config: Look for configuration file in parent directories

Inspired be ESLint's search, it looks for configuration files in all
parent directories up until it reaches the user's home or root.

closes #571

docs/configuration.rst

@@ -15,7 +15,8 @@ If ``-c`` is not provided, yamllint will look for a configuration file in the
 following locations (by order of preference):
 
 - a file named ``.yamllint``, ``.yamllint.yaml``, or ``.yamllint.yml`` in the
-  current working directory
+  current working directory, or a parent directory (the search for this file is
+  terminated at the user's home or filesystem root)
 - a filename referenced by ``$YAMLLINT_CONFIG_FILE``, if set
 - a file named ``$XDG_CONFIG_HOME/yamllint/config`` or
   ``~/.config/yamllint/config``, if present

tests/test_cli.py

@@ -734,3 +734,64 @@ class CommandLineConfigTestCase(unittest.TestCase):
 
                 self.assertEqual((ctx.returncode, ctx.stdout, ctx.stderr),
                                  (0, '', ''))
+
+    def test_parent_config_file(self):
+        workspace = {'a/b/c/d/e/f/g/a.yml': 'hello: world\n'}
+        conf = ('---\n'
+                'extends: relaxed\n')
+
+        for conf_file in ('.yamllint', '.yamllint.yml', '.yamllint.yaml'):
+            with self.subTest(conf_file):
+                with temp_workspace(workspace):
+                    with RunContext(self) as ctx:
+                        os.chdir('a/b/c/d/e/f')
+                        cli.run(('-f', 'parsable', '.'))
+
+                self.assertEqual((ctx.returncode, ctx.stdout, ctx.stderr),
+                                 (0, './g/a.yml:1:1: [warning] missing '
+                                     'document start "---" (document-start)\n',
+                                     ''))
+
+                with temp_workspace({**workspace, **{conf_file: conf}}):
+                    with RunContext(self) as ctx:
+                        os.chdir('a/b/c/d/e/f')
+                        cli.run(('-f', 'parsable', '.'))
+
+                self.assertEqual((ctx.returncode, ctx.stdout, ctx.stderr),
+                                 (0, '', ''))
+
+    def test_multiple_parent_config_file(self):
+        workspace = {'a/b/c/3spaces.yml': 'array:\n'
+                                          '   - item\n',
+                     'a/b/c/4spaces.yml': 'array:\n'
+                                          '    - item\n',
+                     'a/.yamllint': '---\n'
+                                    'extends: relaxed\n'
+                                    'rules:\n'
+                                    '  indentation:\n'
+                                    '    spaces: 4\n',
+                     }
+
+        conf3 = ('---\n'
+                 'extends: relaxed\n'
+                 'rules:\n'
+                 '  indentation:\n'
+                 '    spaces: 3\n')
+
+        with temp_workspace(workspace):
+            with RunContext(self) as ctx:
+                os.chdir('a/b/c')
+                cli.run(('-f', 'parsable', '.'))
+
+        self.assertEqual((ctx.returncode, ctx.stdout, ctx.stderr),
+                         (0, './3spaces.yml:2:4: [warning] wrong indentation: '
+                         'expected 4 but found 3 (indentation)\n', ''))
+
+        with temp_workspace({**workspace, **{'a/b/.yamllint.yml': conf3}}):
+            with RunContext(self) as ctx:
+                os.chdir('a/b/c')
+                cli.run(('-f', 'parsable', '.'))
+
+        self.assertEqual((ctx.returncode, ctx.stdout, ctx.stderr),
+                         (0, './4spaces.yml:2:5: [warning] wrong indentation: '
+                         'expected 3 but found 4 (indentation)\n', ''))

yamllint/cli.py

@@ -141,6 +141,19 @@ def show_problems(problems, file, args_format, no_warn):
     return max_level
 
 
+def find_project_config_filepath(path='.'):
+    for filename in ('.yamllint', '.yamllint.yaml', '.yamllint.yml'):
+        filepath = os.path.join(path, filename)
+        if os.path.isfile(filepath):
+            return filepath
+
+    if os.path.abspath(path) == os.path.abspath(os.path.expanduser('~')):
+        return None
+    if os.path.abspath(path) == os.path.abspath(os.path.join(path, '..')):
+        return None
+    return find_project_config_filepath(path=os.path.join(path, '..'))
+
+
 def run(argv=None):
     parser = argparse.ArgumentParser(prog=APP_NAME,
                                      description=APP_DESCRIPTION)
@@ -185,6 +198,7 @@ def run(argv=None):
     else:
         user_global_config = os.path.expanduser('~/.config/yamllint/config')
 
+    project_config_filepath = find_project_config_filepath()
     try:
         if args.config_data is not None:
             if args.config_data != '' and ':' not in args.config_data:
@@ -192,12 +206,8 @@ def run(argv=None):
             conf = YamlLintConfig(content=args.config_data)
         elif args.config_file is not None:
             conf = YamlLintConfig(file=args.config_file)
-        elif os.path.isfile('.yamllint'):
-            conf = YamlLintConfig(file='.yamllint')
-        elif os.path.isfile('.yamllint.yaml'):
-            conf = YamlLintConfig(file='.yamllint.yaml')
-        elif os.path.isfile('.yamllint.yml'):
-            conf = YamlLintConfig(file='.yamllint.yml')
+        elif project_config_filepath:
+            conf = YamlLintConfig(file=project_config_filepath)
         elif os.path.isfile(user_global_config):
             conf = YamlLintConfig(file=user_global_config)
         else: