Allow Pylance to be used with Python 2 if explicitly chosen (#16210) (#16214)

Author: jakebailey

news/2 Fixes/16204.md

@@ -0,0 +1 @@
+Allow Pylance to be used with Python 2 if "Pylance" is explicitly set as the language server.

src/client/activation/activationService.ts

@@ -212,6 +212,11 @@ export class LanguageServerExtensionActivationService
         return configurationService.getSettings(this.resource).languageServer;
     }
 
+    private getCurrentLanguageServerTypeIsDefault(): boolean {
+        const configurationService = this.serviceContainer.get<IConfigurationService>(IConfigurationService);
+        return configurationService.getSettings(this.resource).languageServerIsDefault;
+    }
+
     private async createRefCountedServer(
         resource: Resource,
         interpreter: PythonEnvironment | undefined,
@@ -240,10 +245,14 @@ export class LanguageServerExtensionActivationService
             }
         }
 
-        if (serverType === LanguageServerType.Node && interpreter && interpreter.version) {
-            if (interpreter.version.major < 3) {
-                sendTelemetryEvent(EventName.JEDI_FALLBACK);
-                serverType = LanguageServerType.Jedi;
+        // If "Pylance" was explicitly chosen, use it even though it's not guaranteed to work
+        // properly with Python 2.
+        if (!this.getCurrentLanguageServerTypeIsDefault()) {
+            if (serverType === LanguageServerType.Node && interpreter && interpreter.version) {
+                if (interpreter.version.major < 3) {
+                    sendTelemetryEvent(EventName.JEDI_FALLBACK);
+                    serverType = LanguageServerType.Jedi;
+                }
             }
         }
 

src/client/common/configSettings.ts

@@ -144,6 +144,8 @@ export class PythonSettings implements IPythonSettings {
 
     public languageServer: LanguageServerType = LanguageServerType.Microsoft;
 
+    public languageServerIsDefault = true;
+
     public logging: ILoggingSettings = { level: LogLevel.Error };
 
     public useIsolation = true;
@@ -288,21 +290,19 @@ export class PythonSettings implements IPythonSettings {
         let userLS = pythonSettings.get<string>('languageServer');
         userLS = systemVariables.resolveAny(userLS);
 
-        let ls: LanguageServerType;
-
         // Validate the user's input; if invalid, set it to the default.
         if (
             !userLS ||
             userLS === 'Default' ||
             !Object.values(LanguageServerType).includes(userLS as LanguageServerType)
         ) {
-            ls = this.defaultLS?.defaultLSType ?? LanguageServerType.Jedi;
+            this.languageServer = this.defaultLS?.defaultLSType ?? LanguageServerType.Jedi;
+            this.languageServerIsDefault = true;
         } else {
-            ls = userLS as LanguageServerType;
+            this.languageServer = userLS as LanguageServerType;
+            this.languageServerIsDefault = false;
         }
 
-        this.languageServer = ls;
-
         this.jediPath = systemVariables.resolveAny(pythonSettings.get<string>('jediPath'))!;
         if (typeof this.jediPath === 'string' && this.jediPath.length > 0) {
             this.jediPath = getAbsolutePath(systemVariables.resolveAny(this.jediPath), workspaceRoot);

src/client/common/types.ts

@@ -200,6 +200,7 @@ export interface IPythonSettings {
     readonly onDidChange: Event<void>;
     readonly experiments: IExperiments;
     readonly languageServer: LanguageServerType;
+    readonly languageServerIsDefault: boolean;
     readonly defaultInterpreterPath: string;
     readonly logging: ILoggingSettings;
     readonly useIsolation: boolean;

src/test/common/configSettings/configSettings.unit.test.ts

@@ -187,7 +187,11 @@ suite('Python Settings', async () => {
         config.verifyAll();
     });
 
-    function testLanguageServer(languageServer: LanguageServerType, expectedValue: LanguageServerType) {
+    function testLanguageServer(
+        languageServer: LanguageServerType,
+        expectedValue: LanguageServerType,
+        isDefault: boolean,
+    ) {
         test(languageServer, () => {
             expected.pythonPath = 'python3';
             expected.languageServer = languageServer;
@@ -200,16 +204,17 @@ suite('Python Settings', async () => {
             settings.update(config.object);
 
             expect(settings.languageServer).to.be.equal(expectedValue);
+            expect(settings.languageServerIsDefault).to.be.equal(isDefault);
             config.verifyAll();
         });
     }
 
     suite('languageServer settings', async () => {
         Object.values(LanguageServerType).forEach(async (languageServer) => {
-            testLanguageServer(languageServer, languageServer);
+            testLanguageServer(languageServer, languageServer, false);
         });
 
-        testLanguageServer('invalid' as LanguageServerType, LanguageServerType.Jedi);
+        testLanguageServer('invalid' as LanguageServerType, LanguageServerType.Jedi, true);
     });
 
     function testExperiments(enabled: boolean) {