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The Hidden Teaching: __bool__ and the Design of Truth

In Python, truth is not a binary of True and False. It is a reflection of meaning. Python asks, “In the context of logic, what does this object say about itself?”

To control how an object behaves in a boolean context, such as in an if statement, you may define the __bool__ method.

Let us begin again.

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Part 1: The Interpreter’s Question

When Python evaluates an object in a boolean context like:

if some_object: 
    ...

It performs this sequence:

1. Call some_object.__bool__(), if defined.

2. If not defined, fall back to some_object.__len__():

   • If length is 0, treat as False.

   • Otherwise, treat as True.

3. If neither are defined, default to True

Thus, even a custom class can define its own measure of truth.

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Part 2: Writing Our Own Truth

Let us construct a simple object: a container of wisdom.

class Scroll:
    def __init__(self, text):
        self.text = text

Now, let us observe its truth:

s = Scroll("Do not seek the truth, only cease to cherish opinions.")

if s:
    print("The scroll speaks.")
else:
    print("The scroll is silent.")

This will always print "The scroll speaks.", because Scroll has no __bool__ or __len__. Python defaults to treating all objects as True unless told otherwise.

Now, let us give it a voice of truth:

class Scroll:
    def __init__(self, text):
        self.text = text

    def __bool__(self):
        return bool(self.text.strip())

Now, observe:

Scroll(" ") # Behaves like False
Scroll("Peace") # Behaves like True

We have taught our object how to express its presence.

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Part 3: Falling Back on __len__

If __bool__ is not defined, Python looks for __len__.

class WisdomBasket:
    def __init__(self):
        self.sayings = []

    def __len__(self):
        return len(self.sayings)

Now:

basket = WisdomBasket()
if basket:
    print("There is wisdom.")
else:
    print("The basket is empty.")

This is useful when your object represents a collection, and truth is tied to its contents.

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Part 4: Truth as a Design Decision

Why does this matter?

Because in real-world code, truthiness is often meaning. Consider:

if response:
    ...

What should response mean?

• Did the HTTP request succeed?

• Did the response body contain useful data?

• Was the status 200?

You decide what __bool__ should convey.

When designing APIs, internal libraries, or user-defined classes, a clear definition of truth can reduce boilerplate and make intent self-evident.

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Part 5: A Caution on Ambiguity

Truthiness should be unambiguous and intuitive.

If your object represents a configuration, a request, or a resource, consider what it means to be “truthy.” Avoid cleverness that makes the logic obscure:

if config:
    # What does this mean?

Better to document, or avoid using truthiness altogether if the semantics are unclear.

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Closing the Circle

The master did not say, “The empty list is False.”
He said, “Not false. Only empty.”

In Python, truth is not fixed. It is a shadow cast by your design. You may choose how your objects reflect light, or the absence of it.

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