Append --pre to black installation command when pipenv is used (microsoft#5393)

* if black is being installed, append pre This is so it can be resolved correctly by pipenv * extra extra read all about it! update news * if black is being installed make sure we pass --pre * Thank author of PR

Author: erinxocon

news/2 Fixes/5171.md

@@ -0,0 +1,2 @@
+Append `--pre` to black installation command so pipenv can properly resolve it.
+(thanks [Erin O'Connell](https://github.com/erinxocon))

src/client/common/installer/pipEnvInstaller.ts

@@ -31,8 +31,12 @@ export class PipEnvInstaller extends ModuleInstaller implements IModuleInstaller
  return interpreters && interpreters.length > 0;
  }
  protected async getExecutionInfo(moduleName: string, _resource?: Uri): Promise<ExecutionInfo> {
+ const args = ['install', moduleName, '--dev'];
+ if (moduleName === 'black') {
+ args.push('--pre');
+ };
  return {
- args: ['install', moduleName, '--dev'],
+ args: args,
  execPath: pipenvName
  };
  }

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

@@ -217,6 +217,9 @@ suite('Module Installer', () => {
  test(`Test args (${product.name})`, async () => {
  setActiveInterpreter();
  const expectedArgs = ['install', moduleName, '--dev'];
+ if (moduleName === 'black') {
+ expectedArgs.push('--pre')
+ }
  await installModuleAndVerifyCommand(pipenvName, expectedArgs);
  });
  }