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Python bool() Method: Everything You Wanted to Know

The Bool() Method In Python

Hey there! Today in this tutorial we are going to learn about the Python bool() method.

So, let us get right into the topic.

The Python bool() Method

The bool() method is a built-in Python method that applies the standard truth testing procedure to the passed object/value and returns a boolean value. Moreover, the bool class cannot be sub-classed. Its only instances are False and True. The syntax for using the method is given below.

bool([x])

Here, x is an optional parameter that can be an object, some value or any expression. The method returns True when True is passed and similarly returns False for a False.

The bool() method returns False for the below-mentioned conditions. Otherwise, it returns True.

  • If the object has a defined __bool__() method, then the boolean result depends on what it returns. Or else if the object has __len__() defined instead of __bool__(), it’s return value is considered.
  • If the value is zero of any type(0, 0.0, 0j, etc.),
  • If the object is an empty collection or sequence like list, string, tuple, dictionary, etc.
  • If the value is False or None constant.

Note: If for any object x, the __bool__() or __len__() methods are not defined, we get True as a result.

Python bool() Working

Now that we have learnt the basics for the bool() method, let us try out some examples to get a better understanding.

1. With Numbers

The example below illustrates the working of the bool() method with numbers.

from fractions import Fraction
from decimal import Decimal

# variable
val = 55
val1 = 0
print("bool(val):", bool(val))
print("bool(val1):", bool(val1))

# expression
print("bool(20 - 4):", bool(20 - 4))
print("bool(20 - 20):", bool(20 - 20))

# float
print("bool(0.0001):", bool(0.0001))
print("bool(0.00):", bool(0.00))

# hex
print("bool(0xF):", bool(0xF))

# Complex Numbers
print("bool(12 - 34j):", bool(12 - 34j))
print("bool(0j):", bool(0j))

# decimal floating point and fractional numbers
print("bool(Decimal(0)):", bool(Decimal(0)))
print("bool(Fraction(0, 2)):", bool(Fraction(0, 2)))

Output:

bool(val): True
bool(val1): False
bool(20 - 4): True
bool(20 - 20): False
bool(0.0001): True
bool(0.00): False
bool(0xF): True
bool(12 - 34j): True
bool(0j): False
bool(Decimal(0)): False
bool(Fraction(0, 2)): False

2. With Strings

For strings, the bool() method returns True until an unless it’s len() is equal to zero(0).

# bool() with strings

string1 = "Python"  # len > 0
print(bool(string1))

string1 = ''  # len = 0
print(bool(string1))

string1 = 'False'  # len > 0
print(bool(string1))

string1 = '0'  # len > 0
print(bool(string1))

Output:

True
False
True
True

3. With built-in Objects

For sequences or collections, the method returns False only if the passed object is empty.

# list
a = [1, 2, 3, 4]
print(bool(a))

# empty objects
a = [] 
print(bool(a))

a = ()
print(bool(a))

a = {}
print(bool(a))

Output:

True
False
False
False

4. With Custom Objects

In the example below, we have defined both __init__() and __bool__() methods for our custom class. We construct two objects x and y with different values.

Note: Even if we had defined __len__() for our custom class, it won’t affect the bool() result as we already have __bool__() defined. __len_()’s return value is only considered when a class does not have it’s __bool__() defined.

class custom():
    val = 0
    def __init__(self, num):
        self.val = num 
    def __bool__(self):
        return bool(self.val)

# custom objects
x = custom(0)
y = custom(52)

print(bool(x))
print(bool(y))

Output:

False
True

Here, the bool() results for both custom objects x and y are indirectly dependent on the passed arguments(0 for x, 52 for y). Hence, we get False for x(bool(0) = False) and True for y(bool(52) = True).

Wrapping Up

That’s it for today. Hope you had a clear understanding of the topic – bool() method in Python. We recommend going through the links mentioned in the references section for more info on the topic.

For any further questions, feel free to use the comments below.

References