These are answers for exercises in the chapters. In programming, there's always more than one way to do things, so if your solution wasn't exactly like mine it's not necessarily wrong. Some Python users say that there should be only one right way, but that goal will never be fully reached.

ThinkPython: The way of the program

1. The strings get added together.
2. We get an error.
3. We get a floating point number.

Using if, else and elif

1. Just ask the word and print word * 1000.

   word = input("Enter a word: ")
   print(word * 1000)

2. Add a space to the word before printing.

   word = input("Enter a word: ")
   word += " "
   print(word * 1000)

   We can also add the space right away on the first line:

   word = input("Enter a word: ") + " "
   print(word * 1000)

   Of course, there are 999 spaces between 1000 words and this will print 1000 spaces instead, so there will be a useless space at the end, but it doesn't matter. If we really want to get rid of the space, we can do something like this instead:

   no_space = input("Enter a word: ")
   yes_space = no_space + " "
   print(yes_space * 999 + no_space)

3. Like this:

   first = input("Enter a word: ")
   second = input("Enter another word: ")
   words = first + " " + second + " "
   print(words * 1000)

4. You can compare the word against an empty string ("" or '') to check if it's empty. In this example, the password is "secret".

   word = input("Enter your password: ")
   
   if word == "secret":
       print("Welcome!")
   elif word == "":
       print("You didn't enter anything.")
   else:
       print("Access denied.")

5. Simply check the username first, then the password in indented blocks of code.

   username = input("Enter your username: ")
   password = input("Enter your password: ")
   
   if username == "foo":
       if password == "biz":
           print("Welcome foo!")
       else:
           print("Wrong password!")
   elif username == "bar":
       if password == "baz":
           print("Welcome bar!")
       else:
           print("Wrong password!")
   else:
       print("Wrong username.")

   Example output:

   >>> ================================ RESTART ================================
   >>> 
   Enter your username: foo
   Enter your password: biz
   Welcome foo!
   >>> ================================ RESTART ================================
   >>> 
   Enter your username: bar
   Enter your password: baz
   Welcome bar!
   >>> ================================ RESTART ================================
   >>> 
   Enter your username: spam
   Enter your password: eggs
   Wrong username.
   >>> ================================ RESTART ================================
   >>> 
   Enter your username: foo
   Enter your password: lol
   Wrong password!
   >>> 
   

Loops

1. For loop over the users, each user is a list that contains a username and a password.

   users = [
       ['foo', 'biz'],
       ['bar', 'baz'],
   ]
   
   username = input("Username: ")
   password = input("Password: ")
   
   logged_in = False
   for user in users:
       if username == user[0] and password == user[1]:
           logged_in = True
           break
   
   if logged_in:
       print("Welcome, " + username + "!")
   else:
       print("Wrong username or password.")

2. Just put the whole thing in a while True:. Remember that a break will always break the innermost loop it's in.

   users = [
       ['foo', 'biz'],
       ['bar', 'baz'],
   ]
   
   while True:
       username = input("Username: ")
       password = input("Password: ")
   
       logged_in = False
       for user in users:
           if username == user[0] and password == user[1]:
               logged_in = True
               break   # break the for loop
   
       if logged_in:
           print("Welcome, " + username + "!")
           break   # break the while loop
       else:
           print("Wrong username or password.")

3. Add a for loop that works as an attempt counter.

   users = [
       ['foo', 'biz'],
       ['bar', 'baz'],
   ]
   
   for attempts_left in [3, 2, 1, 0]:
       if attempts_left == 0:
           print("No attempts left!")
           break   # break the outer for loop
   
       print(attempts_left, "attempts left.")
       username = input("Username: ")
       password = input("Password: ")
   
       logged_in = False
       for user in users:
           if username == user[0] and password == user[1]:
               logged_in = True
               break   # break the inner for loop
   
       if logged_in:
           print("Welcome, " + username + "!")
           break   # break the outer for loop
       else:
           print("Wrong username or password.")

You may use this tutorial freely at your own risk. See LICENSE.

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