Add abstract for python storage client

[googlyrahman]

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Summary of Changes

Hello @googlyrahman, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces an abstract base class, BaseClient, to the Python Storage client library. This refactoring centralizes shared client initialization, connection management, and defines a clear interface for resource operations through abstract methods. The primary goal is to enhance code modularity, improve maintainability, and establish a foundational structure that can support future client implementations or extensions more effectively.

Highlights

• Introduction of BaseClient: A new abstract base class, BaseClient, has been added to google/cloud/storage/abstracts/base_client.py. This class centralizes common client functionalities and serves as a foundation for concrete client implementations.
• Refactored Client Initialization: Core client initialization logic, including credential handling, API endpoint determination (emulator, override, universe domain), and client options processing, has been moved into the BaseClient's __init__ method, reducing duplication.
• Abstract Resource Operations: The BaseClient now defines abstract methods for fundamental resource operations like bucket(), _get_resource(), _list_resource(), _patch_resource(), _put_resource(), _post_resource(), and _delete_resource(), enforcing their implementation in derived classes.
• Client Class Simplification: The existing Client class in google/cloud/storage/client.py has been refactored to inherit from BaseClient, significantly reducing its internal complexity by delegating common logic to the new base class.

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[gemini-code-assist]

Code Review

This pull request introduces an abstract base class BaseClient for the python-storage client, refactoring common logic from the existing Client class. This is a good architectural improvement. My review focuses on ensuring the correctness of the refactoring and adherence to Python best practices.

I've identified a critical issue with the use of NotImplemented instead of NotImplementedError in the new abstract methods. Additionally, there are a couple of high-severity issues: one concerning a mutable default argument in the BaseClient constructor, and another related to potential dead code in the Client class's __init__ method. I've also included a minor style suggestion. After addressing these points, the code will be more robust and maintainable.

[gemini-code-assist]

Using a mutable object like a dictionary as a default argument can lead to unexpected behavior. If this dictionary is modified by a caller, the change will persist across subsequent calls that use the default value. It's safer to use None as the default and create a new dictionary inside the method if None is passed.

To fix this, you can change this line to extra_headers=None, and then add if extra_headers is None: extra_headers = {} at the beginning of the __init__ method.

Suggested change

	extra_headers={},
	extra_headers=None,