ruamel.yaml dump wraps long plain mapping keys, output unloadable
ruamel.yaml 0.19.1 (round-trip YAML(), and typ='safe' with block style) folds a plain mapping key at the line width: with y.width = 20, {'one two three four five six': 'v'} dumps as one two three four \n five six: v, and every parser, ruamel included, raises 'mapping values are not allowed here'. At the default width 80 it takes a key longer than ~80 chars with a space near the limit. Cause: in Emitter.write_plain, the ruamel-only branch 'words longer than line length get a line of their own' calls write_indent() without checking split (False for simple keys), while the space branch does check it. One-line fix: add split and to that condition. Workaround without patching: set y.width = 4096 (or larger than your longest key), or make long keys quoted scalars.
Reproduced with ruamel.yaml 0.19.1 on CPython 3.14.10 (uv venv). With width=20, both YAML() and typ='safe' dump
'one two three four \n five six: v\n', and both ruamel safe load and PyYAML 6.0.3 fail with 'mapping values are not allowed here'. At the default width, a 99-char key of 'word word ...' breaks the same way, and width=4096 round-trips. I applied your one-line fix (split andin front of(len(data) + self.column) > self.best_widthin Emitter.write_plain, line 1699) to a second venv: the key stays on one line ('one two three four five six: \n v\n'), both loaders get the dict back, and the 99-char key loads too. Long plain values still fold exactly as before:{'k': 'one two three four five six seven eight'}at width 20 gives the same 3-line output, patched and stock.