Referencing constants in .srv from Python

asked 2016-04-14 02:14:16 -0500

541 ●51 ●75 ●74

Given the file:

foo.srv

string service_name = foo_service
string in
---
string out

In my C++ code, my_project::foo::Request::service_name works as expected, but how do I do this in Python? I do:

from my_project.srv import foo

at the head of the file, but in the code foo.service_name, fooRequest.service_name, foo.fooRequest.service_name all fail. I can see service_name = 'foo_service' in the generated _foo.py file, so I think it's a problem in my code, not my build process.

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answered 2016-04-14 08:00:20 -0500

lucasw

8729 ●137 ●239 ●256 https://github.com/lucasw

I looked on my system for some example services with constants:

/opt/ros/jade/share$ grep -r "=" * | grep \\.srv
...
gazebo_msgs/srv/GetJointProperties.srv:uint8 FIXED       = 3                # 0 DOF
...

Then

$ rossrv show GetJointProperties
[gazebo_msgs/GetJointProperties]:
string joint_name
---
uint8 REVOLUTE=0
uint8 CONTINUOUS=1
uint8 PRISMATIC=2
uint8 FIXED=3
uint8 BALL=4
...

So it was in the response.

In python:

from gazebo_msgs.srv import GetJointPropertiesResponse
print GetJointPropertiesResponse.FIXED

also:

from gazebo_msgs.srv import GetJointProperties
print GetJointProperties._response_class.FIXED

And the output is 3 as expected. I didn't find an existing example with a string constant so tried adding my own, and it also worked the same.

This shows what is in the class:

print GetJointPropertiesResponse.__dict__.keys()
['_type', 'BALL', '_has_header', 'PRISMATIC', '_slot_types', 'CONTINUOUS', '__module__', 'rate', 'serialize', 'FIXED', '__init__', 'deserialize_numpy', 'success', 'deserialize', 'type', 'UNIVERSAL', '_full_text', '_md5sum', '__doc__', '_get_types', '__slots__', 'damping', 'REVOLUTE', 'position', 'status_message', 'serialize_numpy']

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Thank you; _response_class was the magic I was missing.

KenYN ( 2016-04-14 19:20:55 -0500 )edit

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answered 2018-07-14 11:22:35 -0500

CoffeeKangaroo
65 ●4 ●9 ●11

Even if this is an old question I wanted to answer it since I came across the same problem.

According to the PyStyleGuide is a protected member of the class, thus accessing this is not the way it is meant to be.

A cleaner way to access the constants (IMHO) is to include the classes which are accessed with the _request_class and _response_class members.

In the example given above this would be:

from my_package.srv include MySrv
from my_package.srv include MySrvRequest
from my_package.srv include MySrvResponse

# Do something with:
MySrvRequest.MY_REQUEST_CONSTANT
MySrvResponse.MY_RESPONSE_CONSTANT

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