Intro to Python Control Flow

Learning Objectives

Set Up

1. Create a file inside your working folder for Unit 4 titled control_flow.py.
2. Open the folder in VS Code: $ code .

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!

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 & Falsey in Python

Like JS, every piece of data in Python is considered either truthy or falsey.

Conditional expressions for if statements, etc., rely on an expression evaluating to True/truthy or False/falsey 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 falsey 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)

Note the difference between Python's [] & {} and JavaScript.

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 the first operand is falsey, return it, otherwise return 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'

Control Flow

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 have the ability to run 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 :)

Also note that else is always optional.

There is no switch construct in Python

💪 Branching Exercise (7 minutes)

Add this code to your control_flow.py file:

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

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!

To run the code, open a terminal in VS Code by typing control + backtick, then type $ python3 control_flow.py.

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 preview 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:

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

for (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.

Let's look at the syntax:

while condition:
  # do some stuff
  # do some more stuff

while loops are great for when you don't know how many times you will need to iterate.

Beware of infinite loops! When using while loops, it's important to ensure that the condition will change to a falsey value so that the loop exits.

The break Statement

Just like in JavaScript, the break statement in Python can be used to immediately exit for and while loops and continue executing any statements that may follow them.

💪 Looping Exercise (7 minutes)

Wrap the code in control_flow.py in a while loop such that it continues to prompt for a color until the word quit is entered.

Python Ranges

Purpose of Ranges

Python ranges are a sequence type like lists and tuples.

The range type represents an immutable sequence of numbers 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 only 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

Summary

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

The remainder of the day will provide you with lab time to get some practice performing branching and looping in Python.

But first...

Essential Questions

Take a minute to review before the picker shows up...

❓ What are the two types of control flow discussed in this lesson.

❓ Name three values in Python that are considered to be falsey.

❓ What is returned by the following expression:

25 or 50

❓ What is returned by the following expression:

25 and 50

References

Official Docs for Control Flow in Python