Adds a python formatter

.style.yapf

@@ -0,0 +1,11 @@
+[style]
+based_on_style = yapf
+# Defined in https://github.com/google/yapf/blob/20d0c8f1774cf3843f4032f3e9ab02338bf98c75/yapf/yapflib/style.py#L326
+# Docs and full list of knobs:
+# https://github.com/google/yapf#knobs
+split_before_first_argument = true
+blank_line_before_module_docstring = true
+# dedent_closing_brackets is required by coalesce_brackets
+dedent_closing_brackets = true
+coalesce_brackets = true
+each_dict_entry_on_separate_line = false

DEPS

@@ -173,6 +173,9 @@ deps = {
   'src/third_party/boringssl':
    Var('github_git') + '/dart-lang/boringssl_gen.git' + '@' + Var('dart_boringssl_gen_rev'),
 
+  'src/third_party/yapf':
+  Var('github_git') + '/google/yapf' + '@' + '212c5b5ad8e172d2d914ae454c121c89cccbcb35',
+
   'src/third_party/boringssl/src':
    'https://boringssl.googlesource.com/boringssl.git' + '@' + Var('dart_boringssl_rev'),
 

ci/bin/format.dart

@@ -45,21 +45,24 @@ enum MessageType {
 
 enum FormatCheck {
   clang,
+  gn,
   java,
+  python,
   whitespace,
-  gn,
 }
 
 FormatCheck nameToFormatCheck(String name) {
   switch (name) {
     case 'clang':
       return FormatCheck.clang;
+    case 'gn':
+      return FormatCheck.gn;
     case 'java':
       return FormatCheck.java;
+    case 'python':
+      return FormatCheck.python;
     case 'whitespace':
       return FormatCheck.whitespace;
-    case 'gn':
-      return FormatCheck.gn;
     default:
       throw FormattingException('Unknown FormatCheck type $name');
   }
@@ -69,12 +72,14 @@ String formatCheckToName(FormatCheck check) {
   switch (check) {
     case FormatCheck.clang:
       return 'C++/ObjC';
+    case FormatCheck.gn:
+      return 'GN';
     case FormatCheck.java:
       return 'Java';
+    case FormatCheck.python:
+      return 'Python';
     case FormatCheck.whitespace:
       return 'Trailing whitespace';
-    case FormatCheck.gn:
-      return 'GN';
   }
 }
 
@@ -140,6 +145,15 @@ abstract class FormatChecker {
           allFiles: allFiles,
           messageCallback: messageCallback,
         );
+      case FormatCheck.gn:
+        return GnFormatChecker(
+          processManager: processManager,
+          baseGitRef: baseGitRef,
+          repoDir: repoDir,
+          srcDir: srcDir,
+          allFiles: allFiles,
+          messageCallback: messageCallback,
+        );
       case FormatCheck.java:
         return JavaFormatChecker(
           processManager: processManager,
@@ -149,17 +163,17 @@ abstract class FormatChecker {
           allFiles: allFiles,
           messageCallback: messageCallback,
         );
-      case FormatCheck.whitespace:
-        return WhitespaceFormatChecker(
+      case FormatCheck.python:
+        return PythonFormatChecker(
           processManager: processManager,
           baseGitRef: baseGitRef,
           repoDir: repoDir,
           srcDir: srcDir,
           allFiles: allFiles,
           messageCallback: messageCallback,
         );
-      case FormatCheck.gn:
-        return GnFormatChecker(
+      case FormatCheck.whitespace:
+        return WhitespaceFormatChecker(
           processManager: processManager,
           baseGitRef: baseGitRef,
           repoDir: repoDir,
@@ -665,6 +679,93 @@ class GnFormatChecker extends FormatChecker {
   }
 }
 
+/// Checks the format of any .py files using the "yapf" command.
+class PythonFormatChecker extends FormatChecker {
+  PythonFormatChecker({
+    ProcessManager processManager = const LocalProcessManager(),
+    required String baseGitRef,
+    required Directory repoDir,
+    required Directory srcDir,
+    bool allFiles = false,
+    MessageCallback? messageCallback,
+  }) : super(
+          processManager: processManager,
+          baseGitRef: baseGitRef,
+          repoDir: repoDir,
+          srcDir: srcDir,
+          allFiles: allFiles,
+          messageCallback: messageCallback,
+        ) {
+    yapfBin = File(path.join(
+      repoDir.absolute.path,
+      'tools',
+      'yapf.sh',
+    ));
+    _yapfStyle = File(path.join(
+      repoDir.absolute.path,
+      '.style.yapf',
+    ));
+  }
+
+  late final File yapfBin;
+  late final File _yapfStyle;
+
+  @override
+  Future<bool> checkFormatting() async {
+    message('Checking Python formatting...');
+    return (await _runYapfCheck(fixing: false)) == 0;
+  }
+
+  @override
+  Future<bool> fixFormatting() async {
+    message('Fixing Python formatting...');
+    await _runYapfCheck(fixing: true);
+    // The yapf script shouldn't fail when fixing errors.
+    return true;
+  }
+
+  Future<int> _runYapfCheck({required bool fixing}) async {
+    final List<String> filesToCheck = await getFileList(<String>['*.py']);
+
+    final List<String> cmd = <String>[
+      yapfBin.path,
+      '--style', _yapfStyle.path,
+      if (!fixing) '--diff',
+      if (fixing) '--in-place',
+    ];
+    final List<WorkerJob> jobs = <WorkerJob>[];
+    for (final String file in filesToCheck) {
+      jobs.add(WorkerJob(<String>[...cmd, file]));
+    }
+    final ProcessPool yapfPool = ProcessPool(
+      processRunner: _processRunner,
+      printReport: namedReport('python format'),
+    );
+    final List<WorkerJob> completedJobs = await yapfPool.runToCompletion(jobs);
+    reportDone();
+    final List<String> incorrect = <String>[];
+    for (final WorkerJob job in completedJobs) {
+      if (job.result.exitCode == 1) {
+        incorrect.add('  ${job.command.last}\n${job.result.output}');
+      }
+    }
+    if (incorrect.isNotEmpty) {
+      final bool plural = incorrect.length > 1;
+      if (fixing) {
+        message('Fixed ${incorrect.length} python file${plural ? 's' : ''}'
+            ' which ${plural ? 'were' : 'was'} formatted incorrectly.');
+      } else {
+        error('Found ${incorrect.length} python file${plural ? 's' : ''}'
+            ' which ${plural ? 'were' : 'was'} formatted incorrectly:');
+        incorrect.forEach(stderr.writeln);
+      }
+    } else {
+      message('All python files formatted correctly.');
+    }
+    return incorrect.length;
+  }
+}
+
 @immutable
 class _GrepResult {
   const _GrepResult(this.file, [this.hits = const <String>[], this.lineNumbers = const <int>[]]);

ci/licenses_golden/licenses_flutter

@@ -12,6 +12,7 @@ LIBRARY: txt
 ORIGIN: ../../../flutter/LICENSE
 TYPE: LicenseType.bsd
 FILE: ../../../flutter/.clang-tidy
+FILE: ../../../flutter/.style.yapf
 FILE: ../../../flutter/DEPS
 FILE: ../../../flutter/assets/asset_manager.cc
 FILE: ../../../flutter/assets/asset_manager.h

ci/licenses_golden/tool_signature

@@ -1,2 +1,2 @@
-Signature: 7b0c7d76d9cfc331776ed8d447e52f67
+Signature: 4d80570261409484c1813b3fe5282783
 

tools/licenses/lib/main.dart

@@ -1844,6 +1844,7 @@ class _RepositoryRootThirdPartyDirectory extends _RepositoryGenericThirdPartyDir
         && entry.name != 'mockito' // only used by tests
         && entry.name != 'pymock' // presumably only used by tests
         && entry.name != 'pyyaml' // build-time dependency only
+        && entry.name != 'yapf'  // only used for code formatting
         && entry.name != 'android_embedding_dependencies' // testing framework for android
         && entry.name != 'yasm' // build-time dependency only
         && entry.name != 'binutils' // build-time dependency only

tools/yapf.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+# Copyright 2013 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# Generates objc docs for Flutter iOS libraries.
+
+set -e
+
+# On Mac OS, readlink -f doesn't work, so follow_links traverses the path one
+# link at a time, and then cds into the link destination and find out where it
+# ends up.
+#
+# The function is enclosed in a subshell to avoid changing the working directory
+# of the caller.
+function follow_links() (
+  cd -P "$(dirname -- "$1")"
+  file="$PWD/$(basename -- "$1")"
+  while [[ -h "$file" ]]; do
+    cd -P "$(dirname -- "$file")"
+    file="$(readlink -- "$file")"
+    cd -P "$(dirname -- "$file")"
+    file="$PWD/$(basename -- "$file")"
+  done
+  echo "$file"
+)
+
+SCRIPT_DIR=$(follow_links "$(dirname -- "${BASH_SOURCE[0]}")")
+SRC_DIR="$(cd "$SCRIPT_DIR/../.."; pwd -P)"
+YAPF_DIR="$(cd "$SRC_DIR/third_party/yapf"; pwd -P)"
+
+PYTHONPATH="$YAPF_DIR" python3 "$YAPF_DIR/yapf" "$@"