Module 3: Python Conditionals

Topics

• Review of boolean expressions
• If-statements

A Review of Boolean Expressions

In order to fully understand conditional logic, it’s important to have a solid grasp of boolean expressions in python. Remember, a boolean expression is a statement or a part of a statement that results in a Boolean value, such as True or False. Comparisons are the most obvious and helpful example of boolean expressions:

>>> "rainbow rockstar" == "mermaid unicorn"
False

This expression, which is the comparison of two strings, results in the Boolean value False. We can also compare two numbers. Read the following boolean expressions:

>>> 5 == 5
True
>>> 4 > 1
True
>>> 7 > 99
False

Also, remember that True and False are values in their own right. If we wanted to simply make a variable whose value is True, we could:

>>> my_var = True
>>> print my_var
True

Also, reaching way back to the first module, the string "True" is not the same as the Boolean value True. They are two values that look the same, but are different types.

As a fun aside, we could prove this using a Boolean expression:

>>> type("True") == type(True)
False
>>> "True" == True
False
>>> "True" != True
True

Conditional Logic

We’re about to add a very useful tool to our toolkit. Conditional logic lets us set conditions, or prerequisites, in our code, such that only certain lines of code are run if certain conditions are met. This is similar to allowing yourself to go to the movies if you finish this module of work. Some “psedocode”, or “fake” computer code, for you day might look like this:

work on module
if module is finished:
    go to the movies, have lots of fun!
otherwise:
    stay home and keep working.

In our pseudocode, the line that says if module is finished is called an if-statement. Python if-statements look pretty similar to our pseudocode. Here is some valid Python code that does a similar thing:

module = "incomplete"
if module == "complete":
    print "Time to go to the movies"
else:
    print "Stay home and keep working."

Try typing this out in your Python repl console on your own. What is the output of this code?

>>> module = "incomplete"
>>> if module == "complete":
...     print "Time to go to the movies"
... else:
...     print "Stay home and keep working."
...
Stay home and keep working.

If-Statement Syntax

if family_member1 == family_member2:
    print "You have two family members with the same name."

There are a couple important things to note about the correct way to craft an if-statement.

1. if is another Python keyword. It must be lower-cased.
2. After the if is a Boolean expression followed by a colon– :.
3. Next comes the “body” of the if-statement– the thing to do if the Boolean expression evaluates to True.

if family_member1 == family_member2:
    print "You have two family members with the same name."
    print "Here is family member 1: ", family_member1
    print "Here is family member 2: ", family_member2

4. Optionally, an if-statement can have an else block– what to do if the Boolean expression does not evaluate to True. Here’s another example of an else-block. The “body” of the else-block should also be indented.

if message_recipient == "Balloonicorn":
    print "Balloonicorn, you have a message."
else:
    print "No messages for you, Balloonicorn!"

Using elif

If there are multiple successive conditions you’d like to check, you can utilize the elif keyword. Here’s an example:

if message_recipient == "Balloonicorn":
    print "Balloonicorn, you have a message."
elif message_recipient == "Hackbright":
    print "Ballonicorn, can you take a message for Hackbright?"
else:
    print "No messages for you, Balloonicorn!"

The elif keyword is a way to evaluate an additional condition, after the first has been checked.

It’s important to note that if the first if-statement evaluates to True, neither the elif, nor the else, will ever occur. With just an if and an else, it’s either one or the other. With an if, elif, and and else, it’s only one of the 3 conditions that can occur.

The conditions are checked in the order that they appear: first the if, then the elif, then, if neither the if nor the elif worked out, the else handles it.

Nested If-Statements

If-statements can be nested. That is, another if-statement can be included in the body of an if-statement. Here is an example:

num_pets = 5
fav_animal = "cat"

if num_pets > 5:
    print "Wow, that's a lot of pets!"
    if fav_animal == "cat":
        print "I like cats too!"

Nested if-statements can also have elif blocks and else blocks. To complicate the example above a bit:

num_pets = 5
fav_animal = "cat"

if num_pets > 5:
    print "Wow, that's a lot of pets!"
    if fav_animal == "cat":
        print "I like cats too!"
    else:
        print "Why don't you like cats??"
elif num_pets == 2:
    print "I hope your two pets are friends."
elif num_pets == 0:
    print "Time to get a pet."

Using raw_input with if-statements

So far, our if-statements have been a little bit contrived. What’s the point of checking to see if a variable is equal to another thing if you made the variable yourself, right? If statements are typically used when the result of that Boolean expression is unknown, or varying over time or depending on some input that was provided to a program.

As luck would have it, we do have a function that can take in input into our programs– raw_input!

Using these two concepts, we can make a little advice machine:

>>> print "Would you like some advice?"
>>> print "If yes, type Y."
>>> the_answer = raw_input("What's your answer? ")
>>> if the_answer == "Y":
...     print "A wise person knows when to ask for help."
... else:
...     print "Ok, I won't give you any advice then."

Open a repl Python console and type this out. Remember when this line runs:

>>> the_answer = raw_input("What's your answer? ")

You’ll have to provide the answer yourself. Try it out with two different inputs. So, the first time, type Y in response to What's your answer?, and then type something other than Y.

Directions

Login to your account and start a new repl here. Title your new session module_3_practice.py.

Complete the practice problems below in this repl console. If you’d like to work through the practice in several sittings be sure to log in to Repl.it and save your work in between.

1. Setup: Type the following into your repl Python console.

adjective = "absolutely fabulous"
adjective2 = "supercalifragilisticexpialidocious"
noun = "aardvarks"
noun2 = "billy goats"
verb = "lollygagging"
verb2 = "jogging"

2. Write an if-statement for each of the above variables. The if-statement should check whether the length of the variable’s value is greater than 9 characters. If it is, your code should print “long string”. If it’s not, print “not a long string”.

   Here’s a hint:

   Here’s the first part. It accomplishes half of the problem. However, it doesn’t accomplish printing “not a long string” when the length is not greater than 9 characters.

   if len(adjective) > 9:
      print "long string"
   

3. Using the raw_input function, prompt yourself to type something. Be sure to capture what you type into a variable. Once you have the variable, print the length of whatever you typed.

4. Type the following into the repl Python console:

   >>> answer = raw_input("What is 2 + 2 ?")
   >>> answer_as_integer = int(answer)
   

   Then, write some code that follows these specifications:

   • If the user is correct, tell them they are correct.
   • If the user is incorrect, tell them whether their answer is too high or too low.

   Do this a couple of times, making sure that if you type 1) the wrong answer 2) a “too high” answer and 3) a “too low” answer all show the correct output.

5. Write a conditional statement that checks if the variables verb and verb2 are equal to one another. If they are, print “They are equal!”. If they aren’t, print 3 things: the value of verb, the value of verb2, and the message “These are not the same”

6. Type the following code to prompt the user to rate the movie Wizard of Oz:

   >>> rating = raw_input("On a 1-10 scale, how would you rate Wizard of Oz?")
   >>> rating_as_integer = int(rating)
   

   Then, write an if/elif/else statement that follows these specifications:

   • If they rate less than 5, tell the user “Wow, you hated it!”
   • If they rate less than 7, tell the user “You are meh about this movie.”
   • If they rate more than 7, tell the user “You loved it. There’s no place like home.”

Directions

As a programmer, debugging is a fact of life. There are times you write code that Python doesn’t understand. In these cases, Python will display an error message. The more familiar you are with Python’s many error messages, the faster you’ll be at debugging code. But there’s good news: Python’s error messages are incredibly descriptive and helpful in figuring out what the problem is.

In the following problems, you’ll find code that is invalid or not allowed in some way. Read the code, and see if you can predict what is wrong. When you’re ready, hover over the solution area to reveal the error message that Python shows, along with an explanation of what is going wrong.

1) What’s wrong with this code?

>>> rating = 5
>>> if rating = 5:
...     print "Right in the middle."

>>> if rating = 5:
  File "<stdin>", line 1
    if rating = 5:
              ^
SyntaxError: invalid syntax

2. What’s wrong with this code?

rating = 5
if rating == 5:
print "You rated a 5!"

>>> if rating == 5:
... print "You rated a 5!"
  File "<stdin>", line 2
    print "You rated a 5!"
        ^
IndentationError: expected an indented block

Videos

Let’s Learn Python: Conditionals

Socratica: Python If, Then, Else

Articles

Wikibooks on Conditionals in Python

Python 2 Conditionals Tutorial