1 files changed, 75 insertions, 0 deletions
diff --git a/pre_commit/commands/migrate_config.py b/pre_commit/commands/migrate_config.py
new file mode 100644
index 0000000..842fb3a
--- /dev/null
+++ b/pre_commit/commands/migrate_config.py
@@ -0,0 +1,75 @@
+from __future__ import annotations
+
+import re
+import textwrap
+
+import cfgv
+import yaml
+
+from pre_commit.clientlib import InvalidConfigError
+from pre_commit.yaml import yaml_load
+
+
+def _is_header_line(line: str) -> bool:
+    return line.startswith(('#', '---')) or not line.strip()
+
+
+def _migrate_map(contents: str) -> str:
+    if isinstance(yaml_load(contents), list):
+        # Find the first non-header line
+        lines = contents.splitlines(True)
+        i = 0
+        # Only loop on non empty configuration file
+        while i < len(lines) and _is_header_line(lines[i]):
+            i += 1
+
+        header = ''.join(lines[:i])
+        rest = ''.join(lines[i:])
+
+        # If they are using the "default" flow style of yaml, this operation
+        # will yield a valid configuration
+        try:
+            trial_contents = f'{header}repos:\n{rest}'
+            yaml_load(trial_contents)
+            contents = trial_contents
+        except yaml.YAMLError:
+            contents = f'{header}repos:\n{textwrap.indent(rest, " " * 4)}'
+
+    return contents
+
+
+def _migrate_sha_to_rev(contents: str) -> str:
+    return re.sub(r'(\n\s+)sha:', r'\1rev:', contents)
+
+
+def _migrate_python_venv(contents: str) -> str:
+    return re.sub(
+        r'(\n\s+)language: python_venv\b',
+        r'\1language: python',
+        contents,
+    )
+
+
+def migrate_config(config_file: str, quiet: bool = False) -> int:
+    with open(config_file) as f:
+        orig_contents = contents = f.read()
+
+    with cfgv.reraise_as(InvalidConfigError):
+        with cfgv.validate_context(f'File {config_file}'):
+            try:
+                yaml_load(orig_contents)
+            except Exception as e:
+                raise cfgv.ValidationError(str(e))
+
+    contents = _migrate_map(contents)
+    contents = _migrate_sha_to_rev(contents)
+    contents = _migrate_python_venv(contents)
+
+    if contents != orig_contents:
+        with open(config_file, 'w') as f:
+            f.write(contents)
+
+        print('Configuration has been migrated.')
+    elif not quiet:
+        print('Configuration is already migrated.')
+    return 0