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Python bool() Example | bool() Function In Python Tutorial

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Python bool() is an inbuilt function that converts the value to Boolean (True or False) using the standard truth testing procedure. It’s not mandatory to pass the value to bool(). If you do not pass a value, bool() returns False. Python bool() function returns the boolean value of a specified object. 

Python bool() Example

The boolean builtins are capitalized: True and False.

The object will always return True, unless:

The object is empty, like [], (), {}
The object is False
The object is 0
The object is None

See the following syntax.

bool(object)

The object parameter is like String, List, Number, etc.

See the following example.

app = False
print(bool(app)) 

app = True
print(bool(app)) 

app = 5
data = 10
print(bool(app==data)) 

app = None
print(bool(app)) 
  
app = () 
print(bool(app)) 

app = {}
print(bool(app)) 
  
app = 0.0
print(bool(app)) 

app = 'MillieBobbyBrown'
print(bool(app))

See the following output.

➜  pyt python3 app.py
False
True
False
False
False
False
False
True
➜  pyt

The bool() returns the following output:

  1. False if a value is omitted or false
  2. True if a value is true

See the other examples.

def check(num):
    return(bool(num % 2 == 0))


num = 11
if(check(num)):
    print("Even")
else:
    print("Odd")

See the output.

➜  pyt python3 app.py
Odd
➜  pyt

#Python bool() Standard Rules

Python bool() function uses the standard truth testing rules to convert a specified parameter object to the Boolean value.

The main rules used to return the Boolean value are the following.

  1. Any object Boolean value is considered True if it is not implementing the __bool__() function and __len__() functions.
  2. If the object doesn’t define the __bool__() function but defines the __len__() function, then the __len__() function is used to get a boolean value of the object. If the __len__() returns 0, then bool() function will return False otherwise True.
  3. The boolean value will be False for the None and False constants.
  4. The boolean value will be False for zero value such as the 0, 0.0, 0j, Decimal(0), and Fraction(0, 1).
  5. The boolean value will be False for empty data structures like tupledict, and collections, such as ”, (), [], {} etc.

#Python bool() function with custom object

Let’s see what happens with the custom object. I will not define __bool__() and __len__() functions for the object. See the following code example.

# app.py

class App:
    number = 0

    def __init__(self, i):
        self.number = i


a = App(0)
print(bool(a))
d = App(10)
print(bool(d))

See the following output.

➜  pyt python3 app.py
True
True
➜  pyt

Since in the above example, none of __bool__() and __len__() functions are defined, object boolean value is returned as True. Let’s add __len__() function to the App class.

class App:
    number = 0

    def __init__(self, i):
        self.number = i
    def __len__(self):
        print('len function called')
        if self.number > 0:
            return self.number
        else:
            return 0

a = App(0)
print(bool(a))
d = App(11)
print(bool(d))

See the following output.

➜  pyt python3 app.py
len function called
False
len function called
True
➜  pyt

It’s clear that the __len__() function is called by bool(). When 0 is returned, bool() function is returning False. When positive integer is returned, then bool() function is returning True.

Now let’s add __bool__() function to App class.

class App:
    number = 0

    def __init__(self, i):
        self.number = i

    def __len__(self):
        print('len function called')
        if self.number > 0:
            return self.number
        else:
            return 0

    def __bool__(self):
        print('bool function called')
        return self.number > 0


a = App(0)
print(bool(a))
d = App(11)
print(bool(d))

See the following output.

➜  pyt python3 app.py
bool function called
False
bool function called
True
➜  pyt

So, now it is clear that, if both __bool__() and __len__() functions are defined for the object, then __bool__() function is used to get the Boolean value of an object.

#Python bool() with numbers

Okay, now let’s test the bool() method with numbers.

print(bool(11))
print(bool(11.19))
print(bool(0xF))
print(bool(11 - 10j))

See the following output.

➜  pyt python3 app.py
True
True
True
True
➜  pyt

#Python bool() with Strings

Let’s test the book() method with Strings.

print(bool('Krunal'))
print(bool(''))
print(bool("AppDividend"))

See the following output.

➜  pyt python3 app.py
True
False
True
➜  pyt

Finally, Python bool() Example Tutorial is over.

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