Implement pre-commit migrate-config

pre_commit/commands/autoupdate.py

@@ -11,6 +11,7 @@ import pre_commit.constants as C
 from pre_commit import output
 from pre_commit.clientlib import is_local_repo
 from pre_commit.clientlib import load_config
+from pre_commit.commands.migrate_config import migrate_config
 from pre_commit.repository import Repository
 from pre_commit.util import CalledProcessError
 from pre_commit.util import cmd_output
@@ -104,21 +105,22 @@ def _write_new_config_file(path, output):
 def autoupdate(runner, tags_only):
     """Auto-update the pre-commit config to the latest versions of repos."""
     retv = 0
-    output_configs = []
+    retv |= migrate_config(runner, quiet=True)
+    output_repos = []
     changed = False
 
-    input_configs = load_config(runner.config_file_path)
+    input_config = load_config(runner.config_file_path)
 
-    for repo_config in input_configs['repos']:
+    for repo_config in input_config['repos']:
         if is_local_repo(repo_config):
-            output_configs.append(repo_config)
+            output_repos.append(repo_config)
             continue
         output.write('Updating {}...'.format(repo_config['repo']))
         try:
             new_repo_config = _update_repo(repo_config, runner, tags_only)
         except RepositoryCannotBeUpdatedError as error:
             output.write_line(error.args[0])
-            output_configs.append(repo_config)
+            output_repos.append(repo_config)
             retv = 1
             continue
 
@@ -127,12 +129,14 @@ def autoupdate(runner, tags_only):
             output.write_line('updating {} -> {}.'.format(
                 repo_config['sha'], new_repo_config['sha'],
             ))
-            output_configs.append(new_repo_config)
+            output_repos.append(new_repo_config)
         else:
             output.write_line('already up to date.')
-            output_configs.append(repo_config)
+            output_repos.append(repo_config)
 
     if changed:
-        _write_new_config_file(runner.config_file_path, output_configs)
+        output_config = input_config.copy()
+        output_config['repos'] = output_repos
+        _write_new_config_file(runner.config_file_path, output_config)
 
     return retv

pre_commit/commands/migrate_config.py

@@ -0,0 +1,52 @@
+from __future__ import print_function
+from __future__ import unicode_literals
+
+import io
+
+import yaml
+from aspy.yaml import ordered_load
+
+
+def _indent(s):
+    lines = s.splitlines(True)
+    return ''.join(' ' * 4 + line if line.strip() else line for line in lines)
+
+
+def _is_header_line(line):
+    return (line.startswith(('#', '---')) or not line.strip())
+
+
+def migrate_config(runner, quiet=False):
+    retv = 0
+
+    with io.open(runner.config_file_path) as f:
+        contents = f.read()
+
+    # Find the first non-header line
+    lines = contents.splitlines(True)
+    i = 0
+    while _is_header_line(lines[i]):
+        i += 1
+
+    header = ''.join(lines[:i])
+    rest = ''.join(lines[i:])
+
+    if isinstance(ordered_load(contents), list):
+        # If they are using the "default" flow style of yaml, this operation
+        # will yield a valid configuration
+        try:
+            trial_contents = header + 'repos:\n' + rest
+            yaml.load(trial_contents)
+            contents = trial_contents
+        except yaml.YAMLError:
+            contents = header + 'repos:\n' + _indent(rest)
+
+        with io.open(runner.config_file_path, 'w') as f:
+            f.write(contents)
+
+        print('Configuration has been migrated.')
+        retv = 1
+    elif not quiet:
+        print('Configuration is already migrated.')
+
+    return retv