Control Flow Boolean Logic

Learning objective: By the end of this lesson, students will be able to implement conditional expressions in Python.

Conditional expressions in Python

Control flow can typically be described as different code paths executing according to the evaluation of conditional expressions.

In other words, if the conditional expression evaluates to a truthy value, do something, 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 the same as in JavaScript but are always written with a starting capital letter in Python.

Equality operators

Equality operators are used to compare whether two values are equal.

• == - equal to
• != - not equal to

There’s only one type of equality operator in Python: ==, which is the same as === in JavaScript.

Examples

Check out the following examples:

print(7 == 7)
# prints: True
# 7 is equal to 7

print(7 == "7")
# prints: False 
# 7 is an integer, and "7" is a string

print(7 != 7)
# prints: False
# 7 is equal to 7

print(7 != "7")
# prints: True 
# 7 is an integer, and "7" is a string; therefore, they cannot be equal

Execute the control_flow.py file with this command:

python3 control_flow.py

Comparison operators

Python has all the same comparison operators as JavaScript:

• < - less than
• > - greater than
• <= - less than or equal
• >= - greater than or equal

Examples

Let’s try the following examples together. Use the print() function to print each result in your console.

print(8 > 8)
# prints: False 
# 8 is not greater than 8

print(8 >= 8)
# prints: True 
# 8 is greater than or equal to 8 (equal)

print(8 < 8)
# prints: False 
# 8 is not less than 8

Logical operators

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 &&

Because they behave identically to their JavaScript counterparts, they always return either the first or the second operand as follows:

• or - Returns the first truthy operand, or the last operand.
• and - Returns the first falsy operand, or the last operand.

Python stops evaluating as soon as the result of the operation is determined.

Examples

Let’s try the following examples together. Use the print() function to print each result in your console. Remember that 0 is falsy and non-empty strings are truthy.

# or
# returns the first truthy operand, or the last operand

print(True or False)
# prints: True

print(False or True)
# prints: True

print(False or False)
# prints: False

print('hello' or 0)
# prints: 'hello'

print(0 or 'hello')
# prints: 'hello'

print('hello' or 'tacos')
# prints: 'hello'

# and
# returns the first falsy operand, or the last operand

print(True and False)
# prints: False

print(False and True)
# prints: False

print('hello' and 0)
# prints: 0

print(0 and 'hello')
# prints: 0

print('hello' and 'tacos')
# prints: 'tacos'

Like in JavaScript, more than two conditions may be evaluated at a time. For example:

# or
# returns the first truthy operand, or the last operand

print(True or True or True)
# prints: True

print(True or True or False)
# prints: True

# and
# returns the first falsy operand, or the last operand

print(True and True and True)
# prints: True

print(True and True and False)
# prints: False

You can also mix-and-match and and or:

print(False or True and True)
# prints: True

⚠️ Be careful evaluating more than two items at a time, it can make code more difficult to read.

The not operator

Just like the not operator in JavaScript (!), the not operator in Python flips a truthy expression to a boolean value of False, and a falsy expression to a boolean value of True.

Examples

print(not False)
# prints: True

print(not 'hello')
# prints: False