Check if node is in list comprehension in function defaults

List comprehensions in python 2 do not have scope. If the local scope is
skipped, then generated variables values will raise a false-positive.

Close #1897

4 files changed, 36 insertions, 2 deletions

diff --git a/ChangeLog b/ChangeLog
index aa08a92f6..64f296049 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -27,6 +27,11 @@ Release date: -
 
       Close #1824
 
+    * Fix false-positive ``undefined-variable`` for generated
+      comprehension variables in function default arguments
+
+      Close #1897
+
 What's New in Pylint 1.8.2?
 ==========================
 
diff --git a/doc/whatsnew/1.8.rst b/doc/whatsnew/1.8.rst
index 7cf3b354b..d78ac081f 100644
--- a/doc/whatsnew/1.8.rst
+++ b/doc/whatsnew/1.8.rst
@@ -388,3 +388,6 @@ Other Changes
 
 * Fix false positive ``undefined-variable`` for lambda arguments in
     class definitions
+
+* Fix false-positive ``undefined-variable`` for generated
+  comprehension variables in function default arguments
diff --git a/pylint/checkers/variables.py b/pylint/checkers/variables.py
index 4af7f53bb..5eaf255ca 100644
--- a/pylint/checkers/variables.py
+++ b/pylint/checkers/variables.py
@@ -1099,8 +1099,12 @@ class VariablesChecker(BaseChecker):
         # if the name node is used as a function default argument's value or as
         # a decorator, then start from the parent frame of the function instead
         # of the function frame - and thus open an inner class scope
-        if (utils.is_func_default(node) or utils.is_func_decorator(node)
-                or utils.is_ancestor_name(frame, node)):
+        if ((utils.is_func_default(node) and not utils.in_comprehension(node)) or
+                utils.is_func_decorator(node) or utils.is_ancestor_name(frame, node)):
+            # Do not use the highest scope to look for variable name consumption in this case
+            # If the name is used in the function default, or as a decorator, then it
+            # cannot be defined there
+            # (except for list comprehensions in function defaults)
             start_index = len(self._to_consume) - 2
         else:
             start_index = len(self._to_consume) - 1
diff --git a/pylint/test/unittest_checker_variables.py b/pylint/test/unittest_checker_variables.py
index 1ef4a50d2..1ca08fc22 100644
--- a/pylint/test/unittest_checker_variables.py
+++ b/pylint/test/unittest_checker_variables.py
@@ -179,6 +179,28 @@ class TestVariablesCheckerWithTearDown(CheckerTestCase):
         with self.assertNoMessages():
             self.walk(node)
 
+    def test_scope_in_defaults(self):
+        # Should not emit undefined variable
+        node = astroid.extract_node('''
+        def foof(x=[i for i in [1, 2]]):
+            return x
+        ''')
+        # Trickier to detect
+        node = astroid.extract_node('''
+        def foof(x=[(i, 1) for i in [1, 2]]):
+            return x
+        ''')
+        with self.assertNoMessages():
+            self.walk(node)
+        # Lambdas have their own scope
+        node = astroid.extract_node('''
+        def foof(x=lambda zoo: zoo):
+            return x
+        ''')
+        with self.assertNoMessages():
+            self.walk(node)
+
+
 class TestMissingSubmodule(CheckerTestCase):
     CHECKER_CLASS = variables.VariablesChecker