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Answers

These are my answers for exercises in the chapters. If your solution isn't exactly like mine but it works just fine it's ok, and you can ask me why I didn't do it like you did it.

ThinkPython: The way of the program

  1. With +, the strings get added together, and with * we get an error.
  2. With + we get an error, and with * the string is repeated multiple times.
  3. Python calculates the result and echoes it.

If, else and elif

  1. Problems and solutions:

    • The first line should be print("Hello!") or print('Hello!'), not print(Hello!). Hello! is a piece of text, so we need to tell Python that it's a string by putting quotes around it.
    • The second line will create an error message that says that there's no variable called something. But we are trying to create it here, so we need = instead of ==. = is assigning, == is comparing.
    • The last line should have a comma between the arguments, like print('You entered:', something).

  2. The broken code has mostly the same issues as exercise 1. Here are the problems that exercise 1 doesn't have:

    • The if-elif-else has a blank line at a confusing place. Delete it.
    • After deleting the code, it looks quite dense. Add a new blank line before the if.
    • The elif line is missing a : at the end.
    • On the last line the comma is on the wrong side. "bla bla," is a string that contains a comma, but "bla bla", is a string and a separate comma. In this exercise, the last line should be print("I don't know what", something, "means.")

  3. We can simply ask the word with input and print word * 1000.

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

  4. We can add a space to the word before we print it.

    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)

  5. Like this:

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

  6. We can compare the word against an empty string ("" or '') to check if it's empty. In this example, the password is "seKr3t".

    word = input("Enter your password: ")

if word == "seKr3t":
    print("Welcome!")
elif word == "":
    print("You didn't enter anything.")
else:
    print("Access denied.")

    Again, this is not a good way to ask a real password from the user.

Handy stuff: Strings

  1. The program is not like you might expect it to be. It actually works just fine if we run it, but there's a problem. The last line is really long and it's hard to see what it does.

    The solution is string formatting. I recommend replacing the last line with this:

    print(f"You entered {word1}, {word2}, {word3} and {word4}.")

  2. If we have a look at help(str.upper) it looks like this:

     S.upper() -> str

 Return a copy of S converted to uppercase.

    We have two problems. First of all, the broken code uses message.upper instead of message.upper(). It also expects the message to magically make itself uppercased, but strings can't be changed in-place so it doesn't work.

    The solution is to do message.upper() and save the value we got from that to a variable:

    message = input("What do you want me to say? ")
uppermessage = message.upper()
print(uppermessage, "!!!")
print(uppermessage, "!!!")
print(uppermessage, "!!!")

    Or we can reuse the same variable name:

    message = input("What do you want me to say? ")
message = message.upper()
print(message, "!!!")
print(message, "!!!")
print(message, "!!!")

    Or we can convert the message to uppercase right away on the first line:

    message = input("What do you want me to say? ").upper()
print(message, "!!!")
print(message, "!!!")
print(message, "!!!")

  3. In the code below, palindrome_input[::-1] is the string palindrome_input reversed. For example, if palindrome_input is "hello", then palindrome_input[::-1] is "olleh".

    palindrome_input = input("Enter a string: ")
if palindrome_input == palindrome_input[::-1]:
    print("This string is a palindrome")
else:
    print("This string is not a palindrome")

Lists and tuples

  1. Look carefully. The namelist is written in () instead of [], so it's actually a tuple, not a list. Using confusing variable names is of course a bad idea, but you shouldn't be surprised if someone is doing that. Replace the () with [] and the code will work.

  2. When we run the program we get a weird error:

     Hello!
 Enter your name: my name
 Traceback (most recent call last):
   File "program.py", line 3, in <module>
     print("Your name is " + name + ".")
 TypeError: Can't convert 'tuple' object to str implicitly

    So Python is trying to convert a tuple to a string. But "Your name is " and "." are strings, so maybe name is a tuple? Let's change the last line to just print(name) so our program looks like this:

    print("Hello!")
name = input("Enter your name: "),
print(name)

    Let's run it.

     Hello!
 Enter your name: my name
 ('my name',)

    name is indeed a tuple! The problem is the second line. Look carefully, there's a comma after input("Enter your name: "). Python created a tuple automatically, but that's not what we wanted. If we remove the comma, everything works just fine.

  3. Again, the code gives us a weird error message.

     Enter your name: my name
 Traceback (most recent call last):
   File "program.py", line 3, in <module>
     if input("Enter your name: ") in namelist:
 TypeError: argument of type 'NoneType' is not iterable

    Now we need to remember that when the error messages talk about NoneType they always mean None. So namelist seems to be None. Let's make the program a bit simpler for working on the namelist:

    namelist = ['wub_wub', 'RubyPinch', 'go|dfish', 'Nitori']
namelist = namelist.extend('theelous3')
print(namelist)

    Now fixing the namelist is easier, so I'll just go through the problems and solutions:

    • namelist is None. It should be namelist.extend('theelous3'), not namelist = namelist.extend('theelous3'). See this thing.
    • Now the namelist is like ['wub_wub', ..., 't', 'h', 'e', 'e', ...]. Python treated 'theelous3' like a list so it added each of its characters to namelist. We can use namelist.append('theelous3') or namelist.extend(['theelous3']) instead to solve this problem.

Loops

  1. The problem is that things is a string because we converted it to a string with str, so the for loop loops over the characters [, 1, , and so on. Replace str([1, 2, 3, 4, 5]) with [1, 2, 3, 4, 5].

  2. The code appends each list in before to after, so the number variable actually pointed to a list like [1, 2]. An easy solution is to just write two for loops inside each other:

    before = [[1, 2], [3, 4], [5, 6]]
after = []
for sublist in before:
    for number in sublist:
        after.append(number)
print(after)

    Lists also have an extend method that appends each item from another list, so we can also use that:

    before = [[1, 2], [3, 4], [5, 6]]
after = []
for sublist in before:
    after.extend(sublist)
print(after)

  3. The code has some empty lines in it, and they divide it nicely into three parts. All of these parts have some problems, so I'll go through them one by one.

    The first part makes a variable called input. The problem is that now the rest of the program can't use the input function. It doesn't really matter here because the rest of the program doesn't use it anyway, but I still recommend using some other variable name, like inputlist.

    The second part runs numbers = [] three times. It was probably meant to be ran once before the loop started, like this:

    numbers = []
for string in inputlist:
    numbers.append(int(string))

    The third part calculates result + n but throws away the value. It was probably supposed to do result += n instead.

  4. If you run this program you'll notice that nothing happened to the numbers list. The reason is that the number variable only works one way. It gets its values from the numbers list, but changing it doesn't change the numbers list. In general, thing = stuff changes the thing variable, and that's it. It doesn't change anything else.

    An easy solution is to just create a new list:

    numbers = ['1', '2', '3']
converted_numbers = []
for number in numbers:
    converted_numbers.append(int(number))
print(converted_numbers)
  5. row_count = int(input("Type the number of rows needed:")) 
for column_count in range(1, row_count+1):
    # Print numbers from 1 to column_count
    for number in range(1, column_count+1): 
        print(number, end=" ") 
    print()  # creates a new line for the next row

    If the user enters 5, we want to do a row with 1 column, then 2 columns, and so on until 5 columns. That would be for column_count in range(1, 6), because the end of the range is excluded. In general, we need to specify row_count + 1 so that it actually ends at row_count. The second loop is similar.

    Usually print(number) puts a newline character at the end of the line, so that the next print goes to the next line. To get all numbers on the same line, we use a space instead of a newline character, but we still need print() to add a newline character once we have printed the entire row.

  6. row_count=int(input("Type the number of rows needed:"))
 
for line_number in range(1, row_count+1):
    for number in range(line_number, row_count+1):
        print(number, end=' ')
    print()

    Just like in the previous exercise, if the user enters 5, the first for loop gives the line numbers 1, 2, 3, 4, 5.
    For example, on line 2, we should print numbers from 2 to 5, as in range(2, 6), or in general, range(line_number, row_count+1).

Trey Hunner: zip and enumerate

  1. Read some lines with input into a list and then enumerate it.

    print("Enter something, and press Enter without typing anything",
      "when you're done.")

lines = []
while True:
    line = input('>')
    if line == '':
        break
    lines.append(line)

for index, line in enumerate(lines, start=1):
    print("Line", index, "is:", line)

  2. Let's start by trying out zip with strings:

    >>> for pair in zip('ABC', 'abc'):
...     print(pair)
...
('A', 'a')
('B', 'b')
('C', 'c')
>>>

    Great, it works just like it works with lists. Now let's create the letter printing program:

    uppercase = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
lowercase = 'abcdefghijklmnopqrstuvwxyz'

for upper, lower in zip(uppercase, lowercase):
    print(upper, lower)

  3. This one is a bit more difficult than the other two because we need to combine zip and enumerate. One way to do this is to pass a zip result to enumerate, like this:

    uppercase = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
lowercase = 'abcdefghijklmnopqrstuvwxyz'

for index, letterpair in enumerate(zip(uppercase, lowercase), start=1):
    upper, lower = letterpair
    print(index, upper, lower)

    We can also save the zip result to a variable. I would probably do this.

    uppercase = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
lowercase = 'abcdefghijklmnopqrstuvwxyz'

letterzip = zip(uppercase, lowercase)
for index, letterpair in enumerate(letterzip, start=1):
    upper, lower = letterpair
    print(index, upper, lower)

    Another alternative is to pass an enumerate result to zip. This is a bit more complicated, so I wouldn't do it this way.

    uppercase = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
lowercase = 'abcdefghijklmnopqrstuvwxyz'

for upper, indexlowerpair in zip(uppercase, enumerate(lowercase, start=1)):
    index, lower = indexlowerpair
    print(index, upper, lower)

Defining functions

  1. The problem with the first example is that name is a local variable. I explained how to fix this in the output section:

    def ask_name():
    name = input("Enter your name: ")
    return name

print("Your name is", ask_name())

  2. If you run the next example, you get something like this:

     <function get_greeting at 0xb73a0a04>

    The problem is that we print the actual get_greeting function, but we need to call it like get_greeting():

    def get_greeting():
    return "Hello World!"

print(get_greeting())

  3. See the return or print section.

    The greet function prints a greeting.

    >>> greet("World")
Hello World
>>>

    But it also returns None because we don't tell it to return anything else.

    >>> return_value = greet("World")
Hello World
>>> print(return_value)
None
>>>

    This code from the exercise does the same thing as the code above does, but without a temporary return_value variable:

    >>> print(greet("World"))
Hello World
None
>>>

If you have trouble with this tutorial, please tell me about it and I'll make this tutorial better, or ask for help online. If you like this tutorial, please give it a star.

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

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