`undefined-loop-variable` takes in consideration non-empty iterred objects before emitting

 Close #2039

4 files changed, 66 insertions, 5 deletions

diff --git a/ChangeLog b/ChangeLog
index 4c734505c..807aa2dd8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -4,6 +4,10 @@ Pylint's ChangeLog
 What's New in Pylint 2.0?
 =========================
 
+    * `undefined-loop-variable` takes in consideration non-empty iterred objects before emitting
+
+       Close #2039
+
 	* Add support for nupmydoc optional return value names.
 
 	  Close #2030
diff --git a/doc/whatsnew/2.0.rst b/doc/whatsnew/2.0.rst
index f5fa63b0c..98e1dc3aa 100644
--- a/doc/whatsnew/2.0.rst
+++ b/doc/whatsnew/2.0.rst
@@ -125,3 +125,9 @@ Other Changes
 * Suppress false-positive ``not-callable`` messages from certain staticmethod descriptors
 
 * `singleton-comparison` will suggest better boolean conditions for negative conditions.
+
+* `undefined-loop-variable` takes in consideration non-empty iterred objects before emitting.
+
+  For instance, if the loop iterable is not empty, this check will no longer be emitted.
+
+
diff --git a/pylint/checkers/variables.py b/pylint/checkers/variables.py
index 22468a997..1b960910f 100644
--- a/pylint/checkers/variables.py
+++ b/pylint/checkers/variables.py
@@ -38,6 +38,7 @@ import re
 import astroid
 from astroid import decorators
 from astroid import modutils
+from astroid import objects
 from pylint.interfaces import IAstroidChecker, INFERENCE, INFERENCE_FAILURE, HIGH
 from pylint.utils import get_global_option
 from pylint.checkers import BaseChecker
@@ -930,11 +931,37 @@ class VariablesChecker(BaseChecker):
                 continue
             _astmts.append(stmt)
         astmts = _astmts
-        if len(astmts) == 1:
-            assign = astmts[0].assign_type()
-            if (isinstance(assign, (astroid.For, astroid.Comprehension,
-                                    astroid.GeneratorExp))
-                    and assign.statement() is not node.statement()):
+        if len(astmts) != 1:
+            return
+
+        assign = astmts[0].assign_type()
+        if not (isinstance(assign, (astroid.For, astroid.Comprehension, astroid.GeneratorExp))
+                and assign.statement() is not node.statement()):
+            return
+
+        # For functions we can do more by inferring the length of the iterred object
+        if not isinstance(assign, astroid.For):
+            self.add_message('undefined-loop-variable', args=name, node=node)
+            return
+
+        try:
+            inferred = next(assign.iter.infer())
+        except astroid.InferenceError:
+            self.add_message('undefined-loop-variable', args=name, node=node)
+        else:
+            sequences = (
+                astroid.List,
+                astroid.Tuple,
+                astroid.Dict,
+                astroid.Set,
+                objects.FrozenSet,
+            )
+            if not isinstance(inferred, sequences):
+                self.add_message('undefined-loop-variable', args=name, node=node)
+                return
+
+            elements = getattr(inferred,  'elts', getattr(inferred, 'items', []))
+            if not elements:
                 self.add_message('undefined-loop-variable', args=name, node=node)
 
     def _should_ignore_redefined_builtin(self, stmt):
diff --git a/pylint/test/functional/undefined_loop_variable.py b/pylint/test/functional/undefined_loop_variable.py
index 6c28a4088..3840003b5 100644
--- a/pylint/test/functional/undefined_loop_variable.py
+++ b/pylint/test/functional/undefined_loop_variable.py
@@ -35,3 +35,27 @@ for x in []:
     pass
 for x in range(3):
     VAR5 = (lambda: x)()
+
+
+def do_stuff_with_a_list():
+    for var in [1, 2, 3]:
+        pass
+    return var
+
+
+def do_stuff_with_a_set():
+    for var in {1, 2, 3}:
+        pass
+    return var
+
+
+def do_stuff_with_a_dict():
+    for var in {1: 2, 3: 4}:
+        pass
+    return var
+
+
+def do_stuff_with_a_tuple():
+    for var in (1, 2, 3):
+        pass
+    return var