README.md

Constructors and Invariants (Python)

This module shows how constructors enforce valid object state.

Learning Metadata

• Difficulty: Intermediate.
• Estimated Time: 35-50 minutes.
• Prerequisites: 03-advanced/structs-and-classes.
• Cross-Language Lens: Compare constructor rules, validation points, and object setup guarantees in each track.

Quick Run

python example/main.py

Topics Covered

• Constructor guards that normalize invalid values.
• Defining invariants as always-true object rules.
• Safe update methods that reject invalid transitions.
• Returning explicit operation success values.

Common Pitfalls

• Accepting invalid constructor values and fixing later.
• Writing directly into internal fields from outside methods.
• Ignoring failed updates and assuming state changed.

Exercise Focus

• exercises/01.py: bank account with non-negative balance invariant.
• exercises/02.py: date model with month/day validation.

Exercise Specs

1. exercises/01.py

• Input: initial balance and transaction values.
• Output: updated balance with validity checks.
• Edge cases: negative initial balance; withdrawal beyond balance.

2. exercises/02.py

• Input: month and day values.
• Output: valid/invalid date result.
• Edge cases: month out of range; day out of range for month.

Checkpoint

• I can define clear invariants for a class.
• I can enforce invariants in constructors and methods.
• I can reject invalid updates safely.
• I completed exercises/01.py.
• I completed exercises/02.py.