Skip function arguments prefixed with _ in D417 check

These arguments have been marked private. As such,
it should be okay for the checker to not validate their
existence in the docstring,

docs/release_notes.rst

@@ -4,6 +4,14 @@ Release Notes
 **pydocstyle** version numbers follow the
 `Semantic Versioning <http://semver.org/>`_ specification.
 
+Current Development Version
+---------------------------
+
+New Features
+
+* Skip function arguments prefixed with `_` in D417 check (#440).
+
+
 5.0.2 - January 8th, 2020
 ---------------------------
 

src/pydocstyle/checker.py

@@ -764,6 +764,13 @@ def _check_missing_args(docstring_args, definition):
             # positional argument as it is `cls` or `self`
             if definition.kind == 'method' and not definition.is_static:
                 function_args = function_args[1:]
+            # Filtering out any arguments prefixed with `_` marking them
+            # as private.
+            function_args = [
+                arg_name
+                for arg_name in function_args
+                if not is_def_arg_private(arg_name)
+            ]
             missing_args = set(function_args) - docstring_args
             if missing_args:
                 yield violations.D417(", ".join(sorted(missing_args)),
@@ -996,6 +1003,9 @@ def get_leading_words(line):
     if result is not None:
         return result.group()
 
+def is_def_arg_private(arg_name):
+    """Returns a boolean indicating if the argument name is private."""
+    return arg_name.startswith("_")
 
 def get_function_args(function_string):
     """Return the function arguments given the source-code string."""

src/tests/test_cases/sections.py

@@ -278,7 +278,7 @@ def missing_colon_google_style_section():  # noqa: D406, D407
 @expect("D417: Missing argument descriptions in the docstring "
         "(argument(s) y are missing descriptions in "
         "'test_missing_google_args' docstring)")
-def test_missing_google_args(x=1, y=2):  # noqa: D406, D407
+def test_missing_google_args(x=1, y=2, _private=3):  # noqa: D406, D407
     """Toggle the gizmo.
 
     Args:
@@ -290,7 +290,7 @@ def test_missing_google_args(x=1, y=2):  # noqa: D406, D407
 class TestGoogle:  # noqa: D203
     """Test class."""
 
-    def test_method(self, test, another_test):  # noqa: D213, D407
+    def test_method(self, test, another_test, _):  # noqa: D213, D407
         """Test a valid args section.
 
         Args:
@@ -301,8 +301,8 @@ def test_method(self, test, another_test):  # noqa: D213, D407
 
     @expect("D417: Missing argument descriptions in the docstring "
             "(argument(s) test, y, z are missing descriptions in "
-            "'test_missing_args' docstring)", arg_count=4)
-    def test_missing_args(self, test, x, y, z=3):  # noqa: D213, D407
+            "'test_missing_args' docstring)", arg_count=5)
+    def test_missing_args(self, test, x, y, z=3, _private_arg=3):  # noqa: D213, D407
         """Test a valid args section.
 
         Args:
@@ -313,8 +313,8 @@ def test_missing_args(self, test, x, y, z=3):  # noqa: D213, D407
     @classmethod
     @expect("D417: Missing argument descriptions in the docstring "
             "(argument(s) test, y, z are missing descriptions in "
-            "'test_missing_args_class_method' docstring)", arg_count=4)
-    def test_missing_args_class_method(cls, test, x, y, z=3):  # noqa: D213, D407
+            "'test_missing_args_class_method' docstring)", arg_count=5)
+    def test_missing_args_class_method(cls, test, x, y, _, z=3):  # noqa: D213, D407
         """Test a valid args section.
 
         Args:
@@ -326,8 +326,8 @@ def test_missing_args_class_method(cls, test, x, y, z=3):  # noqa: D213, D407
     @staticmethod
     @expect("D417: Missing argument descriptions in the docstring "
             "(argument(s) a, y, z are missing descriptions in "
-            "'test_missing_args_static_method' docstring)", arg_count=3)
-    def test_missing_args_static_method(a, x, y, z=3):  # noqa: D213, D407
+            "'test_missing_args_static_method' docstring)", arg_count=4)
+    def test_missing_args_static_method(a, x, y, _test, z=3):  # noqa: D213, D407
         """Test a valid args section.
 
         Args:
@@ -340,7 +340,7 @@ def test_missing_args_static_method(a, x, y, z=3):  # noqa: D213, D407
 @expect("D417: Missing argument descriptions in the docstring "
         "(argument(s) y are missing descriptions in "
         "'test_missing_numpy_args' docstring)")
-def test_missing_numpy_args(x=1, y=2):  # noqa: D406, D407
+def test_missing_numpy_args(_private_arg=0, x=1, y=2):  # noqa: D406, D407
     """Toggle the gizmo.
 
     Parameters
@@ -354,7 +354,7 @@ def test_missing_numpy_args(x=1, y=2):  # noqa: D406, D407
 class TestNumpy:  # noqa: D203
     """Test class."""
 
-    def test_method(self, test, another_test, x=1, y=2):  # noqa: D213, D407
+    def test_method(self, test, another_test, _, x=1, y=2, _private_arg=1):  # noqa: D213, D407
         """Test a valid args section.
 
         Parameters
@@ -368,8 +368,8 @@ def test_method(self, test, another_test, x=1, y=2):  # noqa: D213, D407
 
     @expect("D417: Missing argument descriptions in the docstring "
             "(argument(s) test, y, z are missing descriptions in "
-            "'test_missing_args' docstring)", arg_count=4)
-    def test_missing_args(self, test, x, y, z=3, t=1):  # noqa: D213, D407
+            "'test_missing_args' docstring)", arg_count=5)
+    def test_missing_args(self, test, x, y, z=3, t=1, _private=0):  # noqa: D213, D407
         """Test a valid args section.
 
         Parameters