feat(librarian): python can infer output directory from library name (#3729)

It's more fiddly to infer the output directory from an API path than
from a known library name. There's already code to infer an API path
from a library name (not for Python, admittedly), so the expectation is
that the library name is always present.

Author: jskeet

internal/librarian/generate.go

@@ -156,10 +156,12 @@ func postGenerate(ctx context.Context, language string) error {
 	}
 }
 
-func defaultOutput(language, api, defaultOut string) string {
+func defaultOutput(language, name, api, defaultOut string) string {
 	switch language {
 	case languageRust:
 		return rust.DefaultOutput(api, defaultOut)
+	case languagePython:
+		return python.DefaultOutputByName(name, defaultOut)
 	default:
 		return defaultOut
 	}

internal/librarian/library.go

@@ -107,7 +107,7 @@ func libraryOutput(language string, lib *config.Library, defaults *config.Defaul
 	if defaults != nil {
 		defaultOut = defaults.Output
 	}
-	return defaultOutput(language, apiPath, defaultOut)
+	return defaultOutput(language, lib.Name, apiPath, defaultOut)
 }
 
 // applyDefaults applies language-specific derivations and fills defaults.
@@ -126,7 +126,7 @@ func applyDefaults(language string, lib *config.Library, defaults *config.Defaul
 		if lib.Veneer {
 			return nil, fmt.Errorf("veneer %q requires an explicit output path", lib.Name)
 		}
-		lib.Output = defaultOutput(language, lib.APIs[0].Path, defaults.Output)
+		lib.Output = defaultOutput(language, lib.Name, lib.APIs[0].Path, defaults.Output)
 	}
 	return fillDefaults(lib, defaults), nil
 }

internal/librarian/python/generate.go

@@ -290,3 +290,10 @@ func cleanUpFilesAfterPostProcessing(repoRoot string) error {
 
 	return nil
 }
+
+// DefaultOutputByName derives an output path from a library name and a default
+// output directory. Currently this just assumes each library is a directory
+// directly underneath the default output directory.
+func DefaultOutputByName(name, defaultOutput string) string {
+	return filepath.Join(defaultOutput, name)
+}

internal/librarian/python/generate_test.go

@@ -374,6 +374,14 @@ func TestGenerate(t *testing.T) {
 	}
 }
 
+func TestDefaultOutputByName(t *testing.T) {
+	want := "packages/google-cloud-secret-manager"
+	got := DefaultOutputByName("google-cloud-secret-manager", "packages")
+	if diff := cmp.Diff(want, got); diff != "" {
+		t.Errorf("mismatch (-want +got):\n%s", diff)
+	}
+}
+
 func requirePythonModule(t *testing.T, module string) {
 	t.Helper()
 	cmd := exec.Command("python3", "-c", fmt.Sprintf("import %s", module))

internal/librarian/tidy.go

@@ -91,7 +91,7 @@ func tidyLibrary(cfg *config.Config, lib *config.Library) error {
 }
 
 func isDerivableOutput(cfg *config.Config, lib *config.Library) bool {
-	derivedOutput := defaultOutput(cfg.Language, lib.APIs[0].Path, cfg.Default.Output)
+	derivedOutput := defaultOutput(cfg.Language, lib.Name, lib.APIs[0].Path, cfg.Default.Output)
 	return lib.Output == derivedOutput
 }