What is the meaning of single and double underscore before an object name?

Understanding Single and Double Underscores Before Object Names in Python

Navigating Python's naming conventions can feel like deciphering a secret code, but once you get the hang of it, it becomes a powerful tool for writing clean and effective code. Let’s break down what single (_) and double (__) underscores before an object name mean in Python, making it simple and straightforward.

Single Underscore (_) Prefix

Purpose and Usage

A single underscore before an object name is a convention in Python indicating that the object is intended for internal use within a module or class. It serves as a hint to other programmers that this part of the code is meant to be private and should not be accessed directly.

Example:

class MyClass:
    def __init__(self):
        self.public_attribute = "I am public"
        self._protected_attribute = "I am protected"

    def public_method(self):
        print("This is a public method")

    def _protected_method(self):
        print("This is a protected method")

In this example:

• public_attribute and public_method are meant to be accessed freely.
• _protected_attribute and _protected_method are intended for internal use within MyClass.

When to Use

• When you want to indicate that certain methods or attributes are intended for internal use.
• To prevent accidental modification or access from outside the class or module.

Double Underscore (__) Prefix

Purpose and Usage

A double underscore before an object name triggers name mangling in Python. This means the interpreter changes the name of the variable in a way that makes it harder to create subclasses that accidentally override the private methods and attributes. It’s a way to ensure that the attribute is truly private and not easily accessible from outside the class.

Example:

class MyClass:
    def __init__(self):
        self.__private_attribute = "I am private"

    def __private_method(self):
        print("This is a private method")

In this example:

• __private_attribute and __private_method are name-mangled to prevent direct access from outside MyClass.

How It Works

• The interpreter changes __private_attribute to _MyClass__private_attribute.
• This makes it difficult to access the attribute directly from outside the class.

Accessing Name-Mangled Attributes:

obj = MyClass()
print(obj._MyClass__private_attribute)  # Output: I am private

When to Use

• When you need to prevent subclasses from accidentally overriding methods or attributes.
• To enforce strict encapsulation in your classes.

Key Differences Between Single and Double Underscores

Single Underscore (_)

• Convention-Based: Signals that the attribute or method is intended for internal use.
• No Name Mangling: The name remains the same and can be accessed if needed.
• Use Case: Protecting methods and attributes from accidental access, but not strictly enforcing privacy.

Double Underscore (__)

• Name Mangling: Changes the attribute name to include the class name, making it harder to access.
• Stronger Encapsulation: Provides a higher level of privacy by preventing easy access and overriding.
• Use Case: When you need to ensure that an attribute or method remains private and is not easily accessible or overridden.

Trade-Offs Between Single and Double Underscores

Single Underscore

Pros:

• Simple and easy to understand.
• Flexible, allowing access if absolutely necessary.

Cons:

• Relies on developer discipline; doesn't enforce privacy.
• Can still be accessed from outside, leading to potential misuse.

Double Underscore

Pros:

• Provides stronger encapsulation through name mangling.
• Prevents accidental access and modification from subclasses.

Cons:

• More complex and can make debugging harder.
• Makes subclassing more restrictive, potentially limiting flexibility.

When to Choose Which

• Use a Single Underscore when you want to indicate that a method or attribute is intended for internal use but don’t need to enforce strict privacy.
• Use a Double Underscore when you need to ensure that an attribute or method is truly private and protected from being overridden or accessed unintentionally.

Additional Resources

Enhance your understanding of Python’s object-oriented principles and prepare for interviews with these DesignGurus.io courses:

• Grokking the Object Oriented Design Interview here
• Grokking the System Design Interview here
• Grokking the Coding Interview: Patterns for Coding Questions here

Helpful Blogs

Dive deeper into Python and software design principles by visiting DesignGurus.io's blog:

• Essential Software Design Principles You Should Know Before the Interview
• Mastering the FAANG Interview: The Ultimate Guide for Software Engineers

By mastering the use of single and double underscores, you can write more organized, maintainable, and secure Python code. Happy coding!