Add new linter: dict-iter-missing-items

Add a new linter to check against forgotten calls to `.items()` when
iterating through a dictionary in a `for` loop.

Close #2761 

6 files changed, 63 insertions, 3 deletions

diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index 18ae9a83c..cb37e6255 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -272,6 +272,8 @@ contributors:
 
 * Fantix King (UChicago): contributor
 
+* Yory (yory8): contributor
+
 * Thomas Hisch: contributor
 
 * Clément Pit-Claudel : contributor
@@ -283,4 +285,3 @@ contributors:
 * Bluesheeptoken: contributor
 
 * Michael Scott Cuthbert: contributor
-
diff --git a/ChangeLog b/ChangeLog
index 095f040f4..160dcaf1c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -42,6 +42,10 @@ Release date: TBA
   This check is emitted when ``pylint`` finds a class variable that conflicts with a slot
   name, which would raise a ``ValueError`` at runtime.
 
+* Added new check: dict-iter-missing-items (E1141)
+
+  Close #2761
+
 * Fix issue with pylint name in output of python -m pylint --version
 
   Close #2764
@@ -52,7 +56,6 @@ Release date: TBA
 
   Close #2816
 
-
 What's New in Pylint 2.3.0?
 ===========================
 
diff --git a/doc/whatsnew/2.4.rst b/doc/whatsnew/2.4.rst
index 45a6b832a..b96e0992f 100644
--- a/doc/whatsnew/2.4.rst
+++ b/doc/whatsnew/2.4.rst
@@ -13,6 +13,11 @@ Summary -- Release highlights
 New checkers
 ============
 
+* We added a new check message ``dict-iter-missing-items``.
+  This is emitted when trying to iterate through a dict in a for loop without calling its .items() method.
+
+  Closes #2761
+
 * We added a new check message ``missing-parentheses-for-call-in-test``.
   This is emitted in case a call to a function is made inside a test but
   it misses parentheses.
@@ -28,7 +33,6 @@ New checkers
         __slots__ = ('first', 'second')
         first = 1
 
-
 Other Changes
 =============
 
diff --git a/pylint/checkers/typecheck.py b/pylint/checkers/typecheck.py
index fa95d7cc4..61aff8f93 100644
--- a/pylint/checkers/typecheck.py
+++ b/pylint/checkers/typecheck.py
@@ -353,6 +353,12 @@ MSGS = {
         "Emitted when a dict key is not hashable "
         "(i.e. doesn't define __hash__ method).",
     ),
+    "E1141": (
+        "Unpacking a dictionary in iteration without calling .items()",
+        "dict-iter-missing-items",
+        "Emitted when trying to iterate through"
+        "a dict without calling .items()",
+    ),
     "W1113": (
         "Keyword argument before variable positional arguments list "
         "in the definition of %s function",
@@ -1522,6 +1528,28 @@ accessed. Python regular expressions are accepted.",
         if not supported_protocol(inferred):
             self.add_message(msg, args=node.value.as_string(), node=node.value)
 
+    @check_messages("dict-items-missing-iter")
+    def visit_for(self, node):
+        if not isinstance(node.target, astroid.node_classes.Tuple):
+            # target is not a tuple
+            return
+        if not len(node.target.elts) == 2:
+            # target is not a tuple of two elements
+            return
+
+        iterable = node.iter
+        if not isinstance(iterable, astroid.node_classes.Name):
+            # it's not a bare variable
+            return
+
+        inferred = safe_infer(iterable)
+        if not inferred:
+            return
+        if not isinstance(inferred, astroid.node_classes.Dict):
+            # the iterable is not a dict
+            return
+
+        self.add_message("dict-iter-missing-items", node=node)
 
 class IterableChecker(BaseChecker):
     """
diff --git a/pylint/test/functional/dict_iter_missing_items.py b/pylint/test/functional/dict_iter_missing_items.py
new file mode 100644
index 000000000..333f58990
--- /dev/null
+++ b/pylint/test/functional/dict_iter_missing_items.py
@@ -0,0 +1,23 @@
+# pylint: disable=invalid-name, consider-iterating-dictionary, missing-docstring, import-error
+from unknown import Uninferable
+
+d = {1: 1, 2: 2}
+l = [1, 2]
+s1 = {1, 2}
+s2 = {1, 2, 3}
+
+# Positive
+for k, v in d:  # [dict-iter-missing-items]
+    pass
+
+# Negative
+for k, v in d.items():
+    pass
+for k in d.keys():
+    pass
+for i, v in enumerate(l):
+    pass
+for i, v in s1.intersection(s2):
+    pass
+for k, v in Uninferable:
+    pass
diff --git a/pylint/test/functional/dict_iter_missing_items.txt b/pylint/test/functional/dict_iter_missing_items.txt
new file mode 100644
index 000000000..0e939ee33
--- /dev/null
+++ b/pylint/test/functional/dict_iter_missing_items.txt
@@ -0,0 +1 @@
+dict-iter-missing-items:10::Unpacking a dictionary in iteration without calling .items()