Introduction
A Boolean is a data type that has either one of two values representing a conceptual “true” or “false”. Booleans are named after the mathematician George Boole, who was a pioneer of mathematical logic. In programming, they are used for decision-making, (i.e. to determine whether the computer should do something or not.
What is a Boolean?
In Python, Booleans are handled with the bool type, which can hold one of two possible values: True or False. These values are case-sensitive and must be capitalized.
Example:
is_active = True is_logged_in = False
Falsy and Truthy Values
Every value in Python has a Boolean equivalent. This means that any value can be used as or converted to a Boolean.
Data types and classes in Python can define which of their values is equivalent to True or False. In general, values that represent something empty or a kind of “nothing” get False and other values within the type True. For example, for int types, 0 is False and any other int is True. For strings, an empty string "" is False and any other string is True.
Falsy Values
Falsy values are those that evaluate to False in a Boolean context. Common values that are falsy include:
None0(integer zero)0.0(float zero)''(empty string)[](empty list){}(empty dictionary)set()(empty set)Falseitself
Truthy Values
Truthy values are all values that are not falsy. This includes all numbers except 0, non-empty strings, non-empty collections, and True.
The bool() Function
The bool() function is used to convert a value to a Boolean. It returns True if the value is truthy and False if the value is falsy.
Example:
print(bool(42))
print(bool('Hello'))
print(bool(None))
print(bool(''))
print(bool([1,2,3]))Output:
True True False False True
Boolean Operators
Boolean operators perform logical operations on Boolean values. Since Python values have equivalent Booleans (i.e. they’re either truthy or falsy), in Python they can operate on any value. The primary Boolean operators in Python are and, or, and not.
and Operator
The and operator is similar to the logical ‘and’, which returns (evaluates to) true if two things are both true. The Python and adds a twist to this, but let’s first see an example of how it works in the most basic way with Booleans.
Example:
print(True and True) print(True and False) print(False and False)
Output:
True False False
In the example above, line 1 has an and that operates on two True values, and therefore the result is True. Line 2 has only one True and line 3 has no True values, and therefore both result are False.
From the example above, it seems that the Python and operates like the logical and, which returns True when both values are True, and False otherwise. However, what it’s actually doing is more subtle. The and operator returns the value of the operand that has the final determination of the logical outcome.
To understand what that means, let’s look at an example with non-Boolean values.
Example:
print(1 and 2) print(2 and 1) print(3 and 0) print([] and 0) print(0 and [])
Output:
2 1 0 [] 0
On line 1 in the example above, given that the first operand is a 1, the value of the second operand determines the outcome of the logical operator. A falsy value of the second operand makes the entire expression logically false, and a truthy value there makes entire expression logically true. Therefore, in case both operands are truthy, the value that and returns is the value of the second operand, which is 2 in this case. You can see that and indeed returns the value of the second operand when both are truthy, by looking at line 2, for which the returned value is 1.
Line 3 has one falsy value, and when that’s the case, that falsy value is always the one that makes the final determination, and is therefore the one that’s returned.
Lines 4 and 5 have two falsy values. When that’s the case, the first falsy value is the one that makes the determination because the expression is logically false regardless of the value of the second operand, and therefore the first falsy value is the one returned.
Here is a summary of what and returns:
- If both values are truthy, the second value is returned.
- If one value is falsy and the other truthy, the falsy value is returned.
- If both values are falsy, the first falsy value is returned.
or Operator
The or operator is similar to the logical ‘or’, which returns true if at least one value is true.
Example:
print(True or False) print(False or False) print(True or True)
Output:
True False True
The or operator can, of course, operate on all truthy and falsy values as well, not just Booleans. Like the and operator, or also returns the value of the operand that has the final determination of the logical outcome.
Example:
print(1 or 2) print([] or 3) print(0 or [])
Output:
1 3 []
Here is a summary of what or returns:
- If both values are truthy, the first value is returned.
- If one value is falsy and the other truthy, the truthy value is returned.
- If both values are falsy, the second falsy value is returned.
not Operator
The not operator negates the Boolean value, returning True if the operand is falsy, and False if the operand is truthy. Unlike the and and or operators, not always returns a proper Boolean, i.e. either True or False.
Example:
print(not True) print(not False) print(not 'Hello') print(not '') print(not 2) print(not 0)
Output:
False True False True False True
Combining Boolean Operators
Boolean operators can be combined to create more complex logical expressions. For example, you can use parentheses to group operations and enforce a specific order of evaluation.
Example:
has_permission = is_admin or (is_logged_in and has_valid_token)
In the example above, the expression (is_logged_in and has_valid_token) is evaluated first due to the parentheses. If it is True, then the entire expression will be True regardless of the value of is_admin. If it is False, then the value of is_admin will determine the final result.
Comparison Operators
Comparison operators are used to compare two values, and they return a Boolean result (True or False). The standard comparison operators in Python are:
==(equal): Checks whether two values are equal.!=(not equal): Checks whether two values are not equal.>(greater than): Checks whether one value is greater than another.<(less than): Checks whether one value is less than another.>=(greater than or equal): Checks whether one value is greater than or equal to another.<=(less than or equal): Checks whether one value is less than or equal to another.
Examples:
a = 10 b = 20 print(a == b) # --> False print(a != b) # --> True print(a > b) # --> False print(a < b) # --> True print(a >= b) # --> False print(a <= b) # --> True
Output:
False True False True False True
Every data type can define its own way of comparing its values, and some data types do not support all comparison operators.
The complex type only supports the == and != because complex numbers are not ordered.
Strings (str types) support all comparison operators. They are compared in lexicographical order, character by character, from left to right.
Example:
print('apple' < 'banana')
print('apple' == 'Apple') Output:
True False
Combining Comparison Operators
Comparison operators can be combined to test whether an expression is within a certain range. This is done by placing the expression between two comparison operators.
Example:
age = 25 is_age_allowed = 18 <= age <= 65
In the example above, the expression 18 <= age <= 65 evaluates to True if age is between 18 and 65 (inclusive), and False otherwise. The expression 18 <= age <= 65 is effectively shorthand for 18 <= age and age <= 65, and as you can see, Boolean and comparison operators can also be used together.
Short-Circuit Evaluation
Python uses short-circuit evaluation when evaluating Boolean expressions. This means that it will stop evaluating the operands of a Boolean expression as soon as the overall result is determined.
For the and operator, if the first operand is falsy, the expression is immediately known as that, and the second operand is not evaluated. This is because a logical ‘and’ requires only one operand to be false for the whole expression to be false.
Similarly, for the or operator, if the first operand is truthy, the expression is immediately evaluated as that, and the second operand is not evaluated. This is because a logical ‘or’ requires only one operand to be true for the whole expression to be true.
This short-circuit evaluation can be useful for preventing unnecessary computations and potential errors (e.g., dividing by zero) in complex Boolean expressions.
Example:
x, y = 1, 0 print(y != 0 and x/y) x, y = 1, 2 print(y != 0 and x/y)
Output:
False 0.5
In the example above, the y != 0 and part of the expression guards against a division by zero. On line 2, the divisor, y, is 0, and therefore y != 0 evaluates to False. Because of short-circuit evaluation, that’s all the and needs to determine its value, which is False in this case. The division by 0 is not performed and no error occurs.
On line 5, y is 2 and therefore y != 0 evaluates to True. The and needs to evaluate the second operand to determine its value. The division is executed and the and operator returns its result, 0.5.
Naming Boolean Variables
When naming Boolean variables, it’s a good practice to use descriptive names that clearly indicate the condition or state being represented. This makes the code more readable and easier to understand.
Here are some guidelines for naming Boolean variables:
- Use a prefix like
is_,has_,can_, orshould_to indicate the condition being checked. - Use clear and concise names that describe the condition.
- Avoid using negations like
not_orno_in the variable name, as this can make the code harder to read.
Example:
is_valid = True has_permission = False can_proceed = is_valid and has_permission
In the example above, the variable names is_valid, has_permission, and can_proceed clearly describe the conditions they represent, making the code more self-documenting and easier to understand.
Summary & Reference for Python Booleans
A Boolean in Python is a data type that represents one of two values: True or False. These values are case-sensitive and must be capitalized.
is_active = True is_logged_in = False
Booleans are useful in decision-making processes within your code, helping to determine the flow of execution based on conditions.
Every value in Python has a Boolean equivalent. Values that are equivalent to False are called ‘falsy’ and values that are equivalent to True are called ‘truthy’.
Data types and classes in Python can define which of their values is equivalent to True or False. In general, values that represent something empty or a kind of “nothing” get False and other values within the type get True.
Falsy values include: None, 0, 0.0, '' (empty string), [] (empty list), {} (empty dictionary), set() (empty set), and False. Truthy values are all values that are not falsy, including non-zero numbers, non-empty strings, non-empty collections, and True.
The bool() function converts any value to its Boolean equivalent.
bool(42) # --> True bool(None) # --> False
Boolean operators (and, or, not) perform logical operations. They can operate on any value and consider the Boolean equivalents. The and and or operators return the value of the operand that makes the final determination in the value of the logical expression. The not operator returns the Boolean opposite of its operand.
print(1 and []) # --> [] print(1 or 2) # --> 1 print(not "") # --> True
Comparison operators (==, !=, >, <, >=, <=) return Boolean results based on the comparison of two values.
a = 10 b = 20 print(a == b) # False print(a < b) # True
Boolean expressions can be combined as well as use parentheses to enforce a specific order of evaluation.
is_admin = False is_logged_in = True has_valid_token = True has_permission = is_admin or (is_logged_in and has_valid_token) print(has_permission) # --> True
Python uses short-circuit evaluation for Boolean expressions. For and, if the first operand is falsy, the second is not evaluated. For or, if the first operand is truthy, the second is not evaluated.
x, y = 1, 0 print(y != 0 and x / y) # --> False
When naming Boolean variables, use descriptive names that clearly indicate the condition being represented, such as is_valid, has_permission, or can_proceed.
is_valid = True has_permission = False can_proceed = is_valid and has_permission