Notebook 3 - Boolean Comparisons, Boolean Operators, and Expressions¶

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Things you'll learn in this lesson:

• More about types in Python
• Comparison operators
• Boolean operators
• How to combine values, variables, and operators into expressions

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More Data Types¶

In Python, values have a type. We already saw three data types in the previous lesson (ints, floats, and strings). In this lesson we'll learn about some additional types, how to determine a variable's type, how to convert a value from one type to another, and how to combine values, variables, and operators into complex expressions.

The Boolean (bool) Type¶

Python supports a special type called booleans, written bool in Python, which are used to indicate whether something is true or false. Booleans have one of two possible values:

• True
• False

When evaluating the truth value of a number, the following rules apply:

• 0 is False
• 0.0 is False
• all other numerical values are True

When evaluating the truth value of a string, the following rules apply:

• the empty string ("" and '') is False
• all other strings are True

Marc’s Law of Boolean Evaluation in Python: “nothingness is false, somethingness is true”

The None Type¶

Python has a special type called None and it means no value.

It's a good choice when you want to initialize a variable without an obvious choice for the initial value, like this:

salary = None

None always evaluates to False in boolean expressions.

Type Conversion and the type Function¶

Type Conversion Functions¶

These are built-in Python functions to convert a value to a given type. The name of the function mirrors the type you want to convert to, and you pass a variable or value to be converted.

• int(x) - converts x to an integer
• float(x) - converts x to a floating point number
• str(x) - converts x to a string
• bool(x) - converts x to a boolean value

When would you need to convert a type?¶

When you have an expression containing mixed types...

• name + number
• age * 365

Another case is when you call a function that requires a different type than you are passing. For example, the len function returns the length of a string so it expects the value passed to it will be a string.

len(42)

won’t work, but...

len(str(42))

is fine. Try both of those experiments in the code cell below.

Here's an example where type conversion is needed:

birth_year = input("Enter the year you were born:")
age = 2024 - birth_year
print(f"You are {age} years old")

Try running this code in the following cell and, when it fails, see if you can fix it by using a type conversion function.

The type() function¶

The built-in type function returns the type of the passed value. It conveys the type name in a string of the form <class type-name>. Don't worry too much about that format for now - just know that the name after class is the type name.

>>> type(123)
<class 'int'>

Run the next cell to see the type function in action.

Comparison Operators¶

As their name suggests, comparison operators allow us to compare values and result in a boolean type indicating whether the comparison is True or False.

The following table summarizes the most commonly used comparison operators in Python, along with their definition when applied to numbers and strings.

Challenge¶

Which boolean value (True or False) do each of these expressions evaluate to?

• 123 == 10

• 10 == 123

• 123 == 123

• 123 != 321

• 123 != 123

• age == 64

• "andrew" == "marc"

• "marc" == "andrew"

• "marc" == "marc"

• "marc" != "Marc"

> and >=¶

• 123 > 10

• 10 > 123

• 123 > 123

• 123 >= 123

• age >= 65

• age > min_age

• "fred" > "marc"

• "marc" > "fred"

• "marc" >= "marc"

• "marc" > "Marc"

Boolean Operators - and, or, and not¶

Boolean operators are special operators in Python that let you combine boolean values in logical ways corresponding to how we combine truth values in the real world. An example of a boolean and expression would be "I'll buy a new phone if I like the features and the price is low". There are three main boolean operators: and, or, and not. We'll look at examples of each in the next cells.

Boolean and¶

• A and B

is True only true when both A and B are True, otherwise it's False.

Example:

• I ride my bike only when it's both sunny and warm.

In Python...

if is_sunny and is_warm:
    print("ride bike")

We haven't learn about if statements so don't worry if the previous construct looks unfamiliar. It's a simple way of checking the value of a boolean expression, but we'll dive deeper into if statements in our next lesson.

Truth Table for boolean and¶

Boolean or¶

• A or B

is True when either A or B are True, or when both are True, otherwise it's False.

Example:

• I ride my bike when it's sunny, warm, or both.

In Python...

if is_sunny or is_warm:
  print("ride bike")

Truth Table for boolean or¶

Boolean Not¶

not A

• is True when A is False
• is False when A is True

Truth Table for boolean not¶

Expressions¶

• Python lets us combine values, variables, and operators into larger units called expressions.
• When evaluating an expression, Python internally replaces the variable names with the values to which they refer.
• Expressions show up all over the place, for example:
  • numerical calculations
    • year + 1 # we do this every Jan 1
  • assignment statements
    • age = age + 1 # we do this every birthday
  • function calls
    • print(age * 365) # number of days alive
• As we learn more, we'll see expressions popping up all over the place.

Challenge¶

Let's combine input, print, and expressions to build a temperature conversion program. The formulae for converting Celcius to Fahrenheit and vice versa are:

• F = (C * 9/5) + 32
• C = (F - 32) * 5/9

Types of Expressions¶

• arithmetic expressions

tax = income * tax_rate

• comparative expressions

error_count <= 0

• boolean expressions

more_to_do and no_errors

• combinations of the above

valid_ticket and (cur_day - purchase_day) < exp_window

Order of Evaluation¶

How does Python know the correct order to evaluate a complex expression?

Example: 4 + 1 * 5

Is that (4 + 1) * 5, which is 25?
Or is it 4 + (1 * 5), which is 9?

Another example: True or False and False

Is that (True or False) and False, which is False?
Or is it True or (False and False), which is True?

Python uses operator precedence rules to avoid this ambiguity and evaluate expressions in a predictable way.

Simplified Python Precedence Rules¶

This is a subset of the complete rules (in order of highest to lowest precedence):

• parentheses (innermost to outermost, left to right)
• exponentiation (left to right)
• multiplication, division, modulus (left to right)
• addition, subtraction (left to right)
• comparisons (left to right)
• boolean not
• boolean and
• boolean or

The Official Rules

Practical Advice¶

Marc's rule or operator precedence: When in doubt, use parentheses.

I make liberal use of parentheses because then:

• I don't need to remember the precedence rules.
• I don't have to worry about surprises.
• It makes my code more readable.

For example, I could write this expression, which evaluates A and B first, then C and D, and finally takes the boolean or of the two preceding results:

A and B or C and D

but I much prefer to make this explicit so I don't have to think about precedence rules every time I look at this code:

(A and B) or (C and D)

Challenges¶

Evaluate the following expressions mentally, then verify your answers in the following code cell...

• 2 + 3 * 4 + 5
• (2 + 3) * (4 + 5)
• 1 * 2 + 3 * 4 + 5 * 6
• 100 / 25 / 2
• (100 / 33) <= 3
• (100 // 33) <= 3
• True and False and True
• True and True or False and False
• 100 % 99
• (100 / 100) // (100 % 100)

Assuming we have the following variables and functions:

• customer_id - unique identifier per customer
• customer_age - customer age in years
• marketing_budget - remaining dollars available for a marketing campaign
• purchased(customer_id) - returns True if this customer already bought the product and False otherwise

Run the following cell to initialize these four variables.

Your marketing department wants to start a new campaign. Formulate an expression to determine whether it's ok to start a new marketing campaign based on your available budget.

Marketing says you're allowed to offer the discount to adults only. Formulate an expression to determine whether it's ok to start a marketing campaign and the customer is 18 years of age or older.

Marketing says you're bothering too many people with this promotion. Refine the previous boolean expression by excluding people who've already purchased the product.

Write a print statement to verify the previous expression results in the expected type using a built-in function.

Assume you have the following variables:

• birth_year - year customer was born in a string
• cur_year - current year in a string

Formulate an arithmetic expression to calculate and print the customer's current age.

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