PARAM-4001 ros2 runtime_error ai_generated true

rclcpp.exceptions.ParameterAlreadyDeclaredException: parameter 'my_param' has already been declared

ID: ros2/parameter-declare-not-allowed-during-callback

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93%Fix Rate
89%Confidence
1Evidence
2023-12-05First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
ROS2 Humble Hawksbill active — — —
ROS2 Iron Irwini active — — —
ROS2 Jazzy Jalisco active — — —

Root Cause

A node attempts to declare a parameter that was already declared, often due to calling `declare_parameter` twice in the same node, or because the parameter was automatically declared by a parent class (e.g., `use_sim_time`).

generic

中文

节点尝试声明一个已经声明过的参数，通常是由于在同一个节点中两次调用 `declare_parameter`，或者因为该参数已被父类自动声明（例如 `use_sim_time`）。

Official Documentation

https://docs.ros.org/en/humble/Tutorials/Intermediate/Parameters/Understanding-ROS2-Parameters.html

Workarounds

  1. 95% success Check if the parameter already exists before declaring, using `has_parameter` in C++ or Python. This is especially important when inheriting from nodes that declare common parameters like 'use_sim_time'. 
    Check if the parameter already exists before declaring, using `has_parameter` in C++ or Python. This is especially important when inheriting from nodes that declare common parameters like 'use_sim_time'.
  2. 90% success If the parameter is declared by a parent class (e.g., from `rclcpp::Node`), avoid redeclaring it. Instead, set its value via the node options or parameter overrides. 
    If the parameter is declared by a parent class (e.g., from `rclcpp::Node`), avoid redeclaring it. Instead, set its value via the node options or parameter overrides.

中文步骤

  1. Check if the parameter already exists before declaring, using `has_parameter` in C++ or Python. This is especially important when inheriting from nodes that declare common parameters like 'use_sim_time'.
  2. If the parameter is declared by a parent class (e.g., from `rclcpp::Node`), avoid redeclaring it. Instead, set its value via the node options or parameter overrides.

Dead Ends

Common approaches that don't work:

  1. 50% fail

    Wrapping the declare_parameter call in a try-except block to ignore the exception may cause the parameter to have an unintended default value, leading to subtle bugs.

  2. 60% fail

    Changing the parameter name to a different string (e.g., 'my_param_2') avoids the error but doesn't fix the duplicate declaration; it may also break other code expecting the original name.