With the transition to use
IDL for specifying interfaces in ROS 2 Dashing this article has been superseded by the Interface Definition and Language Mapping article.
This article describes the generated Python code for ROS 2 interfaces.
Authors: Dirk Thomas
Date Written: 2016-01
Last Modified: 2019-03
This article specifies the generated Python code for ROS interface types defined in the interface definition article.
All code of a ROS package should be defined in a Python package with the same name. To separate the generated code from other code within the package it is defined in a sub module:
NOTE: The names are currently identical to the ones used in ROS 1. Therefore it is not possible to import both in a Python application.
Following the Python conventions the namespace hierarchy is mapped to a folder structure.
The package and module names and therefore the folder and file names use lowercase alphanumeric characters with underscores for separating words (following PEP 8).
Python files end with
For a message a Python
class with the same name as the message is generated in the file
The Python module
<package_name>.srv exports all message / service classes without the message module name to shorten import statements, e.g.
|ROS type||Python type|
|byte||builtins.bytes with length 1|
|char||builtins.str with length 1|
|ROS type||Python type|
|unbounded dynamic array||builtins.list|
|bounded dynamic array||builtins.list|
The class has same-named properties for every field of the message. The setter of a property will ensure that ROS type constraints beyond the Python type are enforced.
A constant is defined as a class variable with uppercase name. The class variable is considered to be read-only which is ensured by overridding the setter or a magic method.
The init function initializes all members with their default value.
The class constructor supports only keyword arguments for all members. The short reason for this is that if code would rely on positional arguments to construct data objects changing a message definition would break existing code in subtle ways. Since this would discourage evolution of message definitions the data structures should be populated by setting the members separately, e.g. using the setter methods.
TODO: decide which magic methods should be provided
For a service a
class with the same name is generated.
The class contains only two
Requestwhich is the type of the request part of the service
Responsewhich is the type of the request part of the service
The generated code is split across multiple files the same way as message are.
For the request and response parts of a service separate messages are being generated.
These messages are named after the service and have either a
They are are still defined in the
srv sub namespace.