Added a new Python 2/3 check for invalid lambda parameter list, `lambda-parameter-parens`

Close #1995

4 files changed, 28 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 342045db2..d26960237 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -7,6 +7,10 @@ What's New in Pylint 1.9.0?
 
 Release date:
 
+   * Added a new Python 2/3 check for invalid lambda parameter list, `lambda-parameter-parens`
+
+     Close #1995
+
    * Added a new Python 2/3 check for accessing `operator.div`, which is removed in Python 3
 
      Close #1936
diff --git a/doc/whatsnew/1.9.rst b/doc/whatsnew/1.9.rst
index 6ddc30d2d..f9a349961 100644
--- a/doc/whatsnew/1.9.rst
+++ b/doc/whatsnew/1.9.rst
@@ -31,5 +31,13 @@ New checkers
       from six.moves.urllib.parse import urlencode
 
 
+* A new Python 3 checker was added to warn about invalid lambda parameter list when
+  using parantheses. This is invalid in Python 3 and will result in a SyntaxError:
+
+  .. code-block:: python
+
+      lambda (some_param, some_other_param): ...
+
+
 Other Changes
 =============
diff --git a/pylint/checkers/python3.py b/pylint/checkers/python3.py
index 5b9cabaaa..e3854dae1 100644
--- a/pylint/checkers/python3.py
+++ b/pylint/checkers/python3.py
@@ -970,6 +970,11 @@ class Python3TokenChecker(checkers.BaseTokenChecker):
                   'Used when non-ascii bytes literals are found in a program. '
                   'They are no longer supported in Python 3.',
                   {'maxversion': (3, 0)}),
+        'E1611': ('Drop invalid lambda parantheses',
+                  'lambda-parameter-parens',
+                  'Used when a lambda definition contains parantheses for the parameter '
+                  'list. This is invalid syntax in Python 3.X.',
+                  {'maxversion': (3, 0)}),
     }
 
     def process_tokens(self, tokens):
@@ -986,6 +991,12 @@ class Python3TokenChecker(checkers.BaseTokenChecker):
                 if any(elem for elem in token if ord(elem) > 127):
                     self.add_message('non-ascii-bytes-literal', line=start[0])
 
+            if (tok_type == tokenize.NAME
+                    and token == 'lambda'
+                    and idx < len(tokens)
+                    and tokens[idx + 1][0] == tokenize.OP
+                    and tokens[idx + 1][1] == '('):
+                self.add_message('lambda-parameter-parens', line=start[0])
 
 def register(linter):
     linter.register_checker(Python3Checker(linter))
diff --git a/pylint/test/unittest_checker_python3.py b/pylint/test/unittest_checker_python3.py
index a0bad865a..5e3fa7c74 100644
--- a/pylint/test/unittest_checker_python3.py
+++ b/pylint/test/unittest_checker_python3.py
@@ -975,3 +975,8 @@ class TestPython3TokenChecker(testutils.CheckerTestCase):
             tokens = testutils._tokenize_str(code)
             with self.assertNoMessages():
                 self.checker.process_tokens(tokens)
+
+    def test_drop_lambda_parens(self):
+        self._test_token_message("lambda (a): a", "lambda-parameter-parens")
+        self._test_token_message("lambda(a): a", "lambda-parameter-parens")
+        self._test_token_message("lambda  (a, b, c): a", "lambda-parameter-parens")