How to Use a SqlAlchemy BinaryExpression as a Filter on a List of Objects in Python

Learn how to efficiently apply SqlAlchemy `BinaryExpression` filters on Python lists without using a database session. Use this step-by-step guide to implement object filtering with mock database behavior.
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Filtering Lists with SqlAlchemy BinaryExpression

When working with Python, you may find yourself needing to filter a list of objects based on certain criteria, especially when interacting with databases using SQLAlchemy. This can seem challenging when you are trying to take advantage of the powerful querying capabilities of SQLAlchemy but want to filter Python objects without involving a database session. In this post, we will explore how to achieve this using SqlAlchemy's BinaryExpression.

The Problem

Suppose you have a list of dataclass objects defined as follows:

[[See Video to Reveal this Text or Code Snippet]]

Your database model in SqlAlchemy looks something like this:

[[See Video to Reveal this Text or Code Snippet]]

The Solution

Let's break down the solution step-by-step.

Basic Filtering with Built-in Python Operators

To begin with, you can create a simple function to filter your list. Here’s how:

Define your expression (e.g., filtering where attr equals 25):

[[See Video to Reveal this Text or Code Snippet]]

Implement the convert function as follows:

[[See Video to Reveal this Text or Code Snippet]]

Use a list comprehension to filter obj_list:

[[See Video to Reveal this Text or Code Snippet]]

This approach will correctly filter obj_list based on the criteria defined in expr.

Expanding Functionality with Custom Attributes

For more complex scenarios, such as using different SQLAlchemy operations, you can create a fake attribute class that mimics the behavior of SQLAlchemy attributes. Here’s how to do it:

Define the FakeAttribute class to handle various operations:

[[See Video to Reveal this Text or Code Snippet]]

Update the convert function to use FakeAttribute:

[[See Video to Reveal this Text or Code Snippet]]

Example of Use

Now you can use the updated setup with complex expressions:

Filtering Greater Than:

[[See Video to Reveal this Text or Code Snippet]]

Filtering with IN:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By utilizing SqlAlchemy's BinaryExpression capabilities within a standalone context, you can effectively filter lists of dataclass objects in Python without needing to connect to a database session. This method is particularly useful when creating mock databases for testing or custom implementations.

Feel free to experiment with the provided code and adapt it for your specific use case. Your ability to manipulate data in such a manner will greatly enhance your flexibility when working with Python and SQLAlchemy.