Add missing pyyaml dependency

[rotu]

ament_clang_format will crash without this dependency with something like:

➜ ament_clang_format
Traceback (most recent call last):
  File "/Users/dan/temp/env/bin/ament_clang_format", line 11, in <module>
    load_entry_point('ament-clang-format', 'console_scripts', 'ament_clang_format')()
  File "/Users/dan/temp/env/lib/python3.7/site-packages/pkg_resources/__init__.py", line 489, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/Users/dan/temp/env/lib/python3.7/site-packages/pkg_resources/__init__.py", line 2793, in load_entry_point
    return ep.load()
  File "/Users/dan/temp/env/lib/python3.7/site-packages/pkg_resources/__init__.py", line 2411, in load
    return self.resolve()
  File "/Users/dan/temp/env/lib/python3.7/site-packages/pkg_resources/__init__.py", line 2417, in resolve
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "/Users/dan/temp/ament_lint/ament_clang_format/ament_clang_format/main.py", line 26, in <module>
    import yaml
ModuleNotFoundError: No module named 'yaml'

[jacobperron]

LGTM, thanks!

Linux:

[dirk-thomas]

The dependency needs to be declared in the package manifest.

[rotu]

The dependency needs to be declared in the package manifest.

Oops. I figured putting it in setup.py was good enough, since typically setuptools makes sure dependency packages get installed.

What's the relation between the package manifest and the setup.py for Python package dependencies? Is there a way to declare it in one place for DRYer code?

[dirk-thomas]

What's the relation between the package manifest and the setup.py for Python package dependencies?

The dependencies declared in the manifest are used to determine the topological build order as well as during packaging are mapped to e.g. Debian package names.

Is there a way to declare it in one place for DRYer code?

No.

[rotu]

The dependencies declared in the manifest are used to determine the topological build order as well as during packaging are mapped to e.g. Debian package names.

Hmmm. That seems problematic for a multiple-interpreter system. If you have your workspace building on top of python3.7 but apt install python3-yaml installs in python3.6, your workspace won't have the dependency at runtime. Why not just get it from PyPi?

[dirk-thomas]

If you have your workspace building on top of python3.7 but apt install python3-yaml installs in python3.6

If you use a non-default Python interpreter on your system the Debian packages will likely not help you much since they are standardized on the default version shipped in that distribution. In that case you do whatever you like to satisfy your dependencies (e.g. a venv).

Why not just get it from PyPi?

Because a Debian package can't depend on a package from PyPi - it must depend on Debian packages.

[rotu]

If you use a non-default Python interpreter on your system the Debian packages will likely not help you much since they are standardized on the default version shipped in that distribution.

So why doesn't colcon install the dependencies from PyPI as per the setup.py documentation? Colcon knows the current interpreter and the current install location. Presumably pip works on Debian - why use python packaging tools but non-standard python dependency resolution?

Edit: found useful discussion here ros/rosdistro#18840

[dirk-thomas]

So why doesn't colcon install the dependencies

colcon intentionally doesn't install any dependencies. It is explicitly listed as a non-goal in the docs. Why? Because the topic is so complex on its own that it is separated into a different tool: rosdep.

Presumably pip works on Debian

There are severe downsides of using pip to install Python packages if there is an equivalent Debian package. E.g. the pip installed package overlay your Debian package. So if in the future your package manager is automatically updating your Debian packages you might have an incompatible version from pip breaking things.