Added a new check, self-assigning-variable

This check is emitted when we detect that a variable is assigned
to itself, which might indicate a potential bug in the code application.
Close #2930

ChangeLog

@@ -7,6 +7,12 @@ What's New in Pylint 2.4.0?
 
 Release date: TBA
 
+* Added a new check, ``self-assigning-variable``
+
+  This check is emitted when we detect that a variable is assigned
+  to itself, which might indicate a potential bug in the code application.
+  Close #2930
+
 * Added a new check, ``property-with-parameters``.
 
   This check is emitted when we detect that a defined property also

doc/whatsnew/2.4.rst

@@ -20,6 +20,22 @@ New checkers
 
   Close #2905
 
+* Added a new check, ``self-assigning-variable``
+
+  This check is emitted when we detect that a variable is assigned
+  to itself, which might indicate a potential bug in the code application.
+
+  For example, the following would raise this warning::
+
+    def new_a(attr, attr2):
+      a_inst = Aclass()
+      a_inst.attr2 = attr2
+      # should be: a_inst.attr = attr, but have a typo
+      attr = attr
+      return a_inst
+
+  Close #2930
+
 * Added a new check ``property-with-parameters`` which detects when a property
   has more than a single argument.
 

pylint/checkers/base.py

@@ -981,6 +981,11 @@ class BasicChecker(_BasicChecker):
             "Emitted when a conditional statement (If or ternary if) "
             "seems to wrongly call a function due to missing parentheses",
         ),
+        "W0127": (
+            "Assigning the same variable %r to itself",
+            "self-assigning-variable",
+            "Emitted when we detect that a variable is assigned to itself",
+        ),
         "E0111": (
             "The first reversed() argument is not a sequence",
             "bad-reversed-sequence",
@@ -1497,6 +1502,38 @@ def visit_with(self, node):
                         # we assume it's a nested "with"
                         self.add_message("confusing-with-statement", node=node)
 
+    @utils.check_messages("self-assigning-variable")
+    def visit_assign(self, node):
+
+        # Detect assigning to the same variable.
+        rhs_names = []
+        targets = node.targets
+        if isinstance(targets[0], astroid.Tuple):
+            if len(targets) != 1:
+                # A complex assignment, so bail out early.
+                return
+            targets = targets[0].elts
+
+        if isinstance(node.value, astroid.Name):
+            if len(targets) != 1:
+                return
+            rhs_names = [node.value]
+        elif isinstance(node.value, astroid.Tuple):
+            rhs_count = len(node.value.elts)
+            if len(targets) != rhs_count or rhs_count == 1:
+                return
+            rhs_names = node.value.elts
+
+        for target, lhs_name in zip(targets, rhs_names):
+            if not isinstance(lhs_name, astroid.Name):
+                continue
+            if not isinstance(target, astroid.AssignName):
+                continue
+            if target.name == lhs_name.name:
+                self.add_message(
+                    "self-assigning-variable", args=(target.name,), node=target
+                )
+
 
 KNOWN_NAME_TYPES = {
     "module",

tests/functional/self_assigning_variable.py

@@ -0,0 +1,20 @@
+# pylint: disable=missing-docstring,too-few-public-methods
+# pylint: disable=unpacking-non-sequence,attribute-defined-outside-init,invalid-name
+
+class Class:
+    pass
+
+CLS = Class()
+FIRST = 1
+# Not enough values on the right hand side
+FIRST, SECOND = FIRST
+# Not enough values on the left hand side
+FIRST = FIRST, SECOND
+# Not equivalent to a self assignment
+FIRST = (FIRST, )
+# Not assigning to an attribute
+CLS.FIRST = FIRST
+# Not a name on the right hand side
+FIRST = Class()
+FIRST = FIRST # [self-assigning-variable]
+FIRST, SECOND = FIRST, CLS.FIRST # [self-assigning-variable]

tests/functional/self_assigning_variable.txt

@@ -0,0 +1,2 @@
+self-assigning-variable:19::Assigning the same variable 'FIRST' to itself
+self-assigning-variable:20::Assigning the same variable 'FIRST' to itself

tests/functional/unused_import.py

@@ -9,7 +9,7 @@
 from collections import deque, OrderedDict, Counter
 import re, html.parser  # [unused-import]
 DATA = Counter()
-
+# pylint: disable=self-assigning-variable
 from fake import SomeName, SomeOtherName  # [unused-import]
 class SomeClass(object):
     SomeName = SomeName # https://bitbucket.org/logilab/pylint/issue/475