Adds fix-yaml hook

This adds missing --- header to yaml file, which is required for
strict linting via yamllint. Future versions may add other fixes
for yaml files.

Fixes: #348

.pre-commit-hooks.yaml

@@ -130,6 +130,15 @@
     entry: fix-encoding-pragma
     description: 'Add # -*- coding: utf-8 -*- to the top of python files'
     types: [python]
+-   id: fix-yaml
+    name: Fix YAML files
+    language: python
+    entry: fix-yaml
+    description: 'Add --- header to yaml files'
+    types: [yaml]
+    # for backward compatibility
+    files: ''
+    minimum_pre_commit_version: 0.15.0
 -   id: flake8
     name: Flake8
     description: This hook runs flake8.

pre_commit_hooks/fix_yaml.py

@@ -0,0 +1,34 @@
+from __future__ import absolute_import
+from __future__ import print_function
+from __future__ import unicode_literals
+
+import argparse
+import io
+
+from six import text_type
+
+
+def main(argv=None):
+    parser = argparse.ArgumentParser()
+    parser.add_argument('filenames', nargs='*', help='Filenames to fix')
+    args = parser.parse_args(argv)
+
+    retv = 0
+
+    for filename in args.filenames:
+        with io.open(filename, 'r+', encoding="utf-8") as f:
+            line = f.readline().strip()
+            if line != text_type('---'):
+                print("[%s]" % line)
+                f.seek(0, 0)
+                content = f.read()
+                f.seek(0, 0)
+                f.write('---\n' + content)
+                print('{}: Added --- header to YAML file'.format(filename))
+                retv = 1
+
+    return retv
+
+
+if __name__ == '__main__':
+    exit(main())

tests/fix_simple_yaml_test.py

@@ -0,0 +1,16 @@
+from __future__ import absolute_import
+from __future__ import unicode_literals
+
+from pre_commit_hooks import fix_yaml
+
+
+def test_no_change(tmpdir):
+    f = tmpdir.join('f.yaml')
+    f.write('---\nfoo: bar')
+    assert fix_yaml.main((f.strpath,)) == 0
+
+
+def test_change(tmpdir):
+    f = tmpdir.join('f.yaml')
+    f.write('foo: bar')
+    assert fix_yaml.main((f.strpath,)) == 1