--- title: A New Price Without Breaking the Old One • The O in SOLID description: Adding a new pricing rule to a single shared class changed an existing one in this walkthrough of the Open-Closed Principle. A potion shop's apprentice... canonical: https://daily.dev/posts/a-new-price-without-breaking-the-old-one-the-o-in-solid-gbaha0g2f twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: A New Price Without Breaking the Old One • The O in SOLID | daily.dev og:description: Adding a new pricing rule to a single shared class changed an existing one in this walkthrough of the Open-Closed Principle. A potion shop's apprentice... og:url: https://daily.dev/posts/a-new-price-without-breaking-the-old-one-the-o-in-solid-gbaha0g2f og:image: https://api.daily.dev/og/posts/gbAha0G2f.png og:image:alt: A New Price Without Breaking the Old One • The O in SOLID og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # A New Price Without Breaking the Old One • The O in SOLID **[Planet Python](https://daily.dev/sources/planetpython)** · 7 min read · 0 upvotes · 0 comments ## Summary Adding a new pricing rule to a single shared class changed an existing one in this walkthrough of the Open-Closed Principle. A potion shop's apprentice discount correctly produced £108, but after a second programmer added a guild discount inside the same class, the apprentice price shifted to £110. The fix separates each pricing rule into its own class, like ApprenticePricing and GuildPricing, each with a .price() method, called through a shared quote() function. Adding GuildPricing this way left the apprentice calculation untouched, restoring £108 while guild members got £115. The code uses Python 3.15. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### What is the Open-Closed Principle in object-oriented design? The Open-Closed Principle states that software should be open to extension but closed to modification. In practice, this means adding new behavior, like a new pricing rule, through a new class or object rather than editing existing code that already works correctly, reducing the risk of accidentally breaking established behavior. _daily.dev surfaces SOLID design explainers for developers hardening their codebase against regressions._ ### How do you add a new pricing rule in Python without breaking an existing one using OOP? Separate each pricing rule into its own class with a shared method signature, such as a .price(base_price, surcharge) method, then call it through a generic function like quote() that accepts any policy object. In one example, adding a GuildPricing class alongside ApprenticePricing left the apprentice calculation untouched, keeping its £108 result while the guild price correctly came out to £115. _developers refactoring shared logic can find design-pattern writeups like this through daily.dev._ ### Why did adding a new customer discount type break an existing discount calculation in a shared pricing class? Because both discount rules were handled with if-branches inside one method, generalizing the logic to support the new rule changed how the existing rule computed its result. An apprentice price that correctly returned £108 dropped to £110 after a guild discount branch was added, since the new shared calculation applied the discount differently than the original apprentice-specific logic did. _daily.dev helps developers avoid these regression traps when evolving shared business logic._ ## Similar posts on daily.dev - [The Potion Shop That Does Too Much • The S in SOLID](https://daily.dev/posts/the-potion-shop-that-does-too-much-the-s-in-solid-pqygwcav1) · Planet Python · 0 upvotes · 0 comments - [Centralizing Pricing Logic: Stop Scattering Your Business Rules](https://daily.dev/posts/centralizing-pricing-logic-stop-scattering-your-business-rules-rqa51hjgs) · Sergio Lema · 57 upvotes · 7 comments - [The Open-Closed Principle](https://daily.dev/posts/the-open-closed-principle-kyj1lquzw) · Ruby Flow · 0 upvotes · 0 comments - [What SOLID Still Teaches Ruby Programmers](https://daily.dev/posts/what-solid-still-teaches-ruby-programmers-24ppwefj9) · RUBYLAND · 0 upvotes · 0 comments --- Tags: [#general-programming](https://daily.dev/tags/general-programming), [#python](https://daily.dev/tags/python), [#architecture](https://daily.dev/tags/architecture) [View this post on daily.dev](https://daily.dev/posts/a-new-price-without-breaking-the-old-one-the-o-in-solid-gbaha0g2f) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"A New Price Without Breaking the Old One • The O in SOLID","url":"https://daily.dev/posts/a-new-price-without-breaking-the-old-one-the-o-in-solid-gbaha0g2f","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/a-new-price-without-breaking-the-old-one-the-o-in-solid-gbaha0g2f"},"datePublished":"2026-10-26T10:48:26.404Z","dateModified":"2026-10-26T10:59:24.138Z","description":"Adding a new pricing rule to a single shared class changed an existing one in this walkthrough of the Open-Closed Principle. 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