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Variables and Variable Types

Introduce Python variables and common value types.

WHY: Programs are built from values. Clear naming supports readable and maintainable code.

What Is a Variable

In Python, a variable is a name that holds a value.

A variable associates a readable name with a piece of data so it can be reused, updated, and inspected throughout a program.

Common Variable Types

Examples of common value types:

  • str – holds text
  • int – holds a whole number
  • float – holds a decimal number
  • bool – holds either True or False (capitalized)
  • list[T] – ordered collections of values
  • dict[K, V] – key/value mappings

The letters inside square brackets (such as T, K, and V) are type hints. They describe what kind of values the collection is expected to contain.

Type Hints (Why We Use Them)

Type hints make variable types explicit.

name: str = "Abbie"
age: int = 5
weight_lbs: float = 9.8
is_cat: bool = True

Benefits:

  • clearer intent
  • better editor feedback
  • easier collaboration

Type hints do not change how Python runs.

Numeric Values - Include the Units

Units matter when working with numbers. Make units explicit to improve quality and clarity.

weight_lbs: float = 9.8
height_ft: float = 2.4

For currency, use Decimal

Working with currency, precision matters. Use a Decimal.

from decimal import Decimal

adoption_fee_usd: Decimal = Decimal("35.50")

Constants

Values that do not change are indicated by:

  • the way we name it ALL_CAP_SNAKE_CASE
  • adding Final to the type hint

Examples

CAT_NAME: Final[str] = "Abbie"

Why Variables Come First

Variables are introduced early because:

  • programs are composed of values
  • data analysis depends on naming data clearly
  • readable variables make intent visible

Where Variables Are Used

Variables appear throughout Python programs, including:

  • calculations
  • logging output
  • reading data from files
  • building summaries of results

Clear variable names reduce confusion and improve code quality over time.