I recently discovered the GeoAlchemy2 project – a replacement for the original GeoAlchemy package, focused on providing PostGIS support for SQLAlchemy. The SQLAlchemy package is a “Python SQL Toolkit and Object Relational Mapper”. In a nutshell this means you can write Python classes and map them to PostgreSQL tables without the need to write SQL statements – pretty cool!

PostGIS is great for doing spatial stuff, but if you’re using it as back-end for a Python app then you can spend a lot of time writing Python wrappers around SQL statements, and even with the excellent Psycopg2 package this can be tricky. This is especially true if you’re using the OGR Python bindings to handle PostGIS read/writes.

Enter GeoAlchmey2. I’ve been experimenting with it for a week, in that time I’ve learnt this:

For developing geospatial Python apps with PostGIS, GeoAlchemy2 is nothing short of revolutionary.

You can call PostGIS functions in Python, which means you can use them (and the data) directly within your Python application logic. Here’s an example. The SQL statement below uses PostGIS to create a new line geometry between a point, and the closest point on the nearest line.

Now here’s a snippet from a Python script, performing the same process using GeoAlchemy2.

You’ll notice here that we’re actually calling our own Python function “make_link_line” during the query to create the new geometry. This exemplifies how we can move PostGIS objects around inside the script. Once the query runs we can access the returned data in our application from the row variable. Below is the complete script.

Nearest neighbour PostGIS and GeoAlchemy2 script:

The script above is just a simple example, but it shows how powerful GeoAlchemy2 is for embedding PostGIS objects and methods inside Python. I’m really looking forward to digging deeper into the functionality of GeoAlchemy2 and SQLAlchemy to integrate them within my own projects. Check out the official tutorials for more examples: https://geoalchemy-2.readthedocs.org/en/latest/#tutorials

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