07_booleans.md

Booleans

The Boolean data type in Python is the True or False data type. In other words, the only two Boolean values are True and False. Although simple in terms of possible values, don't let their simplicitly suggest to you that they are not important. Booleans play an very important role for nearly every Python application because Booleans are critical for decision-making by your application.

Capitalization matters. In Python, you must capitalize True and False for the Python intepreter to recognize it as a Boolean. On the flip side, if you use True or False without quotes but mean for it to be a string, Python will interpret it as a Boolean. Therefore, if you want to use the word True or False as a string, you need to wrap it in quotes.

Sometimes you will want to check if a value is True or False. A common scenario is having a variable that starts off as empty or 0, which makes it inherently False. For the sake of example, you call that variable number_of_users and give it a value of 0. Therefore, when your application starts, number_of_users = 0 and therefore evaluates to False. Let's say that while number_of_users is False, you want your applicationt to print("Users loading . . .").

After the program starts, something happens that changes the value of number_of_users. Let's say the something that happened is that your application requested from your database the number of users for your application. Upon receiving the response from your database, your applicatoin set the value of number_of_users to 119. Now that number_of_users no longer equals 0, number_of_users evaluates to True. Let's say that while number_of_users is True, you want your applicationt to print(number_of_users).

Here in this example, by evaluating the Boolean value of number_of_users, you can control what your application prints to the console. The next lesson goes into more detail about how to use operators and expressions with Booleans to control your applicaton's flow by checking an object's Boolean value.

One way to check Boolean value of a Python object is to use the built-in bool() function. It represents Boolean type and is a subset of int(). Most values are inherently True. For instance, numbers other than 0, strings that have text in them, and other data types that have non-empty values.

Some values, however, are inherently False. For instance, empty values (i.e., other data types that have no value, like "", (), and {}), the number 0, and None are inherently False.

# example of a values that evaluates to True
print(bool(7))
print(bool("Python is fun"))
print(bool(["Python," "programming", "basics"]))
print(bool(("Python," "programming", "basics")))
print(bool({
    "first": "Python", 
    "second": "programming", 
    "third": "basics"
}))

>>> True
>>> True
>>> True
>>> True
>>> True

# example of a values that evaluates to False
print(bool(0))
print(bool(""))
print(bool([]))
print(bool(()))
print(bool({}))

>>> False
>>> False
>>> False
>>> False
>>> False

You don't have to use the bool() function in an expression to control the flow of your application. For instance, instead of writing if bool(7), you can simply write if 7. You may, however, find that at times using bool() makes your code more expressive and therefore easier for others to understand.

In addition to checking the Boolean value of an object, you can check the Boolean value of a Boolean expression. A Boolean expression is an expression that evaluates to True or False. As for objects, you can use the same built-in bool() function for expressions or you can evalute the expression directly (without using bool()). Whether it returns True or False depends on the details of the expression.

# example of an expression that evaluates to True
print(7 > 3)
print(bool(7 > 3))

>>> True
>>> True

# example of an expression that evaluates to False
print(7 < 3)
print(bool(7 < 3))

>>> False
>>> False

You will see Booleans in action in examples from other sections, like operators, expressions, and conditionals.