add ros2 param describe

[dirk-thomas]

Fixes #362.

I chose to use the parameter name as the item without indentation and all other keys are then indented one level. That works nicely for multiple parameters and doesn't use extra empty lines. See example output:

$ ros2 param describe turtlesim background_r background_g
Parameter name: background_r
  Type: integer
  Description: Red channel of the background color
  Constraints:
    Min value: 0
    Max value: 255
Parameter name: background_g
  Type: integer
  Description: Green channel of the background color
  Constraints:
    Min value: 0
    Max value: 255
    Additional constraints: foo

[mjcarroll]

LGTM once the flake8 blank line is clear.

[jacobperron]

LGTM. Since we can know the list of parameters, I think it would make for a nice feature if no parameter names are provided, then we describe all of them.

[dirk-thomas]

Since we can know the list of parameters, I think it would make for a nice feature if no parameter names are provided, then we describe all of them.

We could query the list with an additional service call. Unfortunately the DescribeParameter service doesn't offer a way to query all parameters without specifying them.

[dirk-thomas]

Sanity linter check:

• Linux

[jacobperron]

We could query the list with an additional service call. Unfortunately the DescribeParameter service doesn't offer a way to query all parameters without specifying them.

But I guess the tool knows about them from an additional service call since auto-completion is working?

[dirk-thomas]

But I guess the tool knows about them from an additional service call since auto-completion is working?

Not sure I understand but currently we don't call the list service. While it is being invoked during completion it is not run during the actual command.

As I said: it can certainly be added (with the extra call time). Feel free to do so in a new PR.