"""
This module demonstrates why MUTATION is DANGEROUS:
Example 1:
-- A function has a error (it mutates a parameter when it shouldn't).
-- That error does not become apparent in testing the function,
but the error messes up ANOTHER function that CALLS the first one.
Example 2:
-- The author constructs a BankAccount with $1,000 savings.
-- She shows her checking and savings balances.
Surely that is safe!
-- But unbeknownst to her, evil David has put nasty code into the
show_balances method, that steals the rest of her savings!
-- So her money is now gone!
Authors: David Mutchler, Vibha Alangar, Dave Fisher, Matt Boutell, Mark Hays,
Mohammed Noureddine, Sana Ebrahimi, Sriram Mohan, and their colleagues.
"""
import rosegraphics as rg
import random
# -----------------------------------------------------------------------------
# Students: Your instructor may ask you to read and run this program, or
# your instructor may use a different way to show why mutation is dangerous.
#
# Do you see how MUTATION made the nasty stuff possible?
# If objects were immutable, the show_balances method simply
# could NOT steal her money!
# -----------------------------------------------------------------------------
def main():
one_example_of_danger()
another_example_of_danger()
def one_example_of_danger():
# -------------------------------------------------------------------------
# Students:
# 1. Read the move_randomly specification below.
# 2. Run run_test_move_randomly.
# 3. Does the code pass the test? (Answer: Yes.)
#
# Then:
# 1. Read the repeat_move_randomly specification below.
# 2. Run run_test_repeat_move_randomly.
# 3. Does the code pass the test? (Answer: No.)
#
# So, you would think that the REPEAT_move_randomly code
# has an error. But look at that code, and you will see that
# it definitely is OK!
#
# Can you figure out what went wrong?
# Once you do so, do you see why mutation is dangerous?
# -------------------------------------------------------------------------
# Un-comment these as you do the above exercise.
run_test_move_randomly()
run_test_repeat_move_randomly()
def run_test_move_randomly():
""" Tests the move_randomly function. """
window = rg.RoseWindow(800, 500)
circle1 = rg.Circle(rg.Point(400, 250), 50)
circle1.fill_color = 'blue'
circle1.attach_to(window)
move_randomly(window, circle1, 50, 0.05)
circle2 = rg.Circle(rg.Point(200, 100), 30)
circle2.fill_color = 'red'
circle2.attach_to(window)
move_randomly(window, circle2, 250, 0.01)
window.close_on_mouse_click()
def move_randomly(window, circle, times_to_move, seconds_per_move):
"""
Repeatedly moves a copy of the given circle a small random distance
on the given window. Does this the given number of times, rendering
and pausing for the given number of seconds after each move.
Includes a small 'drift' to the right.
Preconditions:
:type window: rg.RoseWindow
:type circle: rg.Circle
:type times_to_move: int
:type seconds_per_move: float | int
where the times_to_move and seconds_per_move are non-negative
and where the circle is already attached to the given window.
"""
# -------------------------------------------------------------------------
# Students: The following implementation of the above specification
# has an error.
# 1. Do the tests expose the error?
# 2. Can you spot the error?
# -------------------------------------------------------------------------
for _ in range(times_to_move):
dx = random.randrange(-5, 7)
dy = random.randrange(-5, 6)
circle.move_by(dx, dy)
window.render(seconds_per_move)
def run_test_repeat_move_randomly():
""" Tests the repeat_move_randomly function. """
window = rg.RoseWindow(800, 500)
circle = rg.Circle(rg.Point(400, 250), 50)
circle.fill_color = 'green'
circle.attach_to(window)
repeat_move_randomly(5, circle, window)
window.close_on_mouse_click()
def repeat_move_randomly(n, circle, window):
"""
Runs move_randomly n times using the given circle and window,
each time making 200 random moves with 0 seconds pause after each.
Waits for a mouse click after each of the n trials.
Preconditions:
:type n: int
:type circle: rg.Circle
:type window: rg.RoseWindow
where n is non-negative and the circle is already attached
to the given window.
"""
for _ in range(n):
move_randomly(window, circle, 200, 0)
window.continue_on_mouse_click()
def another_example_of_danger():
# -------------------------------------------------------------------------
# Students:
# 1. Read the code in this function.
# -- Do NOT read the code of the BankAccount class.
# Imagine that you are a USER, not an AUTHOR,
# of the BankAccount class.
# 2. Predict what the last line of this function will print.
# 3. Run the function.
# -- Does the function print what you expected? (Answer: No.)
#
# Can you figure out what went wrong?
# Once you do so, do you see why mutation is dangerous?
# -------------------------------------------------------------------------
my_money = BankAccount(1000, 300) # $1,000 in savings. $300 in checking
print('Savings, checking:', my_money.savings, my_money.checking)
my_money.pay_for_college(100) # Uses $100 of my savings as tuition
my_money.show_balances()
my_money.pay_for_college(100) # Should have plenty left, but ...!
my_money.show_balances()
class BankAccount(object):
def __init__(self, savings, checking):
self.savings = savings
self.checking = checking
def show_balances(self):
print('Your savings balance is: ', self.savings)
print('Your checking balance is: ', self.checking)
# ---------------------------------------------------------------------
# The following line is an "error" inserted by evil David!
# One would NOT expect a SHOW_balances method to mutate
# the object, but there is nothing to prevent that.
# Hence, the danger.
# ---------------------------------------------------------------------
self.give_money_to_david()
def pay_for_college(self, tuition):
print()
message = 'Using {} of your savings to pay for college.'
print(message.format(tuition))
if self.savings >= tuition:
self.savings = self.savings - tuition
else:
print('Yikes! Your savings is not enough! Where did it go?!')
def give_money_to_david(self):
# Imagine that the money sneaks into David's account...
self.savings = 0
# -----------------------------------------------------------------------------
# Calls main to start the ball rolling.
# -----------------------------------------------------------------------------
main()