I have a shapefile with line features. One of my lines was causing problems in a python script and in the QGIS interface it appeared as two segments (one long one with a brief gap between two nodes).

I used the calculator with num_geometries( $geometries) and it showed this entry has two geometries (all the others had one). I imagine this is the problem. Is there any way to fix this and merge them back to a single geometry? Deleting and re-digitizing would be a lot of effort. I didn't see anything in the Validity checker and it didn't complain, but that might be because I messed up a parameter.

The merge tools in QGIS digitizing bar is not relevant. It is for merging multiple features, not multiple gometries in one feature -- it complains I have only one feature selected.

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    Try if explode lines (your selected line) and afterwards a re-merging solves the problem. Multipart to single-part-geometry might be worth a try as well. – Babel May 9 '20 at 18:39

You have a multi-line geometry.

You can use the Merge lines algorithm after you have connected the the two parts, so that they form a continuous line.

You can also split the multi-geometry into two (simple) geometries using the Multipart to single parts algorithm, join them (using the Vertex Tool in the Digitizing Toolbar) to form a continuous line and then combine them into one line using the Merge Selected Features tool in the *Advanced digitizing toolbar".

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