style(nav): add type hints

nav.py

@@ -6,6 +6,7 @@ Usage: python3 nav.py DESTINATION
 """
 
 import sys
+from collections.abc import Iterator
 
 # Distances between neighboring waypoints, in million km.
 DISTANCES = {
@@ -74,20 +75,20 @@ FUEL_PER_MKM = 0.142  # tonnes of fuel per million km
 LAUNCH_FUEL = 380.0  # tonnes of fuel to escape Earth's gravity
 
 
-def distance(a, b):
+def distance(a: str, b: str) -> float:
     """Return the distance between two neighboring waypoints."""
     if (a, b) in DISTANCES:
         return DISTANCES[(a, b)]
     return DISTANCES[(b, a)]
 
 
-def legs(route):
+def legs(route: list[str]) -> Iterator[tuple[str, str]]:
     """Yield the legs (pairs of consecutive waypoints) of a route."""
     for i in range(1, len(route) - 1):
         yield route[i], route[i + 1]
 
 
-def plan(destination):
+def plan(destination: str) -> tuple[list[str], float, float, float]:
     """Return route, distance, travel time (days) and fuel (t)."""
     route = ROUTES[destination]
     total = 0.0
@@ -101,12 +102,12 @@ def plan(destination):
     return route, total, total / CRUISE_SPEED, fuel
 
 
-def asteroid_route(belt):
+def asteroid_route(belt: str) -> list[str]:
     """Return a route through the asteroid belt (currently: via Ceres)."""
     return ROUTES["ceres"] + [belt]
 
 
-def print_plan(destination):
+def print_plan(destination: str) -> None:
     """Print the flight plan for a destination."""
     route, total, days, fuel = plan(destination)
     print("Route: " + " -> ".join(NAMES[w] for w in route))
@@ -115,7 +116,7 @@ def print_plan(destination):
     print(f"Fuel required: {round(fuel)} t")
 
 
-def main():
+def main() -> None:
     if len(sys.argv) != 2:
         sys.exit(__doc__)
     if sys.argv[1] == "--list":