Add some type hints to aid code completion in IDEs.

[robfalck]

Desired capability or behavior.

Type hints have been available in Python since 3.5. They have no impact on code execution, but can aid IDEs for autocompletion. OpenMDAO should add some hints for certain attributes and methods for which introspection is difficult.

For the first round of type hinting, OpenMDAO should provide hints for:

• Problem.driver: typing.Type[Driver]
• Problem.model: typing.Type[System]
• Sytem.nonlinear_solver: typing.Type[NonlinearSolver]
• System.linear_solver: typing.Type[LinearSolver]
• NonlinearSolver.linesearch: typing.Type[LinesearchSolver]

And Group.add_subsystem, which returns the same type input

T = TypeVar("T", bound="System")

def self.add_subsystem(self, subsys=typing.Type[T], ...) -> T:

Note that the build-in function type replaces typing.Type as of 3.8, but we still support older versions.

Testing

Various linters are available to detect that these types are being applied correctly. mypy is common for static analysis, but others are available and may work better in our CI system.

Associated POEM

N/A