Python Basics II
Description: This lesson describes the basics of flow control statements including:
and the basics of writing functions:
This is part 2 of 3 in the series Python Basics that will prepare you to do text analysis using the Python programming language.
Use Case: For Learners (Detailed explanation, not ideal for researchers)
Completion Time: 90 minutes
Knowledge Recommended: None
Data Format: None
random to generate random numbers
Research Pipeline: None
In Python Basics I, you learned about expressions, operators, variables, and a few native Python functions. We wrote programs that executed line-by-line, starting at the top and running to the bottom. This approach works great for simple programs that may execute a few tasks, but as you begin writing programs that can do multiple tasks you'll need a way for your programs to decide which action comes next. We can control when (or if) code gets executed with flow control statements. If a program is a set of steps for accomplishing a task, then flow control statements help the program decide the next action.
Flow control statements work like a flowchart. For example, let's say your goal is to hang out and relax with friends. There are a number of steps you might take, depending on whether your friends are available or you feel like making some new friends.
Each diamond in our flowchart represents a decision that has to be made about the best step to take next. This is the essence of flow control statements. They help a program decide what the next step should be given the current circumstances.
One way we to create flow control statements is with boolean values that have two possible values: True or False. In our example above, we could consider a "Yes" to be "True" and a "No" to be "False." When we have the data we need to answer each question, we could store that answer in a variable, like:
are_friends_available = False
make_new_friends = True
new_friend_available = True
# Note, the first letter of a boolean value must always be capitalized in Python are_friends_available = false print(are_friends_available)
# The boolean values **True** and **False** cannot be used for variable names. # Treating the boolean value True as a variable will create an error True = 7
Now that we have a way to store integers, floats, strings, and boolean values in variables, we can use a comparison operator to help make decisions based on those values. We used the comparison operator
== in Python Basics I. This operator asks whether two expressions are equal to each other.
# Comparing two values with the comparison operator == 67 == 67
# Note, a comparison operator uses == # Do not confuse with variable assignment statement which uses = 67 = 67
# Using the "Not equal to" operator 67 != 32
# Using the "Not equal to" operator 67 != 67
# Using the "equal to" operator with strings 'hello world' == 'hello world'
# Using the "equal to" operator to compare a string with an integer '55' == 55 # A string cannot be equal to a float or an integer
# Using the "equal to" operator to compare an integer with a float 55 == 55.0 # An integer can be equal to a float
# Using a comparison operator on a variable number_of_dogs = 0 # Creating a variable number_of_dogs number_of_dogs >= 1 # Checking whether number_of_dogs is greater than or equal to 1
# If condition one is True AND condition two is True # What will the evaluation be? True and True
# If condition one is True AND condition two is False # What will the evaluation be? True and False
In order for an
and expression to evaluate to True, every condition must be True. Here is the "Truth Table" for every pair:
|True and True||True|
|True and False||False|
|False and True||False|
|False and False||False|
and expressions require all conditions to be True, they can easily result in False evaluations.
or operator determines whether any condition is True.
# Is expression one True OR is expression two True? True or False
# Is condition one True OR is condition two True? False or False
or expression evaluates to True if any condition is True. Here is the "Truth Table" for every pair:
|True or True||True|
|True or False||True|
|False or True||True|
|False or False||False|
or expressions only require a single condition to be True, they can easily result in True evaluations.
not operator only operates on a single expression, essentially flipping True to False or False to True.
# The not operator flips a True to False not False
# Evaluating two conditions with integers at once (3 < 22) and (60 == 34) # What does each condition evaluate to?
# Evaluating two conditions with integers at once (3 == 45) or (3 != 7) # What does each condition evaluate to?
So far, we have evaluated one or two conditions at once, but we could compare even more at once. (In practice, this is rare since it creates code that can be difficult to read.)
# Evaluating four conditions at once (3 < 7) and ('Hello' != 'Goodbye') and (17 == 17.000) and (2 + 2 != 4) # What does each condition evaluate to?
In this condition:
perform this action
Let's return to part of our flowchart for hanging out with friends.
We can imagine a flow control statement that would look something like:
if have_homework == True:
The condition is given followed by a colon (:). The action clause then follows on the next line, indented into a code block.
A code block is a snippet of code that begins with an indentation. A code block can be a single line or many lines long. Blocks can contain other blocks forming a hierarchal structure. In such a case, the second block is indented an additional degree. Any given block ends when the number of indentations in the current line is less than the number that started the block.
|Statement||Means||Condition for execution|
||if||if the condition is fulfilled|
||else if||if no previous conditions were met and this condition is met|
||else||if no condition is met (no condition is supplied for an
||while||while condition is true|
||for||execute in a loop for this many times|
||try||try this and run the
Let's take a look at each of these flow control statement types.
if statement begins with an expression that evaluates to True or False.
In practice, the form looks like this:
if this is True:
perform this action
Let's put an
if statement into practice with a very simple program that asks the user how their day is going and then responds. We can visualize the flow of the program in a flowchart.
Our program will use a single
if statement. If the user types "Yes" or "yes", then our program will send a response.
# A program that responds to a user having a good day print('Are you having a good day? (Yes or No)') # Ask user if they are having a good day having_good_day = input() # Define a variable having_good_day to hold the user's input in a string if having_good_day == 'Yes' or having_good_day == 'yes': # If the user has input the string 'Yes' or 'yes' print('Glad to hear your day is going well!') # Print: Glad to hear your day is going well!
Our program works fairly well so long as the user inputs 'Yes' or 'yes'. If they type 'no' or something else, it simply ends. If we want to have our program still respond, we can use an
else statement does not require a condition to evaluate to True or False. It simply executes when none of the previous conditions are met. The form looks like this:
perform this action
Our updated flowchart now contains a second branch for our program.
# A program that responds to whether the user is having a good or bad day print('Are you having a good day? (Yes or No)') # Ask user if they are having a good day having_good_day = input() # Define a variable having_good_day to hold the user's input if having_good_day == 'Yes' or having_good_day == 'yes': # If the user has input the string 'Yes' or 'yes' print('Glad to hear your day is going well!') # Print: Glad to hear your day is going well! else: # Execute this if none of the other branches executes print('I wish your day was going better.') # Note that we can use double quotations in our string because it begins and ends with single quotes
Our new program is more robust. The new
else statement still gives the user a response if they do not respond "Yes" or "yes". But what if we wanted to add an option for when a user says "No"? Or when a user inputs something besides "Yes" or "No"? We could use a series of
elif statement, short for "else if," allows us to create a list of possible conditions where one (and only one) action will be executed.
elif statements come after an initial
if statement and before an
if condition A is True:
perform action A
elif condition B is True:
perform action B
elif condition C is True:
perform action C
elif condition D is True:
perform action D
perform action E
For example, we could add an
elif statement to our program so it responds to both "Yes" and "No" with unique answers. We could then add an
else statement that responds to any user input that is not "Yes" or "No".
# A program that responds to whether the user is having a good or bad day print('Are you having a good day? (Yes or No)') # Ask user if they are having a good day having_good_day = input() # Define a variable having_good_day to hold the user's input if having_good_day == 'Yes' or having_good_day == 'yes': # If the user has input the string 'Yes' or 'yes' print('Glad to hear your day is going well!') # Print: Glad to hear your day is going well! elif having_good_day == 'No' or having_good_day == 'no': # else if the user has input the string 'No' print('Sorry to hear that. I hope it gets better.') # Print: Sorry to hear that. I hope it gets better. else: # Execute this if none of the other branches executes print('Sorry, I only understand "Yes" or "No"') # Note that we can use double quotations in our string because it begins and ends with single quotes
elif condition is met, all other
elif statements are skipped over. This means that one (and only one) flow control statement is executed when using
elif statements. The fact that only one
elif statement is executed is important because it may be possible for multiple flow control statements to evaluate to True. A series of
elif statements evaluates from top-to-bottom, only executing the first
elif statement whose condition evaluates to True. The rest of the
elif statements are skipped over (whether they are True or False).
In practice, a set of mutually exclusive
if statements will result in the same actions as an
if statement followed by
elif statements. There are a few good reasons, however, to use
elifstatements helps someone reading your code understand that a single flow control choice is being made.
elifstatements will make your program run faster since other conditional statements are skipped after the first evaluates to True. Otherwise, every
ifstatement will be evaluated.
ifstatements can be very complex.
Expanding on the concept of our "How is your day going?" program, let's take a look at an example that asks the user "How is your week going?" It will take two inputs: the day of the week (
day_of_week) and how the user feels the week is going (
# A program that responds to the user's input for the day of the week and how their week is going. print('What day of the week is it?') day_of_week = input() print('Are you having a good week?') having_good_week = input() if day_of_week == 'Friday' or day_of_week == 'friday': print('Enjoy the weekend!') if having_good_week == 'Yes' or having_good_week == 'yes': print('I hope the rest of the week is good too!') if having_good_week == 'No' or having_good_week == 'no': print('Sorry to hear that. I hope the rest of the week is better.') else: print('Sorry, I only understand "Yes" or "No"')
In the program above, try changing the
elif statements to
if statements. What happens if the user inputs 'Friday' and 'Yes'?
So far, we have used flow control statements like decision-making branches to decide what action should be taken next. Sometimes, however, we want a particular action to loop (or repeat) until some condition is met. We can accomplish this with a
while loop statement that takes the form:
take this action
After the code block is executed, the program loops back to check and see if the
while loop condition has changed from True to False. The code block stops looping when the condition becomes False.
In the following program, the user will guess a number until they get it correct.
# A program that asks the user to guess a number secret_number = 4 # The secret number is set here by the programmer. print('I am thinking of a number between 1-10. Can you guess it?') # Ask the user to make a guess guess = input() # Take the user's first guess while guess != str(secret_number): # While the users guess does not equal secret_number print('Nope. Guess again!') # Print "Nope. Guess Again!" guess = input() # Allow the user to change the value of guess print('You guessed the secret number, ' + str(secret_number)) # Print a congratulations message with the secret number
When using a
while loop, it is possible to accidentally create an infinite loop that never ends. This happens because the
while condition never becomes False.
If you accidentally write code that infinitely repeats, you can stop the execution by selecting Interrupt from the Kernel menu. (Alternatively, you can press the letter i twice on your keyboard.) You may also want to remove the output of the rogue cell. You can do this from the Cell menu.
# An infinite loop while True: print('Oh noes!')
# A simple program that prints out 1, 2, 3 number = 0 while number < 3: number = number + 1 # We can also write an equivalent shortcut: number += 1 print(number)
forLoop Statements with a
An abbreviated way to write a
while loop that repeats a specified number of times is using a
for loop with a
range() function. This loop takes the form:
for i in range(j):
take this action
The starting value of
i is 0. After each loop,
i increases by one until it reaches
j. The loop then stops. The variable names
j are merely conventions. Using a different name may make the purpose of your code clearer to readers.
# A `for` loop that repeats 'What?' five times. Changing the `range()` number will change the number of repetitions. for i in range(3): print('What?')
# A `for` loop that prints the value of the current iteration, here called `i`. for i in range(5): print(i)
# A `for` loop that starts looping at 27 and stops looping at 32 for i in range(27,32): print(i)
# A `for` loop that starts looping at -13 and stops looping at 7 for i in range(-3, 7): print(i)
We can also specify the size of each step for the
range() function. In the above examples, the function adds one for each increment step, but we can add larger numbers or even specify negative numbers to have the
range() function count down. The general form is:
for i in range(start, stop, increment):
loop this action
# A `for` loop that counts down from ten to one, followed by printing `Go!` for i in range(10, 0, -1): print(i) print('Go!')
while loops and
for loops can also use
break statements to affect flow control.
continuestatement immediately restarts the loop.
breakstatement immediately exits the loop.
Let's return to our secret number guessing program. We will write the same program workflow using
# A program that asks the user to guess a number guess = 0 secret_number = str(4) # The secret number is set here by the programmer. Notice it is turned into a string so it can be easily compared with user inputs. print('I am thinking of a number between 1-10.') # Ask the user to make a guess while True: print('What is your guess?') guess = input() if guess == secret_number: break else: continue print('You guessed the secret number, ' + secret_number) # Print a congratulations message with the secret number
When running code that may create an error, we can use
except statements to stop a program from crashing.
# Try running the first code block, if there's an error then run the `except` code # Divide 100 by a number the user chooses print('100 divided by...') user_number = input() try: print(100 / int(user_number)) except: print("You can't divide by zero.")
# Dividing by zero causes an error 100 / 0
We have used several Python functions already, including
range(). You can identify a function by the fact that it ends with a set of parentheses
() where arguments can be passed into the function. Depending on the function (and your goals for using it), a function may accept no arguments, a single argument, or many arguments. For example, when we use the
print() function, a string (or a variable containing a string) is passed as an argument.
Functions are a convenient shorthand, like a mini-program, that makes our code more modular. We don't need to know all the details of how the
print() function works in order to use it. Functions are sometimes called "black boxes", in that we can put an argument into the box and a return value comes out. We don't need to know the inner details of the "black box" to use it. (Of course, as you advance your programming skills, you may become curious about how certain functions work. And if you work with sensitive data, you may need to peer in the black box to ensure the security and accuracy of the output.)
While Python comes with many functions, there are thousands more that others have written. Adding them all to Python would create mass confusion, since many people could use the same name for functions that do different things. The solution then is that functions are stored in modules that can be imported for use. A module is a Python file (extension ".py") that contains the definitions for the functions written in Python. These modules (individual Python files) can then be collected into even larger groups called packages and libraries. Depending on how many functions you need for the program you are writing, you may import a single module, a package of modules, or a whole library.
The general form of importing a module is:
print('Waiting 5 seconds...') import time # We import the `time` module time.sleep(5) # We run the sleep() function from the time module using `time.sleep()` print('Done')
print('Waiting 5 seconds...') from time import sleep # We import just the sleep() function from the time module sleep(5) # Notice that we just call the sleep() function, not time.sleep print('Done')
do this task
After the function is defined, we can call it as many times as we want without having to rewrite its code. In the example below, we call
# Creating a simple function to double a number print('I will double any number. Give me a number.') inputnumber = input() def my_function(): outputnumber = int(inputnumber) * 2 print(outputnumber) my_function() print('Give me a new number.') inputnumber = input() my_function() print('Give me one last number.') inputnumber = input() my_function()
Using functions also makes it easier for us to update our code. Let's say we wanted to change our program to square our
inputnumber instead of doubling it. We can simply change the function definition one time to make the change everywhere. See if you can make the change. (Remember to also change your program description in the first line!)
# Creating a simple function to raise a number to the second power. print('I will raise any number to the second power. Give me a number.') inputnumber = input() def my_function(): outputnumber = int(inputnumber) ** 2 print(outputnumber) my_function() print('Give me a new number.') inputnumber = input() my_function() print('Give me one last number.') inputnumber = input() my_function()
By changing our function one time, we were able to make our program behave differently in three different places. Generally, it is good practice to avoid duplicating program code to avoid having to change it in multiple places. When programmers edit their code, they may spend time deduplicating to make the code easier to read and maintain.
do this task
In the pseudo-code above,
input_variable is a parameter because it is being used within the context of a function definition. When we actually call and run our function, the actual variable or value we pass to the function is called an argument.
# A program to greet the user by name def greeting_function(user_name): #`user_name` here is a parameter since it is in the definition of the `greeting_function` print('Hello ' + user_name) greeting_function('Sam') # 'Sam' is an argument that is being passed into the `greeting_function`
# A program to greet the user by name def greeting_function(user_name): #`user_name` here is a parameter since it is in the definition of the `greeting_function` print('Hello ' + user_name) print('What is your name?') answer = input() greeting_function(answer) # `answer` is an argument that is being passed into the `greeting_function`
Whether or not a function takes an argument, it will return a value. If we do not specify that return value in our function definition, it is automatically set to
None, a special value like the Boolean
False that simply means null or nothing. (
None is not the same thing as, say, the integer
0.) We can also specify return values for our function using a flow control statement followed by
return in the code block.
Let's write a function for telling fortunes. We can call it
fortune_picker and it will accept a number (1-6) then return a string for the fortune.
# A fortune-teller program that contains a function `fortune_picker` # `fortune_picker` accepts an integer (1-6) and returns a fortune string def fortune_picker(fortune_number): # A function definition statement that has a parameter `fortune_number` if fortune_number == 1: return 'You will have six children' elif fortune_number == 2: return 'You will become very wise' elif fortune_number == 3: return 'A new friend will help you find yourself' elif fortune_number == 4: return 'Do not eat the sushi' elif fortune_number == 5: return 'That promising venture... it is a trap.' elif fortune_number == 6: return 'Sort yourself out then find love.' print(fortune_picker(5))
In our example, we passed the argument
3 that returned the string
'A new friend will help you find yourself'. To change the fortune, we would have to pass a different integer into the function. To make our fortune-teller random, we could import the function
randint() that chooses a random number between two integers. We pass the two integers as arguments separated by a comma.
# A fortune-teller program that uses a random integer from random import randint # import the randint() function from the random module def fortune_picker(fortune_number): # A function definition statement that has a parameter `fortune_number` if fortune_number == 1: return 'You will have six children' elif fortune_number == 2: return 'You will become very wise' elif fortune_number == 3: return 'A new friend will help you find yourself' elif fortune_number == 4: return 'Do not eat the sushi' elif fortune_number == 5: return 'That promising venture... it is a trap.' elif fortune_number == 6: return 'Sort yourself out then find love.' random_number = randint(1, 6) # Choose a random number between 1 and 6 and assign it to a new variable `random_number` print(fortune_picker(random_number))
We have seen that functions make maintaining code easier by avoiding duplication. One of the most dangerous areas for duplication is variable names. As programming projects become larger, the possibility that a variable will be re-used goes up. This can cause weird errors in our programs that are hard to track down. We can alleviate the problem of duplicate variable names through the concepts of local scope and global scope.
We use the phrase local scope to describe what happens within a function. The local scope of a function may contain local variable, but once that function has completed the local variable and their contents are erased.
# Demonstration of global variable being use in a local scope # The program crashes when a local variable is used in a global scope global_secret_number = 7 def share_number(): local_secret_number = 13 print('The global secret number is ' + str(global_secret_number)) print('The local secret number is ' + str(local_secret_number)) share_number() print('The global secret number is ' + str(global_secret_number)) print('The local secret number is ' + str(local_secret_number))
The code above defines a global variable
global_secret_number with the value of 7. A function, called
share_number, then defines a local variable
local_secret_number with a value of 13. When we call the
share_number function, it prints the local variable and the global variable. After the
share_number() function completes we try to print both variables in a global scope. The program prints
global_secret_number but crashes when trying to print
local_secret_number in a global scope.
It's a good practice not to name a local variable the same thing as a global variable. If we define a variable with the same name in a local scope, it becomes a local variable within that scope. Once the function is closed, the global variable retains its original value.
# A demonstration of global and local scope using the same variable name secret_number = 7 def share_number(): secret_number = 10 print(secret_number) share_number() print(secret_number)
# Using a global statement in a local scope to change a global variable locally secret_number = 7 def share_number(): global secret_number # The global statement indicates this the global variable, not a local variable secret_number = 10 print(secret_number) share_number() print(secret_number)