This may depend on RedKnot, but with type information, this could hit a lot of code. If we know retval is a list, consider converting:

retval.extend([x]) 

to

retval.append(x)

And similarly, merge

retval.extend([x, y])
retval.append(v)
retval.extend([z, w])

etc.

It may not seem like merging will catch a lot of cases, but having the Ruff action available makes it available in my editor, which is a great convenience.