Add 'implicit-map-evaluation' to Python 3 porting checker.

This warning is emitted
 when encountering the use of map builtin, without explicit evaluation,
as in "map(func, args)" on a single line. This behaviour is removed in Python 3, where
map is a generator and must be explicitly evaluated.

3 files changed, 29 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index a877b66..cc970e5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -182,6 +182,9 @@ ChangeLog for Pylint
 
     * Add 'old-octal-literal' to Python 3 porting checker, emitted when
       encountering octals with the old syntax.
+
+    * Add 'implicit-map-evaluation' to Python 3 porting checker, emitted
+      when encountering the use of map builtin, without explicit evaluation.
       
 
 
diff --git a/checkers/python3.py b/checkers/python3.py
index e428a60..472e328 100644
--- a/checkers/python3.py
+++ b/checkers/python3.py
@@ -237,6 +237,14 @@ class Python3Checker(checkers.BaseChecker):
                   'Used when a __cmp__ method is defined '
                   '(method is not used by Python 3)',
                   {'maxversion': (3, 0)}),
+        'W1631': ('map is used as implicitly evaluated call',
+                  'implicit-map-evaluation',
+                  'Used when the map builtin is used as implicitly '
+                  'evaluated call, as in "map(func, args)" on a single line. '
+                  'This behaviour will not work in Python 3, where '
+                  'map is a generator and must be evaluated. '
+                  'Prefer a for-loop as alternative.',
+                  {'maxversion': (3, 0)}),
     }
 
     _missing_builtins = frozenset([
@@ -285,6 +293,14 @@ class Python3Checker(checkers.BaseChecker):
             if isinstance(arg, astroid.Tuple):
                 self.add_message('parameter-unpacking', node=arg)
 
+    @utils.check_messages('implicit-map-evaluation')
+    def visit_discard(self, node):
+        if (isinstance(node.value, astroid.CallFunc) and
+                node.value.func.name == 'map'):
+            module = node.value.func.lookup('map')[0]
+            if getattr(module, 'name', None) == '__builtin__':
+                self.add_message('implicit-map-evaluation', node=node)
+
     def visit_name(self, node):
         """Detect when a built-in that is missing in Python 3 is referenced."""
         found_node = node.lookup(node.name)[0]
diff --git a/test/unittest_checker_python3.py b/test/unittest_checker_python3.py
index b57fc96..f7eead9 100644
--- a/test/unittest_checker_python3.py
+++ b/test/unittest_checker_python3.py
@@ -200,6 +200,16 @@ class Python3CheckerTest(testutils.CheckerTestCase):
         with self.assertAddsMessages(message):
             self.checker.visit_callfunc(node)
 
+    @python2_only
+    def test_implicit_map_evaluation(self):
+        node = test_utils.extract_node('map(str, [1, 2, 3])')
+        discard = node.parent
+        message = testutils.Message('implicit-map-evaluation', node=discard)
+        with self.assertAddsMessages(message):
+            # Use node.parent because extract_node returns the value
+            # of a discard node, not the discard itself.
+            self.checker.visit_discard(discard)
+
     def test_not_next_method(self):
         arg_node = test_utils.extract_node('x.next(x)  #@')
         stararg_node = test_utils.extract_node('x.next(*x)  #@')