LCM ppath / package-prefix mix results in unexpected behaviour

[gregor-pr]

My project looks like this:

src
  lcm
    message1.lcm
    message2.lcm
  python
    interfaces
      lcm_generated

I want to generate python messages into lcm_generated. When I run
lcm-gen -p --ppath src/python --package-prefix interfaces.lcm_generated src/lcm-types/*.lcm, I get an odd result, where the __init__.py in lcm_generated/package_name/__init__.py contains a subset of the LCM types, for example:

"""LCM package __init__.py file
This file automatically generated by lcm-gen.
DO NOT MODIFY BY HAND!!!!
lcm-gen 1.5.1
"""

from .message1 import foo as foo

if I run the same command with a broken package prefix (notice the / instead of . to separate modules), e.g. lcm-gen -p --ppath src/python --package-prefix interfaces/lcm_generated src/lcm-types/*.lcm,

I get a complete list of the LCM types, such as

"""LCM package __init__.py file
This file automatically generated by lcm-gen.
DO NOT MODIFY BY HAND!!!!
lcm-gen 1.5.1
"""

from .message1 import foo as foo
from .message2 import foo as foo

but the actual message1/2.py files will contain the broken prefix:

import interfaces/lcm_generated.std

if I set the prefix to just one module, the generated files are fine:
lcm-gen -p --ppath src/python --package-prefix lcm_generated src/lcm-types/*.lcm but they'll be generated in the wrong folder. I can extend the ppath like
lcm-gen -p --ppath src/python/interfaces --package-prefix lcm_generated src/lcm-types/*.lcm but that has other implications for my code.

No good solution other than keeping the generated code in the top level python path folder 🤷