Fix false positive undefined-variable for lambda arguments in classes

Close #1824

4 files changed, 32 insertions, 2 deletions

diff --git a/ChangeLog b/ChangeLog
index 4e915a9d4..aa08a92f6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -22,6 +22,11 @@ Release date: -
 
       Close #1794 (second part)
 
+    * Fix false positive undefined-variable for lambda argument in
+        class definitions
+
+      Close #1824
+
 What's New in Pylint 1.8.2?
 ==========================
 
diff --git a/doc/whatsnew/1.8.rst b/doc/whatsnew/1.8.rst
index 422aa70f6..7cf3b354b 100644
--- a/doc/whatsnew/1.8.rst
+++ b/doc/whatsnew/1.8.rst
@@ -385,3 +385,6 @@ Other Changes
 * Fix false positive ``inconsistent-return-statements`` message by
   avoiding useless exception inference if the exception is not handled.
   (backport from 2.0)
+
+* Fix false positive ``undefined-variable`` for lambda arguments in
+    class definitions
diff --git a/pylint/checkers/variables.py b/pylint/checkers/variables.py
index 9d856c3b3..4af7f53bb 100644
--- a/pylint/checkers/variables.py
+++ b/pylint/checkers/variables.py
@@ -961,6 +961,10 @@ class VariablesChecker(BaseChecker):
     def _is_variable_violation(node, name, defnode, stmt, defstmt,
                                frame, defframe, base_scope_type,
                                recursive_klass):
+        # node: Node to check for violation
+        # name: name of node to check violation for
+        # frame: Scope of statement of node
+        # base_scope_type: local scope type
         maybee0601 = True
         annotation_return = False
         use_outer_definition = False
@@ -974,8 +978,13 @@ class VariablesChecker(BaseChecker):
         else:
             # we are in a local scope, check the name is not
             # defined in global or builtin scope
-            # skip this lookup if name is assigned later in function scope
-            forbid_lookup = isinstance(frame, astroid.FunctionDef) and _assigned_locally(node)
+            # skip this lookup if name is assigned later in function scope/lambda
+            # Note: the node.frame() is not the same as the `frame` argument which is
+            # equivalent to frame.statement().scope()
+            forbid_lookup = ((isinstance(frame, astroid.FunctionDef) or
+                              isinstance(node.frame(), astroid.Lambda)) and
+                             _assigned_locally(node))
+
             if not forbid_lookup and defframe.root().lookup(name)[1]:
                 maybee0601 = False
                 use_outer_definition = (
diff --git a/pylint/test/unittest_checker_variables.py b/pylint/test/unittest_checker_variables.py
index 23142ba38..1ef4a50d2 100644
--- a/pylint/test/unittest_checker_variables.py
+++ b/pylint/test/unittest_checker_variables.py
@@ -165,6 +165,19 @@ class TestVariablesCheckerWithTearDown(CheckerTestCase):
         with self.assertNoMessages():
             self.walk(node)
 
+    def test_lambda_in_classdef(self):
+        # Make sure lambda doesn't raises
+        # Undefined-method in class function
+
+        # Issue 1824
+        # https://github.com/PyCQA/pylint/issues/1824
+        node = astroid.parse('''
+        class MyObject(object):
+            method1 = lambda func: func()
+            method2 = lambda function: function()
+        ''')
+        with self.assertNoMessages():
+            self.walk(node)
 
 class TestMissingSubmodule(CheckerTestCase):
     CHECKER_CLASS = variables.VariablesChecker