When optimizing Python code for multi-threading, it's important to understand the Global Interpreter Lock (GIL) (b) and how it can impact performance. Using global variables (a) for sharing data in multi-threaded code can lead to synchronization issues and should generally be avoided. The choice between multi-threading and multi-processing (c) depends on the nature of the tasks. Using the same thread for all tasks (d) would not take advantage of multi-threading.

In Python, to represent a constant property in a class, you would typically use the readonly keyword. It ensures that the property cannot be modified after it's assigned a value.

To obtain all the keys from a Python dictionary, you should use the keys() method. This method returns a view object that displays a list of all the keys in the dictionary.

To find the shortest path in a graph with non-negative edge weights, you can use Dijkstra's algorithm. This algorithm efficiently calculates the shortest path from a source vertex to all other vertices in a weighted graph.

To enable database migrations in Flask, the Flask-Migrate extension is used. Flask-Migrate is an extension that integrates the Alembic database migration framework with Flask applications, allowing you to manage database schema changes easily.

To define a metaclass, you generally override the __new__ method. This method is responsible for creating the class itself and can be customized to modify class attributes and behavior during creation.

In Python, to define a generator, you use the yield keyword within a function. A generator function produces an iterator, which can be used to generate a sequence of values lazily, conserving memory and improving performance.

In Python, the pdb module is used for debugging. It allows you to set breakpoints, step through code, and inspect variable values during program execution.

In Django, you create new URL patterns by adding them to the urls.py file of your app. This file maps URLs to view functions, allowing you to define the routing of your web application.

To convert a list into a set in Python, you can use the set() constructor. It takes an iterable, such as a list, as an argument and creates a new set containing the unique elements from the list.

In Python, you can concatenate two tuples using the + operator. The + operator is used for concatenating sequences, including tuples. For example, tuple1 + tuple2 will combine the elements of both tuples.

The process of visiting all the nodes of a tree and performing an operation is known as tree traversal. Tree traversal is essential for various tree-based algorithms and operations like printing the contents of a tree.