Implement pre-commit migrate-config

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