10. Bool type#

Author: Tue Nguyen

10.1. Outline#

  • Why bool type?

  • How to get a bool?

  • Conversion to bool

  • When uses bool?

  • Operations on bool

10.2. Why bool type?#

  • Motivation: used to represent binary data

  • Real-life data: True/False, Yes/No, Success/Failure, Male/Female, Good/Bad

  • Python implementation:

    • Type: bool

    • Possible values: True and False

  • Note that Python is case-sensitive. Thus, True, TRUE, and true are different

10.3. How to get a bool?#

a) Ex 1: from True and False literals

# Init two bool variables
x = True
y = False

# Print values
print(x)
print(y)

True
False

# Print types
print(type(x))
print(type(y))

<class 'bool'>
<class 'bool'>

b) Ex 2: from an expression that produces a bool

A bool is often produced from

  • A comparison

  • A call to a function that returns a bool

b1) Get a bool from a comparison

# Init two variable
x = 1000
y = 2000

# Check equal
x == y

False

# Check NOT equal
x != y

True

# Another way to check NOT equal
not(x == y)

True

# Check if IDs are the same
x is y

False

# Check if IDs are NOT the same
x is not y

True

# Check greater than
x > y

False

# Check greater than or equal
x >= y

False

# Check less than
x < y

True

# Check less than ro equal
x <= y

True

b2) Get a bool from a function call

# Init 2 strings
s1 = "Hello"
s2 = "HELLO"

# Check if all chacters in s1 are uppercase
s1.isupper()

False

# Check if all chacters in s2 are uppercase
s2.isupper()

True

10.4. Conversion to bool#

  • Type conversion is the action of converting a value of one type to a different type

  • Other names: typecasting,type-coercion

  • Think of it as converting USD to Euro

  • To convert a value to bool, we use bool() function.

  • Some rules to remember when converting a value of another type to bool

    • 0, None, and values considered as “empty” will produce False

    • None-zero and non-empty values will produce True

a) Ex 1: from NoneType

bool(None)

False

b) Ex 2: from int

bool(0)

False

bool(100)

True

bool(-20)

True

c) Ex 3: from float

bool(0.0)

False

bool(1.5)

True

bool(1.2)

True

d) Ex 4: from collections

# Empty list
bool([])

False

# Empty dict
bool({})

False

# Empty string
bool("")

False

# Non-empty list
bool([1, 2, 3])

True

# Non-empty dict
bool({"name": "John", "age": 20})

True

# Non-empty string
bool("hello")

True

e) Ex 5: implicit typecasting

  • Python sometimes does this conversion implicitly

  • Ex: in a if statement where it expects a bool value to make a decision

  • Consider the following example

students = []
if students:
    print("There is at least 1 student")
else:
    print("There is no student")

There is no student

What happened?

  • Here, if expect a bool, but it receives an empty list

  • Thus, Python converts this empty list to bool and ends up a False

  • Since the condition is False, Python execute the statement under else

10.5. When use bool?#

  • A bool is normally used as the condition for branching in an if statement

  • We will learn more about if later in the “Control flows” sections

  • For now, consider the following simple example

grade = 8

if grade >= 4:
    print("Passed")
else:
    print("Failed")

Passed

What happened?

  • As you can see, "Passed" was printed out because grade >= 4 produces True (since 8 > 4)

  • so Python executes the statements under if

  • If we change 8 to 3, then grade >= 4 will produces False, and Python will run the statements under else

  • Let’s confirm this

grade = 3

if grade >= 4:
    print("Passed")
else:
    print("Failed")

Failed

10.6. Operations on bool#

  • Since bool type is also so simple, there are not much operations

  • Here is all you need to know

    • Logical operations

    • Since bool is a sub-type of int, we can also perform

      • Math operations

      • Comparison operations

a) Ex 1: logical operations

There are 2 logical operators: and, or, not

a1) Simple examples on bool literals

True and True

True

True and False

False

True or True

True

True or False

True

not True

False

not False

True

a2) More interesting examples on bool expression

# Init 2 variables
age = 18
health = "bad"

# Rule 1: if age >= 18 AND good health, then you can buy a vodka
if (age >= 18) and (health == "good"):
    print("Congrats. You can buy a vodka")
else:
    print("Sorry. You cannot buy a vodka")

Sorry. You cannot buy a vodka

# Rule 2: if age >= 18 OR good health, then you can buy a vodka
if (age >= 18) or (health == "good"):
    print("Congrats. You can buy a vodka")
else:
    print("Sorry. You cannot buy a vodka")

Congrats. You can buy a vodka

b) Ex 2: treat bool values as integers

Remember

  • True is equivalent to 1

  • False is equivalent to 0

# True + True
True + True

2

# True + False
True + False

1

# True > False
True > False

True

10.7. Summary#

Why bool type?

  • Used to represent binary data

  • Used as conditions in branching statements

Conversion to bool

  • Use bool()

  • Rules

    • 0 and empty values will produces False

    • Non-zero and non-empty values will produce True

  • Sometimes Python does implicit type conversion to bool as in an if statement

How to get a bool?

  • From True or False literals

  • From an expression that produces a bool such as

    • Comparison: ==, !=, >=, <=, >, <, is, is not

    • Typecasting: bool(x)

    • Logical operations on bools: (age >= 18) and (health == "good"

    • A call to a function that returns a bool: x.isupper(), isinstance(x, float)

When uses bool?

  • When representing binary data

  • As conditions in branching statements

Operations on bool

  • Logical operations: and, or, not

  • Other math and logical operations when bool values are treated as int values

10.8. Practice#

10.8.1. Exercise 1#

Do the following

  • Initialize a variable x with value True

  • Print the value associated with x

  • Print the data type of x

10.8.2. Exercise 2#

Do the following

  • Initialize a variable x with value 10

  • Initialize a variable y with a value from a comparison that checks whether x is greater than 5 or not

  • Print the value associated with y

  • Print the data type of y

10.8.3. Exercise 3#

Do the following

  • Initialize a variable x with value True

  • Initialize a variable y with value 100

  • Check if x is of type bool (Hint: type ?isinstance to see how you can use it)

  • Similarly, check if y is of type bool

10.8.4. Exercise 4#

Do the following

  • Initialize a variable x with value 7

  • Check if x is greater than or equal to 5. If true, print "Above average". Otherwise, print "Below average".

10.8.5. Exercise 5#

Replicate Ex 4, but this time, use not

10.8.6. Exercise 6#

Give three typecasting examples that produce True and three others that produce False.

10.8.7. Exercise 7#

Do the following

  • Initialize a variable x with an arbitrary value

  • If x is even, print "Even". Otherwise, print "Odd"

  • Try your code with different values of x

10.8.8. Exercise 8#

Do the following

  • Replicate Ex 7, but this time assign 5.5 to x

  • What happens?

  • What is your opinion?