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TIL: Plotly Dash uses the order of the pages in the pages registry to evaluate whether the requested URL matches a page's `path_template` value. By default, pages are ordered alphabetically based on the pages filename.

In my case, I had two pages defined with the following `path_template` values:
- `/datasets/<dataset_id>` in `pages/dataset.py`
- `/datasets/<dataset_id>/labelsets/<labelset_id>` in `pages/labelset.py`

When trying to open a URL matching the second template, it was matching the first one and using the remainder of the URL as `dataset_id`.

Setting the `order` parameter in the `dash.register_page()` function changes the order of the pages in the page registry, thus changing the path matching order. To match the longest/nested `path_template`, the corresponding page has to be registered with a lower `order` value.

That makes sense in hindsight but the docs do not mention that the path evaluation is affected by the page order.

https://dash.plotly.com/urls

#Plotly #PlotlyDash #Dash #Python #WebDev