Best way to add a topic to an existing rosbag?

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Based on the rosbag API link (which only provides the 'w' option for writing, and this erases an existing bag) and the merge code, which reads the old bag and re-writes to a new one... the answer is actually "you can't add a topic to an existing bag," right?

In other words, the first sentence is misleading and I think should be "You can't do this in place. You have to create a new bag in w mode, open the existing bag in r mode, re-write all old messages to the new bag, adding what you want on top."

I believe "misleading" is a bit strong: only very few storage formats that support streaming interfaces / infrastructure will allow you to append data to the end and still have everything work correctly (ie: stream the contents in chronological order). It's simply not something that ..

.. works that way. And rosbag is no exception.

What you write in your comment ("read, write while adding") is at least implied by the "open [..], and write it out while adding the new messages for the topic".

I interpreted the question to be about modifying a file. I haven't tried, and wouldn't recommend, but foo = bag(file.bag, 'r') and bar = bag(file.bag, 'w') and then writing foo into bar + extra could be possible. You'd lose the original forever.

For the answer, it seems simpler to write the details of the method in addition to linking vs. just linking with ambiguous phrasing. This suggests the links could hold the answer. I know how to re-write... and I was hoping to append, so I expected to be enlightened.

So that's what I meant by misleading. The answer isn't wrong, it just dangled hope before me but when I looked at the links I realized it's what I've already been doing (and could have been spared time reading). Python can append to files, so it's reasonable to think bags might allow it, too.

The question posted by the OP did not ask for code, it only asked for "the best way". There is no point in repeatedly posting code for the same thing so a hint about where to look for information seems much more appropriate.