doc: autogenerate python interpreter table (#313408)

* doc: autogenerate python interpreter table

This serves as a practical example on generating documentation by
inspection of the evaluated Nixpkgs tree.

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

doc/default.nix

@@ -105,7 +105,15 @@ in pkgs.stdenv.mkDerivation {
     ln -s ${optionsDoc.optionsJSON}/share/doc/nixos/options.json ./config-options.json
   '';
 
-  buildPhase = ''
+  buildPhase = let
+    pythonInterpreterTable = pkgs.callPackage ./doc-support/python-interpreter-table.nix {};
+    pythonSection = with lib.strings; replaceStrings
+      [ "@python-interpreter-table@" ]
+      [ pythonInterpreterTable ]
+      (readFile ./languages-frameworks/python.section.md);
+  in ''
+    cp ${builtins.toFile "python.section.md" pythonSection} ./languages-frameworks/python.section.md
+
     cat \
       ./functions/library.md.in \
       ${lib-docs}/index.md \

doc/doc-support/python-interpreter-table.nix

@@ -0,0 +1,63 @@
+# For debugging, run in this directory:
+#     nix eval --impure --raw --expr 'import ./python-interpreter-table.nix {}'
+{ pkgs ? (import ../.. { config = { }; overlays = []; }) }:
+let
+  lib = pkgs.lib;
+  inherit (lib.attrsets) attrNames filterAttrs;
+  inherit (lib.lists) elem filter map naturalSort reverseList;
+  inherit (lib.strings) concatStringsSep;
+
+  isPythonInterpreter = name:
+    /* NB: Package names that don't follow the regular expression:
+      - `python-cosmopolitan` is not part of `pkgs.pythonInterpreters`.
+      - `_prebuilt` interpreters are used for bootstrapping internally.
+      - `python3Minimal` contains python packages, left behind conservatively.
+      - `rustpython` lacks `pythonVersion` and `implementation`.
+    */
+    (lib.strings.match "(pypy|python)([[:digit:]]*)" name) != null;
+
+  interpreterName = pname:
+    let
+      cuteName = {
+        cpython = "CPython";
+        pypy = "PyPy";
+      };
+      interpreter = pkgs.${pname};
+    in
+    "${cuteName.${interpreter.implementation}} ${interpreter.pythonVersion}";
+
+  interpreters = reverseList (naturalSort (
+    filter isPythonInterpreter (attrNames pkgs.pythonInterpreters)
+  ));
+
+  aliases = pname:
+    attrNames (
+      filterAttrs (name: value:
+        isPythonInterpreter name
+        && name != pname
+        && interpreterName name == interpreterName pname
+      ) pkgs
+    );
+
+  result = map (pname: {
+    inherit pname;
+    aliases = aliases pname;
+    interpreter = interpreterName pname;
+  }) interpreters;
+
+  toMarkdown = data:
+    let
+      line = package: ''
+        | ${package.pname} | ${join ", " package.aliases or [ ]} | ${package.interpreter} |
+      '';
+    in
+    join "" (map line data);
+
+  join = lib.strings.concatStringsSep;
+
+in
+''
+  | Package | Aliases | Interpeter |
+  |---------|---------|------------|
+  ${toMarkdown result}
+''

doc/languages-frameworks/python.section.md

@@ -4,16 +4,7 @@
 
 ### Interpreters {#interpreters}
 
-| Package    | Aliases         | Interpreter |
-|------------|-----------------|-------------|
-| python27   | python2, python | CPython 2.7 |
-| python39   |                 | CPython 3.9 |
-| python310  |                 | CPython 3.10 |
-| python311  | python3         | CPython 3.11 |
-| python312  |                 | CPython 3.12 |
-| python313  |                 | CPython 3.13 |
-| pypy27     | pypy2, pypy     | PyPy2.7 |
-| pypy39     | pypy3           | PyPy 3.9 |
+@python-interpreter-table@
 
 The Nix expressions for the interpreters can be found in
 `pkgs/development/interpreters/python`.