# Documentation standards for petsc4py

Subject to exceptions given below, new contributions to petsc4py must include `type annotations` for function parameters and results, and docstrings on every class, function and method.

The documentation should be consistent with the corresponding C API documentation, including copying text where this is appropriate. More in-depth documentation from the C API (such as extended discussions of algorithmic or performance factors) should not be copied.

## Docstring standards

Docstrings are to be written in `numpydoc:format` format.

The first line of a class, function or method docstring must be a short description of the method in imperative mood (“Return the norm of the matrix.”) “Return” is to be preferred over “Get” in this sentence. A blank line must follow this description. Use one-liner descriptions for properties.

If the corresponding C API documentation of a method lists a function as being collective, then this information must be repeated on the next line of the docstring. Valid strings are: “Not collective.”, “Logically collective.”, “Collective.”, “Neighborwise collective.”, or “Collective the first time it is called”.

The initial description section can contain more information if this is useful. In particular, if there is a PETSc manual chapter about a class, then this should be referred to from here.

Use double backticks around literals (like strings and numbers), e.g., ``2``, ``”foo”``.

Reference PETSc functions simply using backticks, e.g., `KSP` refers to the PETSc C documentation for KSP. Do not use URLs in docstrings. Always use Intersphinx references.

The following sections describe the use of numpydoc sections. Other sections allowed by numpydoc may be included if they are useful.

### Parameters

This is required for methods unless there are no parameters, or it will be completely obvious to even a novice user what the parameters do.

If a class has a non-trivial constructor, the arguments of the constructor and their types must be explicitly documented within this section.

For methods, types should only be specified in this section if for some reason the types provided by typing prove to be inadequate. If no type is being specified, do not include a colon (`:`) to the right of the parameter name.

Use `Sys.getDefaultComm` when specifying the default communicator.

### Returns

This should only be specified if the return value is not obvious from the initial description and typing.

If a “Returns” section is required, the type of the returned items must be specified, even if this duplicates typing information.

### See Also

If any of the following apply, then this section is required. The order of entries is as follows. Other links are permitted in this section if they add information useful to users.

Every `setFromOptions` must include the link `petsc_options`.

Any closely related part of the petsc4py API not already linked in the docstring should appear (e.g. setters and getters should cross-refer).

If there is a corresponding C API documentation page, this must be linked from the “See also” section, e.g. `petsc.MatSetValues`.

End docstring with an empty line - “closing three quotation marks must be on a line by itself, preferably preceded by a blank line”

## Type hint standards

If returning `self`, use `-> Self` in function signature.

Type hints are not required when the static type signature includes a PETSc type (e.g. `Vec x`). These will be automatically generated. This will also work for `= None`. When using type hints, use spacing around the equals in any `= None`.

Communicators in type signatures must use Python typing instead of c-typing (i.e. `comm: Comm` not `Comm comm`). This is because communicators can come from `mpi4py` and not just the `petsc4py.PETSc.Comm` class.

For petsc4py native types that are strings, the type is `argument: KSP.Type | str` (not e.g.: `KSPType argument`). If the type is strictly an enum the `| str` can be omitted. Full signature example:

    def setType(self, ksp_type: KSP.Type | str) -> None:

If a NumPy array is returned, use `ArrayBool`/`ArrayInt`/`ArrayReal`/`ArrayScalar` as the return type.