Functions:

    • Code Statement Example: print("Hello, world!")
    • Function Mechanics: The print function describes how to show information on the screen.
    • Abstraction Concept: The print function serves as an example of abstraction by encapsulating the lower-level mechanics of displaying output on a screen.

Variables and Memory Storage

  • Variable Definition and Storage:

    • A variable acts as storage that holds a value in memory.
    • Variables allow programmers to store and reuse text or values throughout code.

  • Variable Assignment and Execution:

    • Text Reuse Example:
    • Variable Assignment: greeting = "Hello, world!"
    • Output Call: print(greeting)
    • Program Output: Hello, world!
    • Direct Value Assignment:
    • Code Example: my_value = 8
    • Concept: Assigns the specific integer value 88 directly to the variable my_value.

  • Expression Assignment and Evaluation:

    • A variable can be assigned an expression instead of a simple literal value.
    • Code Example: my_value = 3 + 5
    • Evaluation Mechanics: When the program runs, the expression 3+53 + 5 evaluates to the single value 88, which is then stored in my_value.
    • Performing Computations with Variables:
    • Step 1: Assign expression to variable: my_value = 3 + 5 (evaluates to 88).
    • Step 2: Pass expression with variable to print: print(my_value + 2)
    • Program Output: 1010 (computed as 8+2=108 + 2 = 10).

Variable Naming Rules and Conventions

  • Allowed Characters:

    • Variable names can contain letters, digits (00 through 99), and underscores (_).

  • Numerical Placement Restrictions:

    • A variable name cannot start with a number.
    • Variable names are allowed to have numbers later in the name.

  • Exact Identifier Matching:

    • Variable identifiers are distinct and must match accurately when called.
    • The identifier my-value is different from my_value. Calling my-value when my_value was defined will not work.

  • Human vs. Computer Meaning:

    • A variable name does not carry semantic meaning to the computer; it is processed strictly as a label.
    • Meaningful variable names are selected solely to help humans read and comprehend the code.

Type Operations and Constraints

  • String and Number Combination Restriction:
    • Combining a string and a number directly using addition/concatenation is not permitted in Python.
    • Invalid Attempt Example: greeting = "hello" + 3 followed by print(greeting).
    • Note: Attempting to combine a string and an integer directly causes an error (to be gone over in detail later).