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to Robot Simulator in the Python Track

Test suite

Write a robot simulator.

A robot factory's test facility needs a program to verify robot movements.

The robots have three possible movements:

  • turn right
  • turn left
  • advance

Robots are placed on a hypothetical infinite grid, facing a particular direction (north, east, south, or west) at a set of {x,y} coordinates, e.g., {3,8}, with coordinates increasing to the north and east.

The robot then receives a number of instructions, at which point the testing facility verifies the robot's new position, and in which direction it is pointing.

  • The letter-string "RAALAL" means:
    • Turn right
    • Advance twice
    • Turn left
    • Advance once
    • Turn left yet again
  • Say a robot starts at {7, 3} facing north. Then running this stream of instructions should leave it at {9, 4} facing west.

Exception messages

Sometimes it is necessary to raise an exception. When you do this, you should include a meaningful error message to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. Not every exercise will require you to raise an exception, but for those that do, the tests will only pass if you include a message.

To raise a message with an exception, just write it as an argument to the exception type. For example, instead of raise Exception, you should write:

raise Exception("Meaningful message indicating the source of the error")

Running the tests

To run the tests, run the appropriate command below (why they are different):

  • Python 2.7: py.test robot_simulator_test.py
  • Python 3.4+: pytest robot_simulator_test.py

Alternatively, you can tell Python to run the pytest module (allowing the same command to be used regardless of Python version): python -m pytest robot_simulator_test.py

Common pytest options

  • -v : enable verbose output
  • -x : stop running tests on first failure
  • --ff : run failures from previous test before running other test cases

For other options, see python -m pytest -h

Submitting Exercises

Note that, when trying to submit an exercise, make sure the solution is in the $EXERCISM_WORKSPACE/python/robot-simulator directory.

You can find your Exercism workspace by running exercism debug and looking for the line that starts with Workspace.

For more detailed information about running tests, code style and linting, please see the help page.


Inspired by an interview question at a famous company.

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


import unittest

from robot_simulator import Robot, NORTH, EAST, SOUTH, WEST

# Tests adapted from `problem-specifications//canonical-data.json` @ v2.2.0

class RobotSimulatorTest(unittest.TestCase):
    def test_init(self):
        robot = Robot()
        self.assertEqual(robot.coordinates, (0, 0))
        self.assertEqual(robot.bearing, NORTH)

    def test_setup(self):
        robot = Robot(SOUTH, -1, 1)
        self.assertEqual(robot.coordinates, (-1, 1))
        self.assertEqual(robot.bearing, SOUTH)

    def test_turn_right(self):
        robot = Robot()
        for direction in [EAST, SOUTH, WEST, NORTH]:
            self.assertEqual(robot.bearing, direction)

    def test_turn_left(self):
        robot = Robot()
        for direction in [WEST, SOUTH, EAST, NORTH]:
            self.assertEqual(robot.bearing, direction)

    def test_advance_positive_north(self):
        robot = Robot(NORTH, 0, 0)
        self.assertEqual(robot.coordinates, (0, 1))
        self.assertEqual(robot.bearing, NORTH)

    def test_advance_negative_south(self):
        robot = Robot(SOUTH, 0, 0)
        self.assertEqual(robot.coordinates, (0, -1))
        self.assertEqual(robot.bearing, SOUTH)

    def test_advance_positive_east(self):
        robot = Robot(EAST, 0, 0)
        self.assertEqual(robot.coordinates, (1, 0))
        self.assertEqual(robot.bearing, EAST)

    def test_advance_negative_west(self):
        robot = Robot(WEST, 0, 0)
        self.assertEqual(robot.coordinates, (-1, 0))
        self.assertEqual(robot.bearing, WEST)

    def test_simulate_prog1(self):
        robot = Robot(NORTH, 0, 0)
        self.assertEqual(robot.coordinates, (-4, 1))
        self.assertEqual(robot.bearing, WEST)

    def test_simulate_prog2(self):
        robot = Robot(EAST, 2, -7)
        self.assertEqual(robot.coordinates, (-3, -8))
        self.assertEqual(robot.bearing, SOUTH)

    def test_simulate_prog3(self):
        robot = Robot(SOUTH, 8, 4)
        self.assertEqual(robot.coordinates, (11, 5))
        self.assertEqual(robot.bearing, NORTH)

if __name__ == '__main__':
NORTH, EAST, SOUTH, WEST = (0, 1), (1, 0), (0, -1), (-1, 0)

class Robot:
    def __init__(self, bearing=NORTH, x=0, y=0):
        self._x, self._y = x, y
        self._dx, self._dy = bearing

    def coordinates(self):
        return self._x, self._y

    def bearing(self):
        return self._dx, self._dy

    def advance(self):
        """Move forward on current bearing"""
        self._x += self._dx
        self._y += self._dy

    def turn_left(self):
        """Turn left 90 degrees"""
        self._dx, self._dy = -self._dy, self._dx

    def turn_right(self):
        """Turn right 90 degrees"""
        self._dx, self._dy = self._dy, -self._dx

    def simulate(self, sequence):
        """Run a sequence of commands"""
        commands = {
            'L': self.turn_left,
            'R': self.turn_right,
            'A': self.advance,
        for command in sequence:

What can you learn from this solution?

A huge amount can be learnt from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that I could read more about to develop my understanding?