Week 3
Solutions to Lists, Dictionaries, Error Handling and List Slicing

Exercise 1 - Login System MKII

Here we create a new and improved login system, by making use of the different data structures we have learnt about, dictionaries and lists.
week3_solutions_ex1.py
# Store the username and password as key-value pairs 'username : password'
users = {"alex" : "1234", "joe" : "5678", "luke" : "0000"}
​
# store the bad passwords in a list
bad_passwords = ["password", "pass", "word", "1234"]
​
# store custom welcome messages for each user in users
welcome_msgs = {"alex" : "Greetings Alex",
"joe" : "Howdy Joe",
"luke" : "Good day Luke"}
​
# Add a captcha by getting the user to answer a question
answer = input("What is 4 + 21? ") # always reads a string
answer = int(answer) # convert string input to integer
​
# if captcha answer was wrong, print error message and quit program
if answer != 25:
print("You are not human!")
exit() # stops the script from being run
​
# Now get user to enter their name
input_name = input("Enter your username: ")
input_name = input_name.lower() # Convert to lowercase
​
# check if the username exists in our users-password dictionary
if input_name in users.keys():
# input_name exists so check password matches
input_password = input("Enter your password:")
if input_password == users[input_name]:
# password is correct, print custom welcome message
print(welcome_msgs[input_name])
​
# check if the password is in the bad list
# use lower() to ignore case when checking for bad passwords
if input_password.lower() in bad_passwords:
# password is bad so give the option to change it
change_password = input("Warning: Password sucks, change?(y/n):")
if change_password == "y":
# user wants to change, so get a new one
new_password = input("Enter new password:")
users[input_name] = new_password # store for that user
print("Password changed to:" + new_password)
else:
# password invalid
print("You have entered an incorrect password")
else:
# input_name is not in the users dictionary
print("Unknown username")

Extensions

The code for the extensions can be inserted into the solution above between the preceding and subsequent lines of code -- see the comments in the code for which lines these are.

Extension 1 - Incompetent User Warning

We put the integer conversion into a try except block so that we can stop the program from reporting an error when a non-integer answer is entered.
week3_solutions_ext1.py
.............
answer = input("What is 4 + 21? ") # preceding line
# input always returns a string, so we need to convert it to an integer
​
# attempt to convert the inputted string to an integer
try:
answer = int(answer)
except ValueError:
print("You are incompetent! That was not an integer!")
exit()
​
if(answer != 25): # subsequent line
.............

Extension 2 - List Slicing Fun

We take the poem as a string and split it up using the split(' ') method, splitting on spaces. Then we get every sixth word in the list using list slicing, rejoin the list together and print the result.
week3_solutions_ext2.py
poem = "hidden from all to see, trapped in an eternal night, those shadows plain surrounding me, no sunrise in sight"
​
# split the poem into a list of words
poem_list = poem.split(' ')
​
# split the list on every 6th word
new_poem_list = poem_list[::6]
​
# create a new poem from the list of words
new_poem = " " # space is our separator
new_poem = new_poem.join(new_poem_list)
print(new_poem)

Extension 3 - Forced Palindromes

We ask the user to input a word, and print out a palindrome version of the word, using list slicing by reversing the word and not including the first reversed character
week3_solutions_ext3.py
# Get a word from the user
user_input = input("Enter a word:")
​
# reverse the word and don't include the first reverse character
new_word = user_input + user_input[::-1][1:]
print(new_word)

Extension 4 - Random Captcha

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Exercise 1 - Login System MKII
Extensions
Extension 1 - Incompetent User Warning
Extension 2 - List Slicing Fun
Extension 3 - Forced Palindromes
Extension 4 - Random Captcha