Some sample tests I threw together for my talk on Property Based Testing

• Facts​
• Inverses or duals​
• Known good (oracle)​

​Practice makes perfect

• Something general about the result​

  • (Not) null / negative / empty ​
  • Same result doing it once or multiple times (Idempotent)​
    • “Flag invoice not paid in time”​
    • Add/remove with set semantics​

• Something about the result in relation to the inputs​

  • (Not) greater / equal / less in length / complexity ​

• Something about observable side effects​

  • Value has (not) changed ​
  • “Place order increases total order value”​
  • “A delivery increases total stock value”​

• Something general about the execution ​

  • Should not crash (fuzzing) ​
  • Should (not) throw an exception ​

• Anything you'd write an assertion about ​

  • A property to verify it is always / never hit ​
  • Design by Contract (Bertrand Meyer) ​

• Encode/decode​
• Write/read​
• Encrypt/decrypt​
• Serialize/deserialize​
• Write/delete ​
• Do/undo​
• Order/cancel order​
• Invoice/credit invoice​
• ...​

• Old (mostly correct) version​
• 3rd party library​
• Shell out to some command line tool ​
• Collect samples from existing app, compare in new app​

• You implement two versions​

• Real + One that is more easily verified to be correct​

• Idealistic​

  • A Database can be emulated as an in-memory dictionary (Stateful PBT)​
  • maybe not thread safe, insecure, not persistent, all sorts of compromises​

• Unoptimized​

  • naive implementation​
  • brute-force algorithm
  • single-threaded version​