FrançaisGet started

DRY, KISS, YAGNI

Three rules of restraint in code: do not repeat the same knowledge (DRY), prefer the simplest solution (KISS), do not build what is not needed yet (YAGNI), and their limits.

  • 25 cards
  • 5 topics
  • Free
Study this deck

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

  1. Concepts
  2. Questions
  3. Sort and link
  4. Scenarios
  5. Understand

A few cards from the deck

  • DRY: what is it?

    Show the answer

    The rule that each piece of knowledge in the system (a business rule, a rate, a format) has a single authoritative representation.

    The acronym stands for “Don't Repeat Yourself”. It targets duplicated knowledge, not every resemblance between two pieces of code.

  • Which fix applies DRY to this code?

    ```python
    def prix_ttc_dvd(prix):
    return prix * 1.2

    def prix_ttc_jeu(prix):
    return prix * 1.2

    def facture(total):
    return total + total * 0.2
    `

    • Define a constant TVA = 0.2 and use it in the three functions
    • Merge prix_ttc_dvd and prix_ttc_jeu into one function, leaving facture alone
    • Add a # VAT 20% comment above each function
    • Rewrite each function on a single line
    Show the answer

    Define a constant TVA = 0.2 and use it in the three functions

    The VAT rate is one piece of knowledge written three times, in two forms (1.2 and 0.2). Merging two functions leaves the repetition in facture; a comment adds one more to keep up to date.

  • If the invoice-number format is coded again in four modules, changing it means finding them all: DRY asks that each piece of … have a single … in the system.

    Show the answer

    If the invoice-number format is coded again in four modules, changing it means finding them all: DRY asks that each piece of knowledge have a single representation in the system.

    A named function or constant becomes the single source of that format: it is changed in one place, with no risk of missing a module.

  • KISS: what is it?

    Show the answer

    The rule that, between two solutions that meet the need, you keep the one that is simplest to read, test and maintain.

    The acronym stands for “Keep It Simple, Stupid”. Simple does not mean short: an 80-character regular expression is brief and unreadable.

  • Shipping fees and the late-payment penalty were both 5%. They were merged into one taux() function. The penalty rises to 8% after a legal decision, and the shipping fees change too. What is the cause, and the fix?

    • Accidental duplication: two different pieces of knowledge; separate them again
    • DRY violation: the VAT rate should have been merged too
    • Premature abstraction: add a type parameter to taux() to tell the cases apart
    • KISS violation: the 5 should have been written directly everywhere
    Show the answer

    Accidental duplication: two different pieces of knowledge; separate them again

    The two values were equal by chance, and do not change for the same reason (transport, the law). A type parameter keeps the artificial link while hiding it. DRY targets identical knowledge, not identical values.

Study this deck