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vspec Design Decisions, Architecture and Limitations

Overlays

Each overlay is treated as a separate tree and merged (after expansion) on the existing tree. This means that descriptions, comments, datatype, unit and similar attributes gets overwritten if existing.

Expansion

Expansion is the process where a branch with instances is replaced with multiple branches.

This will result in that e.g. Vehicle.Cabin.Door is expanded to the following branches

  • Vehicle.Cabin.Door.Row1.Left
  • Vehicle.Cabin.Door.Row1.Right
  • Vehicle.Cabin.Door.Row2.Left
  • Vehicle.Cabin.Door.Row2.Right

For some exporters expansion can be suppressed by using the --no-expand option. Then instance information will be represented by other means in the resulting output.

Deletion / Node removal

Nodes can be removed from the tree by using the delete element in the overlay. This is useful when a signal is not used in a next version of the specification or if you simply want to delete a node or branch from the specification, e.g. you are not using a signal of the base specification. Please note, that the deletion for branches will delete all nodes that are connected to that branch (which is desired behavior). Also, if a branch node is deleted, all nodes that are connected to that branch will be deleted irrespective of what their delete element value is.

We chose three examples to show what you can do with the delete element. Let's say we have the following excerpt from the base vehicle signal specification:

Vehicle.Service:
  description: Service data.
  type: branch

Vehicle.Service.DistanceToService:
  datatype: float
  description: Remaining distance to service (of any kind). Negative values indicate service overdue.
  type: sensor
  unit: km

Vehicle.Service.IsServiceDue:
  datatype: boolean
  description: Indicates if vehicle needs service (of any kind). True = Service needed now or in the near future. False = No known need for service.
  type: sensor

Vehicle.Service.TimeToService:
  datatype: int32
  description: Remaining time to service (of any kind). Negative values indicate service overdue.
  type: sensor
  unit: s

Now if you want to delete the Vehicle.Service.TimeToService node from the specification, you can do this by adding the delete element to your overlay like this:

Vehicle.Service.TimeToService:
  type: sensor
  datatype: int32
  delete: true

Let's say you now want to delete the whole branch Vehicle.Service from the specification. You can do this by adding:

Vehicle.Service:
  type: branch
  delete: true

Also, the delement element can be used on instances after they have been expanded. If you want to delete a node or branch that has been expanded using instances you can add the delete element to the overlay, let's say the vehicle only has two doors in the front. In this case we would like to delete the signals for the rear doors:

Vehicle.Cabin.Door.Row2:
  type: branch
  delete: true

By adding the delete: true to a node or branch all nodes and branches connected to it are deleted by vss-tools when converting to a different format.

Please note that for branches you need to provide at least the type element to the overlay. For nodes you at least have to provide the type and datatype elements. Currently, the elements provided do not have to match the previously given elements in the base specification.

Expansion and Overlays

Sometimes an overlay only refers to a signal in a specific branch, like:

Vehicle.Cabin.Door.Row2.Right.NewSignal:
  datatype: int8
  type: sensor
  unit: km
  description: A new signal for just one door.

We do not want this signal expanded, and the tooling prevents expansion by taking the instance Row2 and comparing with instances declared for Door.

Door:
  type: branch
  instances:
    - Row[1,2]
    - [ "Left","Right" ]
  description: d1
  comment: c1

It will do this by taking the instances on first level (Row1 and Row2) and if comparing with current branch (Row2). If they match it will repeat the check for Rightand finally merge NewSignal with other signals in Vehicle.Cabin.Door.

Description, comments and other data defined for a specific instance (like Vehicle.Cabin.Door.Row2.Right.NewSignal above) have precedence over data defined for the unexpanded signal Vehicle.Cabin.Door.NewSignal.

The merge algorithm tries to be smart, so that if you use Row* it assume it is an instantiated branch if branch has an instance declaration of type Row[m,n], even if the the value of Row* is outside the range. It will however in that case not inherit values from the base branch.

CLI Design

The command line interface is designed with click. We are making use of subcommands and evaluating them lazy. That means that we can define the cli interface of subcommands in their own files. The main cli entrypoint for vspec is defined in vspec.py. There you can also see all the subcommands referenced. When building a new exporter tool, this is the place that needs to be adapted as well as creating a new file similar to the other exporters in vssexporters.

It is also possible to build a complete different script entrypoint and not hook into the current interface. All cli options can be imported and used via cli_options.py. Only a few are left out which are defined on the toplevel command just as --log-level, --log-file and --version.

Linters and Static Code Checkers

MyPy

Mypy is used for static type checking of code. General ambition is to resolve errors reported by mypy, but when not feasible errors are suppressed by using # type: ignore[<type>] as comment on the concerned line. Continuous Integration requires that no Mypy errors are reported.

To run manually:

mypy src tests contrib

Suppressed error categories include:

  • Template types in constants.py have expectations on subclasses that may not be fulfilled in the general case, but are fulfilled in our cases.
  • Some dependencies like anytree and deprecation does not have library stubs or py.typed and can thus not be analyzed.
  • Mypy does not like "method variables" like load_flat_model.include_index

Flake8

Flake8 is used as linter. It is also configured to be used as pre-commit hook. The Continuous Integration currently accepts errors reported by Flake8, this might change in the future. New or changed code shall not produce any Flake8 issues.

The project use the following deviations compared to default settings:

  • Max line length is 120

To enable the hook to check when creating a commit:

pre-commit install

To run manually:

flake8 src tests contrib