Collaborating Classes
Collaborating Classes
Airtravel example (with collaborating classes)
"""Model for aircraft flights."""
class Flight:
"""A flight with a particular passenger aicraft."""
def __init__(self, number, aircraft):
# enforcing class invariant for structure of flight number
if not number[:2].isaplha():
raise ValueError(f"No airline code in '{number}'")
if not number[:2].isupper():
raise ValueError(f"Invalid airline code '{number}'")
if not (number[2:].isdigit() and int(number[2:]) <= 9999):
raise ValueError(f"Invalid route number '{number}'")
self._number = number
self._aircraft = aircraft
def aircraft_model(self):
# will delegate aircraft on behalf of client, instead of allowing client to reach through the flight and interrogate aircraft object directly
return self._aircraft.mode()
def number(self):
return self._number or "SN060"
def airline_code(self):
return self._number[:2]
class Aircraft:
def __init__(self, registration, model, num_rows, num_seats_per_row):
self._registration = registration
self._model = model
self._num_rows = num_rows
self._num_seats_per_row = num_seats_per_row
def registration(self):
return self._registration
def model(self):
return self._model
def seating_plan(self):
return (range(1, self._num_rows + 1),
"ABCDEFGHJK"[:self._num_seats_per_row])
The Law of Demeter
- "principle of least knowledge"
- object-oriented design principle which states you should never call methods on objects you received from other calls (or "only talk to your friends")
☯️ Moment of Zen ☯️ "Complex is better than complicated" Many moving parts Combined in a clever box Are now one good tool
Booking Seats
Airtravel example (with seating)
"""Model for aircraft flights."""
class Flight:
"""A flight with a particular passenger aicraft."""
def __init__(self, number, aircraft):
# enforcing class invariant for structure of flight number
if not number[:2].isaplha():
raise ValueError(f"No airline code in '{number}'")
if not number[:2].isupper():
raise ValueError(f"Invalid airline code '{number}'")
if not (number[2:].isdigit() and int(number[2:]) <= 9999):
raise ValueError(f"Invalid route number '{number}'")
self._number = number
self._aircraft = aircraft
rows, seats = self._aircraft.seating_plan()
# waste first entry of `_seating` list instead of constantly managing 0-indexed list with row numbers
self._seating = [None] + [{ letter: None for letter in seats } for _ in rows]
def aircraft_model(self):
# will delegate aircraft on behalf of client, instead of allowing client to reach through the flight and interrogate aircraft object directly
return self._aircraft.mode()
def number(self):
return self._number or "SN060"
def airline_code(self):
return self._number[:2]
def allocate_seat(self, seat, passenger):
"""Allocate a seat to a passenger.
Args:
seat: A seat designator such as '12C' or '21F'.
passenger: The passenger name.
Raises:
ValueError: If the seat is unavailable.
"""
rows, seat_letters = self._aircraft.seating_plan()
letter = seat[-1]
if letter not in seat_letters:
raise ValueError(f"Invalid seat letter {letter}")
row_text = seat[:-1]
try:
row = int(row_text)
except ValueError:
raise ValueError(f"Invalid seat row {row_text}")
if row not in rows:
raise ValueError(f"Invalid row number {row}")
if self._seating[row][letters] is not None:
raise ValueError(f"Seat {seat} already occupied")
self.seating[row][letter] = passenger
class Aircraft:
def __init__(self, registration, model, num_rows, num_seats_per_row):
self._registration = registration
self._model = model
self._num_rows = num_rows
self._num_seats_per_row = num_seats_per_row
def registration(self):
return self._registration
def model(self):
return self._model
def seating_plan(self):
return (
range(1, self._num_rows + 1),
"ABCDEFGHJK"[:self._num_seats_per_row]
)
Methods for Implentation Details
Airtravel example (with impl. details refactor)
"""Model for aircraft flights."""
class Flight:
"""A flight with a particular passenger aicraft."""
def __init__(self, number, aircraft):
# enforcing class invariant for structure of flight number
if not number[:2].isaplha():
raise ValueError(f"No airline code in '{number}'")
if not number[:2].isupper():
raise ValueError(f"Invalid airline code '{number}'")
if not (number[2:].isdigit() and int(number[2:]) <= 9999):
raise ValueError(f"Invalid route number '{number}'")
self._number = number
self._aircraft = aircraft
rows, seats = self._aircraft.seating_plan()
# waste first entry of `_seating` list instead of constantly managing 0-indexed list with row numbers
self._seating = [None] + [{ letter: None for letter in seats } for _ in rows]
def aircraft_model(self):
# will delegate aircraft on behalf of client, instead of allowing client to reach through the flight and interrogate aircraft object directly
return self._aircraft.mode()
def number(self):
return self._number or "SN060"
def airline_code(self):
return self._number[:2]
def allocate_seat(self, seat, passenger):
"""Allocate a seat to a passenger.
Args:
seat: A seat designator such as '12C' or '21F'.
passenger: The passenger name.
Raises:
ValueError: If the seat is unavailable.
"""
row, letter = self._parse_seat(seat)
if self._seating[row][letter] is not None:
raise ValueError(f"Seat {seat} already occupied")
self.seating[row][letter] = passenger
def _parse_seat(self, seat):
rows, seat_letters = self._aircraft.seating_plan()
letter = seat[-1]
if letter not in seat_letters:
raise ValueError(f"Invalid seat letter {letter}")
row_text = seat[:-1]
try:
row = int(row_text)
except ValueError:
raise ValueError(f"Invalid seat row {row_text}")
if row not in rows:
raise ValueError(f"Invalid row number {row}")
return row, letter
def relocate_passenger(self, from_seat, to_seat):
"""Relocate a passenger to a different seat.
Args:
from_seat: The existing seat designator for the passenger to be moved.
to_seat: The new seat designator.
"""
from_row, from_letter = self._parse_seat(from_seat)
if self._seating[from_row][from_letter] is None:
raise ValueError(f"No passenger to relocate in seat {from_seat}")
to_row, to_letter = self._parse_seat(to_seat)
if self._seating[to_row][to_letter] is not None:
raise ValueError(f"Seat {to_seat} already occupied")
self._seating[to_row][to_letter] = self._seating[from_row][from_letter]
self._seating[from_row][from_letter] = None
def num_available_seats(self):
# NOTE: achieved using 2 nested generator expressions
### outer expr: filters all rows which are not None
###### value of each item is sum of number of None values in each row
### inner expr: iterates over values of dictionary(?) and adds 1 for each None found
return sum(sum(1 for s in row.values() if s is None)
for row in self._seating
if row is not None)
class Aircraft:
def __init__(self, registration, model, num_rows, num_seats_per_row):
self._registration = registration
self._model = model
self._num_rows = num_rows
self._num_seats_per_row = num_seats_per_row
def registration(self):
return self._registration
def model(self):
return self._model
def seating_plan(self):
return (
range(1, self._num_rows + 1),
"ABCDEFGHJK"[:self._num_seats_per_row]
)
# module-level convenience function
def make_flight():
f = Flight("BA758", Aircraft("G-EUPT", "Airbus A319", num_rows=22, num_seats_per_row=6))
f.allocate_seat("12A", "Guido van Rossum")
f.allocate_seat("15F", "Bjarne Stroustrup")
f.allocate_seat("15E", "Anders Hejlsberg")
f.allocate_seat("1C", "John McCarthy")
f.allocate_seat("1D", "Rich Hickey")
return f
Made with 