ros2-foxy-angles from_degrees Python example?

asked 2021-07-13 16:33:27 -0500

RobotDreams
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updated 2021-07-14 08:39:39 -0500

ROS nub

I found https://index.ros.org/p/angles/ and ros2-foxy-angles got installed by rosdep from my package.xml:

  <exec_depend>angles</exec_depend>

The git says it hasn't changed but there are no foxy "API Docs" and the ROS1 docs only show C++.

Where can I find a Python example for what to import?

# get degrees to radians func
from ??? import from_degrees

        self.msg_field_of_view = from_degrees(25.0)     # +/- 12.5 degree FOV (~60cm at max range)

I'm starting to think Foxy does not support angles in Python. (the ROS2 setup.py refers to catkin ) My temp workaround since I only need the degrees to radians func at the moment:

# get degrees to radians func
from math import radians

        self.msg_field_of_view = radians(25.0)     # +/- 12.5 degree FOV (~60cm at max range)

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If all else fails, it's easiest to look at the source code.

Given that angles isn't really that complex (it's essentially a single file) and all functions are pretty well documented, that should provide you with the information you're looking for.

gvdhoorn ( 2021-07-14 04:57:41 -0500 )edit

Thank you for responding, but nowhere in the source code does anything tell me where ROS2 installs the source code.

Case in point: looking at the source for std_msgs.msg.Float32 nowhere does it say to:
- place <exec_depends>std_msgs</exec_depends> in package.xml
- run rosdep install -i --from-path src in your ros2 workspace root
- place "from std_msgs.msg import Float32" in your Python program
and life will be great, but I figured that out from the source code AND the similarity to the ROS2 pub/sub tutorial which publishes a "String" by "from std_msgs.msg import String"

ROS2 angles python setup codeseems to indicate the Python package is "angles", and suggest "from angles import from_degrees" BUT - build succeeds - execution says:

from angles import from_degrees
ModuleNotFoundError: No module named 'angles'

RobotDreams ( 2021-07-14 08:27:00 -0500 )edit

but nowhere in the source code does anything tell me where ROS2 installs the source code.

Not the specific link I referred you to, no.

But angles/CMakeLists.txt does. And so does angles/setup.py, but that's indeed pre-ROS 2.

But in any case: I did not point you "the source code" to figure out where things are installed. I understood your question as asking "what functionality is offered by this package?".

gvdhoorn ( 2021-07-14 09:07:04 -0500 )edit

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answered 2021-07-14 08:59:26 -0500

gvdhoorn
86574 ●283 ●1432 ●1054 http://cor.tudelft.nl/

I'm not sure I understand what you're asking in this case. From your question it appeared you were wondering how to use the angles package, once you've installed it (you wrote "ros2-foxy-angles got installed by rosdep from my package.xml").

I don't understand how std_msgs.msg.Float32 is related.

The link to the source repository is displayed on the ROS Index page you already linked. See #q381559.

but nowhere in the source code does anything tell me where ROS2 installs the source code. [..] I'm starting to think Foxy does not support angles in Python.

While there is nothing in the package itself (ie: the actual business code) that would make it impossible to use this package on ROS 2, it does currently not install the Python-side. The setup.py is indeed pre-ament (meaning: catkin).

The angles package installed by rosdep for you is the C++ side.

If you have a need to use functionality right now, you could copy the relevant function(s) from the __init__.py and use them like that.

It would be nicer however if the Python side could be included in the next release.

I would recommend you open an issue on the tracker to discuss what would be needed to get it released. I would expect it would only take an update of the setup.py and perhaps the CMakeLists.txt.

It could also be the maintainers would like to split the Python and C++ versions of the packages. That would require slightly more changes, but would still not be too much work.

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Thank you for looking deeper into my original question: "Where can I find a Python example for what to import? (after installing ros2-angles-foxy)"

Indeed - the answer is "there is no example, and ros2 foxy does not support Python interface to angles"

As a ROS2 nub, tackling the port of ROS1 gopigo3_node to ROS2 seems to be exposing me to some ROS2 bleeding edges (migrating launch arguments documentation, and now Python angles not yet.)

RobotDreams ( 2021-07-14 09:24:16 -0500 )edit

"Where can I find a Python example for what to import? (after installing ros2-angles-foxy)"

just to be extra clear: there would be nothing special about angles necessarily. It would just be another Python package. The fact it's used with ROS (1 or 2) does not really matter.

gvdhoorn ( 2021-07-14 09:37:22 -0500 )edit

And it seems you opened an issue on the tracker: ros/angles#26.

gvdhoorn ( 2021-07-14 09:38:14 -0500 )edit

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answered 2021-07-14 08:12:03 -0500

BTables
1 ●1

updated 2021-07-14 10:17:52 -0500

Your question seems to indicate you're only looking to use a single function from the angles package. If that's the case it's really simpler, and better design, to just implement the function yourself.

def from_degrees(degrees):  
    return degrees * math.pi / 180.0;

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Would you say that re-implementing something which already exists somewhere else in the form of a reusable library is better design?

gvdhoorn ( 2021-07-14 10:01:23 -0500 )edit

Sorry, by M_PI do you mean import math.pi as M_PI?

I prefer the workaround:
from math import radians
var_rads = radians(var_degrees)

RobotDreams ( 2021-07-14 10:14:11 -0500 )edit

Because the entire package package now depends on a single one-line function that does 2 operations. This makes the package uselessly and needlessly less portable for no reason.

BTables ( 2021-07-14 10:17:13 -0500 )edit

@RobotDreams you're correct. I've been writing c++ all morning and it apparently leaked over here. I've edited my reply.

BTables ( 2021-07-14 10:19:39 -0500 )edit

Because the entire package package now depends on a single one-line function that does 2 operations. This makes the package uselessly and needlessly less portable for no reason.

angles is a very lightweight dependency. And I was referring to the general case.

Your answer -- pre-edit -- was easily understood as "just write something yourself, that's always better design".

And re: your suggestion: if you're going to write something custom for this, at the very least just use math.radians(..). math is part of the Python standard runtime environment.

gvdhoorn ( 2021-07-14 10:22:29 -0500 )edit

Lightweight or not, that package is now tied to a version of ros that has a release of angles. I also never meant to imply it's always better to design to write something yourself. That's why I said I was referencing this specific situation.

BTables ( 2021-07-14 10:36:31 -0500 )edit

angles does much more than from_degrees(..). There's functionality in there which you don't want to duplicate, if only for the fact it has been hardened by years of use.

I'm not really sure I understand how depending on angles is different than rclcpp or anything else. The whole point of ROS is the component based development it allows. If we start to copy / reimplement things everywhere, we negate it.

gvdhoorn ( 2021-07-14 10:42:33 -0500 )edit

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