Append --allow-prereleases to black installation command when Poetry is used (#5967)

* if black is being installed, append --allow-prereleases, fixes #5756
* update news
* add unit test for installing black with poetry

Author: notpushkin

news/2 Fixes/5756.md

@@ -0,0 +1 @@
+Append `--allow-prereleases` to black installation command so pipenv can properly resolve it.

src/client/common/installer/poetryInstaller.ts

@@ -60,8 +60,12 @@ export class PoetryInstaller extends ModuleInstaller implements IModuleInstaller
     }
     protected async getExecutionInfo(moduleName: string, resource?: Uri): Promise<ExecutionInfo> {
         const execPath = this.configurationService.getSettings(resource).poetryPath;
+        const args = ['add', '--dev', moduleName];
+        if (moduleName === 'black') {
+            args.push('--allow-prereleases');
+        }
         return {
-            args: ['add', '--dev', moduleName],
+            args,
             execPath
         };
     }

src/test/common/installer/poetryInstaller.unit.test.ts

@@ -122,4 +122,15 @@ suite('Module Installer - Poetry', () => {
 
         assert.deepEqual(info, { args: ['add', '--dev', 'something'], execPath: 'poetry path' });
     });
+    test('Get executable info when installing black', async () => {
+        const uri = Uri.file(__dirname);
+        const settings = mock(PythonSettings);
+
+        when(configurationService.getSettings(uri)).thenReturn(instance(settings));
+        when(settings.poetryPath).thenReturn('poetry path');
+
+        const info = await poetryInstaller.getExecutionInfo('black', uri);
+
+        assert.deepEqual(info, { args: ['add', '--dev', 'black', '--allow-prereleases'], execPath: 'poetry path' });
+    });
 });