DOP-4945: Default tab available for drivers tabs (#629)

* added default_tabs to page options

* add default-tabid option

* use in syntax, remove example

* PR feedback

* Added diagnostic if default tab id is not present

* PR feedback - type certainty, correct docs

snooty/diagnostics.py

@@ -1042,3 +1042,19 @@ def __init__(
             start,
             end,
         )
+
+
+class UnknownDefaultTabId(Diagnostic):
+    severity = Diagnostic.Level.warning
+
+    def __init__(
+        self,
+        unknown_id: str,
+        start: Union[int, Tuple[int, int]],
+        end: Union[None, int, Tuple[int, int]] = None,
+    ):
+        super().__init__(
+            f"Option :default-tabid: '{unknown_id}' is not present in tabs on page.",
+            start,
+            end,
+        )

snooty/postprocess.py

@@ -63,6 +63,7 @@
     SubstitutionRefError,
     TargetNotFound,
     UnexpectedDirectiveOrder,
+    UnknownDefaultTabId,
     UnnamedPage,
     UnsupportedFormat,
 )
@@ -578,6 +579,7 @@ def exit_node(self, fileid_stack: FileIdStack, node: n.Node) -> None:
 class TabsSelectorHandler(Handler):
     def __init__(self, context: Context) -> None:
         super().__init__(context)
+        self.default_tabs: Dict[str, str] = {}
         self.selectors: Dict[str, List[Dict[str, MutableSequence[n.Text]]]] = {}
         self.scanned_pattern: List[str] = []
         self.target_pattern = ["tabs", "tabs", "procedure"]
@@ -619,6 +621,9 @@ def enter_node(self, fileid_stack: FileIdStack, node: n.Node) -> None:
                 )
                 return
 
+            if tabset_name == "drivers" and "default-tabid" in node.options:
+                self.default_tabs[tabset_name] = node.options.get("default-tabid", "")
+
             self.selectors[tabset_name] = []
             return
 
@@ -677,6 +682,26 @@ def exit_page(self, fileid_stack: FileIdStack, page: Page) -> None:
                     for tabid, title in tabsets[0].items()
                 }
 
+                # If default_tabs are present, append to page options
+                if tabset_name in self.default_tabs:
+                    default_tab_is_in_selectors = (
+                        self.default_tabs[tabset_name]
+                        in page.ast.options["selectors"][tabset_name].keys()
+                    )
+                    if not default_tab_is_in_selectors:
+                        self.context.diagnostics[fileid_stack.current].append(
+                            UnknownDefaultTabId(self.default_tabs[tabset_name] or "", 0)
+                        )
+                        return
+
+                    if not page.ast.options.get("default_tabs"):
+                        page.ast.options["default_tabs"] = {}
+
+                    assert isinstance(page.ast.options["default_tabs"], Dict)
+                    page.ast.options["default_tabs"][tabset_name] = self.default_tabs[
+                        tabset_name
+                    ]
+
 
 class TargetHandler(Handler):
     def __init__(self, context: Context) -> None:

snooty/rstspec.toml

@@ -344,6 +344,10 @@ options.glob = "flag"
 [directive.tabs-selector]
 help = """Add dropdown selector used to select the specified tabset."""
 argument_type = "string"
+options.default-tabid = "string"
+example = """.. tabs-selector:: ${1: string}
+   :default-tabid: ${2:id of the default tab (Optional for drivers)}
+"""
 
 [directive.tabs-pillstrip]
 deprecated = true

snooty/test_postprocess.py

@@ -32,6 +32,7 @@
     TabMustBeDirective,
     TargetNotFound,
     UnexpectedDirectiveOrder,
+    UnknownDefaultTabId,
 )
 from .n import FileId
 from .util_test import (
@@ -4529,3 +4530,76 @@ def test_footnote_id_numbers() -> None:
 </root>
 """,
         )
+
+
+def test_default_tabs() -> None:
+    with make_test(
+        {
+            Path(
+                "source/index.txt"
+            ): """
+===================
+Heading of the page
+===================
+
+.. tabs-selector:: drivers
+   :default-tabid: python
+
+.. tabs-drivers::
+
+   .. tab::
+      :tabid: c
+
+      C
+
+   .. tab::
+      :tabid: nodejs
+
+      Node.js
+
+   .. tab::
+      :tabid: python
+
+      Python
+""",
+        }
+    ) as result:
+        page = result.pages[FileId("index.txt")]
+        assert (page.ast.options.get("default_tabs")) == {"drivers": "python"}
+
+
+def test_default_tabs_not_present() -> None:
+    with make_test(
+        {
+            Path(
+                "source/index.txt"
+            ): """
+===================
+Heading of the page
+===================
+
+.. tabs-selector:: drivers
+   :default-tabid: no-language
+
+.. tabs-drivers::
+
+   .. tab::
+      :tabid: c
+
+      C
+
+   .. tab::
+      :tabid: nodejs
+
+      Node.js
+
+   .. tab::
+      :tabid: python
+
+      Python
+""",
+        }
+    ) as result:
+        diagnostics = result.diagnostics[FileId("index.txt")]
+        assert len(diagnostics) == 1
+        assert isinstance(diagnostics[0], UnknownDefaultTabId)