Legacy Modernization

Incremental migration of running systems. The constraint that shapes every decision here: the system is in production and must keep working while it changes.

The sequence

Order matters more than the individual techniques — each step exists to make the next one safe.

1. Characterize before changing. Write tests that pin current behaviour, bugs included. You are not testing what the code should do; you are recording what it does, so a behaviour diff becomes visible. When an assertion looks wrong, that is the point — write it down and say so:

   def test_domestic_lightweight():
       order = {"weight": 5, "destination": "domestic", "priority": False}
       # Current behaviour: nothing charged under 10kg. Looks like a bug —
       # pin it here, fix it as its own change after the migration lands.
       assert calculate_shipping_cost(order) == 0.0
   

   Golden-master or approval tests (approvaltests, syrupy) get coverage over gnarly code fastest, because they capture output wholesale instead of requiring you to understand it first.