Python-OGR: nested loop only loops once

Python-OGR: Nested Loops Acting Up? Here’s Why (and How to Fix It!)

Ever been wrestling with geospatial data in Python, using OGR (that part of the GDAL library), and run into a weird problem where your nested loop only seems to loop once on the inside? Trust me, you’re not alone. It’s a super common head-scratcher, and it all boils down to how OGR handles reading features. Once you get the hang of it, though, it’s a pretty easy fix, and you’ll save yourself a ton of debugging.

The Culprit: OGR’s One-Way Ticket Through Features

Basically, OGR reads features from a layer in order, like reading a book page by page. It keeps track of where it is with an internal “read position.” So, when you loop through a layer using a for loop in Python, OGR moves that read position forward each time it grabs a feature. The gotcha? After the inner loop finishes its first full run, that read position is sitting right at the end of the layer. The next time the outer loop rolls around and the inner loop tries to start again, OGR’s already at the end of the line – nothing left to read! That’s why it only loops once.

Let’s look at a quick example:

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