# Python: Truthiness of Objects

### Introduction:

This post is a brief overview of how Python evaluates the truthiness of objects. In Python, any object can be tested for truth value!

### Built-in Objects:

The following built-in objects evaluate to *False*:

> 
- constants defined to be false: None and False.
> 
- zero of any numeric type: 0, 0.0, 0j, Decimal(0), Fraction(0, 1)
> 
- empty sequences and collections: '', (), [], {}, set(), range(0)

If a built-in object is not one of these values, it will evaluate to *True*. Below are examples of what some built-in objects evaluate to *(the bool() method is a built-in method used to evaluate the truth value of objects)*:

```
bool(0)
False

bool(5)
True

bool([])
False

bool('hello')
True

bool(None)
False
``` 

### Non Built-in Objects:
Non built-in objects are evaluated to *True* by default.

```
class Book:

    def __init__(self, pages: int) -> None:
        self.pages = pages

book_1 = Book(100)

bool(book_1)
True
``` 
To override this default behavior, the ```__bool__```  method can be defined within a class. Within this dunder method, conditions can be set for when an object evaluates to *True* or *False*.

```
class Book:

    def __init__(self, pages: int) -> None:
        self.pages = pages
    
    def __bool__(self) -> bool:
        if self.pages < 0:
            return False
        return True

book_1 = Book(100)
book_2 = Book(-100)

bool(book_1)
True

bool(book_2)
False
``` 
If the ```__bool__``` method is not defined, Python will then look for the ```__len__``` method within the class. If the ```__len__``` method is defined, then an object will evaluate to *False* if the ```__len__``` method would return a value of 0 for that object.

```
class Book:

    def __init__(self, pages: int) -> None:
        self.pages = pages
    
    def __len__(self) -> int:
        return self.pages

book_1 = Book(100)
book_2 = Book(0)

bool(book_1)
True

bool(book_2)
False
``` 
In summary, the ```__bool__``` method will take precedence over the ```__len__``` method if both are defined in a class. If neither the ```__bool__``` nor  ```__len__``` method are defined, all objects of the class will evaluate to *True*. 

### Functions & Methods as Objects:
Functions & methods, as objects, will always evaluate *True*.

```
class Book:

    def __init__(self, author: str, available: bool, pages: int) -> None:
        self.available = available
        self.author = author
        self.pages = pages
    
    def set_to_unavailable(self) -> None:
        self.available = False

def indepedent_function() -> None:
    print('I am a function.')

book_1 = Book('Mitch', True, 100)

bool(book_1.available)
True

bool(indepedent_function)
True 
``` 
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Hopefully this article has clarified how Python evaluates the truthiness of objects!