Dependency Inversion Principle

Depend on abstractions, not concretions.

Intent

High-level modules (policy) and low-level modules (details) both depend on abstractions. Details implement those abstractions.

Problem

Research wants John’s children. If it walks relationships.relations tuples directly, it is glued to how storage is laid out. Change the list → break research.

Solution

Introduce RelationshipBrowser.find_all_children_of.
Relationships implements it. Research only talks to the browser.

Research→RelationshipBrowser←Relationships

Structure

Piece	Role
Research	High-level use case
RelationshipBrowser	Abstraction
Relationships	Low-level store
Person / Relationship	Domain bits

Compare code

Violation — dig into relations storage

class Research:
    def __init__(self, relationships: Relationships):
        for r in relationships.relations:
            if r[0].name == "John" and r[1] == Relationship.PARENT:
                print(f"John has a child called {r[2].name}")

Compliant — this repo

class Research:
    def __init__(self, browser: RelationshipBrowser):
        for p in browser.find_all_children_of("John"):
            print(f"John has a child called {p}")

Walkthrough

John is parent of Chris and Matt. Relationships stores parent/child pairs.

find_all_children_of hides the tuple list. Callers never touch .relations.

Research(relationships) works because Relationships is a RelationshipBrowser.

Quick check

Which change follows DIP?

Run locally

uv run dip

Expected:

John has a child called Chris
John has a child called Matt

Source: main.py · tutorial README

Takeaway

• Policy must not scrape infrastructure internals
• Share an abstraction both sides depend on
• Details implement the abstraction — they don’t own the high-level