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Control Flow in Python

Learning Objectives​

Students Will Be Able To:
Describe what is truthy & falsy in Python
Perform multi-path branching using a if...elif...else statement
Perform looping using while statements
Use a range and a for statement to loop through a range of integers

Road Map​

  1. Setup
  2. Review of Control Flow
  3. Conditional Expressions in Python
  4. Branching
  5. Looping
  6. Python Ranges
  7. Summary
  8. Essential Questions

1. Setup​

Create a Python-based repl in https://replit.com/ - you can name it "Control Flow in Python".

2. Review of Control Flow​

As you may recall, control flow refers to the order in which code executes in a program as determined by the use of constructs in the code.

In JavaScript, we saw how statements such as if and switch were used to perform branching.

We also used statements such as for and while to perform looping.

The good news is that most programming languages, including Python, share these same control flow constructs.

It's worth pointing out that programming concepts such as variables, data types, control flow, functions, etc. apply to programming languages in general thus making learning additional languages far easier than the first one!

3. Conditional Expressions in Python​

Control flow typically comes down to different code paths executing according to the evaluation of conditional expressions.

In other words, if the conditional expression evaluates to truthiness, do this stuff, optionally, do something else.

Let's review some of the logic and fundamentals of conditional expressions.

Boolean Values​

Python has two logical boolean values: True and False.

Most logical operations result in one of these two values. They work exactly the same as in JS but are always written with a starting capital letter in Python.

Truthy & Falsy in Python​

Like JS, every expression in Python is considered to be either truthy or falsy.

Conditional expressions for if statements, etc., rely on an expression evaluating to True/truthy or False/falsy to determine which path the code will follow.

Just like in JavaScript, most things in Python are considered to be truthy.

Here's what is falsy in Python:

  • False
  • None
  • Zero in any numeric type: 0, 0.0 and 0j
  • Empty sequences or collections:
    • '' (empty string)
    • [] (empty list)
    • () (empty tuple)
    • {} (empty dictionary)
    • range(0) (empty range)
caution

👀 [] and {} in JavaScript are truthy, not falsy as in Python.

Comparison Operators​

Python has all the same comparison operators as JavaScript:

  • < - less than
  • > - greater than
  • <= - less than or equal
  • >= - greater than or equal
  • == - equal to
  • != - not equal to

Note that in Python, there's only one equality operator. The == in Python is the same as === in JavaScript.

Examples​

8 > 8
# => False — 8 is not greater than 8.

8 >= 8
# => True — This checks if 8 is greater than or equal to 8, and they are equal.

8 < 8
# => False — 8 is not less than 8.

7 == 7
# => True — 7 is equal to 7.

7 == "7"
# => False — One is a number and the other is a string.

7 != 7
# => False — This checks if they aren't equal. Because does 7 equal 7, it's `False`.

6 != 7
# => True — 6 is not equal to 7.

Logical Operators​

Luckily, the amazing logical operators we used in JavaScript work the same way in Python except Python uses English words instead of symbols:

  • or is the same as ||
  • and is the same as &&

Again, they work just like they did in JS, which means they always return either the first or the second operand as follows:

or​

If the first operand is truthy, return it, otherwise return the second operand.

and​

If this first operand is falsy, we will return its value, if the first operand is truthy, we will then return the value of the second operand.

Examples​
True or False
# => True

False or True
# => True

'hello' or 0
# => 'hello'

0 or 'hello'
# => 'hello'

True and False
# => False

False and True
# => False

'hello' and 0
# => 0

0 and 'hello'
# => 0

'hello' and 'tacos'
# => 'tacos'

not​

"Flips" the truthiness/falsyness of its operand, and returns True or False

not True
# => False

not 123
# => False

not []
# => True

4. Branching​

Indentation​

Before we start looking at control flow, it's important to realize that Python uses indentation to define blocks of code - not curly braces.

It has always been recommended to use indentation in languages for readability purposes, however, in Python, proper indentation is mandatory!

Branching with the if Statement​

Just like in JavaScript, we can use an if statement to branch to one of several code paths depending upon the result of conditional expression(s).

Single path if statement:

floor = "sticky"
walls = "clean"
if floor == "sticky":   # don't forget the colon
  print("Clean the floor! It's sticky!")
  # more lines of code in this code
  # block need to be indented as well
if walls == "sticky":
  print("Clean the walls! They're sticky!")

Yup, no parentheses are required around the conditional expression.

Dual path if..else statement:

if condition:
  # do something
else:
  # do something else
  # do something else

Multi-path if..elif..else statement:

if condition1:
  # do something
  # do something
elif condition2:
  # do something else
  # do something else
  # do something else
elif condition3 and condition4:
  # do another thing entirely
  # do another thing entirely
else:
  # else do this stuff

The elif is not a typo :)

As in JS, including an else is optional.

As of Python 3.10.0, for switch construct Python has an official syntactical equivalent called match.

The basic syntax is:

match value:
    case condition:
        action(s)
    ...

A concrete example is:

my_value = 10
match my_value:
    case 10:
        print("The number is ten")
    case 2*10:
        print("The number is the double of ten")
    case 100:
        print("The number is one hundred")
    case _:
        # this is the default handler if none
        # of the above cases match.
        print("The number is none of 10, 2*10 or 100")

👉 You Do - Branching Exercise (5 minutes)​

Copy/paste the following code that accepts text input from the user, converts it to lowercase and prints it:

color = input('Enter "green", "yellow", "red": ').lower()
print(f'The user entered {color}')

❓ What was the function we used in JavaScript to get input from the user?

input()

Below that code, write an if...elif...else statement that prints out one of the following messages:

  • If green is entered, print the message Go!
  • If yellow is entered, print the message Slow Down!
  • If red is entered, print the message Stop!
  • If anything else is entered, print the message Bogus!

Test the code by running it in the repl.

5. Looping​

The for Statement​

Python's for statement is not designed like the one you first used in JavaScript:

// A JavaScript for loop
for (let i = 0; i < 10; i++) {
  // do stuff
}

Instead, the Python for loop always iterates over the items in a sequence, similar to JavaScript's for...in and for...of loops.

We'll learn about sequences soon, but here's a taste of how Python's for loop is used to loop through a list (Python's array):

names = ["Tom", "Deborah", "Murray", "Axel"]

for name in names:
  print(name)

The above Python code is like the following ES2015 JS:

const names = ["Tom", "Deborah", "Murray", "Axel"];

for (let name of names) {
  console.log(name);
}

The while loop​

Python also has a while loop construct that will continue to iterate while a given condition is truthy - just like in JavaScript.

Let's look at the syntax:

while condition:
  # do some stuff
  # continue to do stuff

while loops are the go to when the number of times you will need to iterate is unknown.

danger

👀 Beware of infinite loops! When using while loops, it's important to ensure that the condition will become falsy as some point so that the loop exits.

The break and continue Statements​

Just like in JavaScript...

The break statement is used to immediately exit for and while loops and continue executing any statements that may follow them.

The continue statement is used to immediately return to the top of the for or while loop. In the case of a while loop, its conditional expression is once again evaluated to determine if the loop should continue.

👉 You Do - Looping Exercise (3 minutes)​

Wrap the "color" code from the branching exercise in a while loop such that it continues to prompt for a color and print the response until the word quit is entered.

tip

Hint: Same gotcha as when you did this exercise in JS 😁 If you get an error, read it - you got this!

6. Python Ranges​

Purpose of Ranges​

Python ranges are a sequence type like lists and tuples.

The range type represents an immutable sequence of integers and is commonly used for looping a specific number of times in for loops.

Ranges have a class (type) of range.

Ranges - Basic Syntax​

Ranges can be created by invoking the range() class:

for num in range(5):
  print(num)
> 0
> 1
> 2
> 3
> 4

Notice that by default, the sequence starts at 0 and goes up to, but does not including the integer passed in.

Ranges can also generate sequences with a start and a step:

for even in range(2, 10, 2):
  print(even)
> 2
> 4
> 6
> 8

When not passed in, the start value defaults to 0 and the step defaults to 1.

Ranges can also be used to create lists and tuples:

nums = list(range(10))
print(nums)
> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
odds = tuple(range(1, 10, 2))
print(odds)
> (1, 3, 5, 7, 9)

Ranges - Negative Step​

If the step is a negative integer, the sequence counts down:

for num in range(5, 0, -1):
  print(num)
> 5
> 4
> 3
> 2
> 1

7. Summary​

As you have seen today so far, Python is not all that different than JavaScript.

Applying what we did in this lesson in the lab will provide you with some practice performing branching and looping in Python.

But first...

8.❓ Essential Questions (1 minute)​

(1) What are the two types of control flow discussed in this lesson.

Branching and Looping

(2) Name three things in Python that are considered to be falsy.

Any three of the following:

False, None,
0, 0.0 and 0j (zero)
Empty sequences or collections:

  • '' (empty string)
  • [] (empty list)
  • () (empty tuple)
  • {} (empty dictionary)
  • range(0) (empty range)
25 or 50

(3) What is returned by the above expression?

25

25 and 50

(4) What is returned by the above expression?

50

References​

Official Docs for Control Flow in Python