bpo-34850: Emit a warning for "is" and "is not" with a literal.

Doc/whatsnew/3.8.rst

@@ -305,6 +305,12 @@ Changes in Python behavior
   in the leftmost :keyword:`for` clause).
   (Contributed by Serhiy Storchaka in :issue:`10544`.)
 
+* The compiler will emit a syntax warning when the ``is`` or ``is not``
+  operator is used with a literal (except singletons ``None``, ``False``,
+  ``True`` and ``...``). It will became a :exc:`SyntaxError` if run the
+  interpreter with the ``-W error::SyntaxWarning`` option.
+  (Contributed by Serhiy Storchaka in :issue:`34850`.)
+
 
 Changes in the Python API
 -------------------------

Lib/idlelib/iomenu.py

@@ -40,7 +40,7 @@
             # resulting codeset may be unknown to Python. We ignore all
             # these problems, falling back to ASCII
             locale_encoding = locale.nl_langinfo(locale.CODESET)
-            if locale_encoding is None or locale_encoding is '':
+            if locale_encoding is None or locale_encoding == '':
                 # situation occurs on Mac OS X
                 locale_encoding = 'ascii'
             codecs.lookup(locale_encoding)
@@ -50,7 +50,7 @@
             # bugs that can cause ValueError.
             try:
                 locale_encoding = locale.getdefaultlocale()[1]
-                if locale_encoding is None or locale_encoding is '':
+                if locale_encoding is None or locale_encoding == '':
                     # situation occurs on Mac OS X
                     locale_encoding = 'ascii'
                 codecs.lookup(locale_encoding)

Lib/test/test_ast.py

@@ -1098,11 +1098,12 @@ def test_stdlib_validates(self):
         tests = [fn for fn in os.listdir(stdlib) if fn.endswith(".py")]
         tests.extend(["test/test_grammar.py", "test/test_unpack_ex.py"])
         for module in tests:
-            fn = os.path.join(stdlib, module)
-            with open(fn, "r", encoding="utf-8") as fp:
-                source = fp.read()
-            mod = ast.parse(source, fn)
-            compile(mod, fn, "exec")
+            with self.subTest(module):
+                fn = os.path.join(stdlib, module)
+                with open(fn, "r", encoding="utf-8") as fp:
+                    source = fp.read()
+                mod = ast.parse(source, fn)
+                compile(mod, fn, "exec")
 
 
 class ConstantTests(unittest.TestCase):

Lib/test/test_grammar.py

@@ -5,6 +5,7 @@
 import inspect
 import unittest
 import sys
+import warnings
 # testing import *
 from sys import *
 
@@ -989,7 +990,7 @@ def g(): f((yield from ()))
         def g(): f((yield from ()), 1)
         # Do not require parenthesis for tuple unpacking
         def g(): rest = 4, 5, 6; yield 1, 2, 3, *rest
-        self.assertEquals(list(g()), [(1, 2, 3, 4, 5, 6)])
+        self.assertEqual(list(g()), [(1, 2, 3, 4, 5, 6)])
         check_syntax_error(self, "def g(): f(yield 1)")
         check_syntax_error(self, "def g(): f(yield 1, 1)")
         check_syntax_error(self, "def g(): f(yield from ())")
@@ -1099,6 +1100,14 @@ def testAssert2(self):
         else:
             self.fail("AssertionError not raised by 'assert False'")
 
+        with self.assertWarnsRegex(SyntaxWarning, 'assertion is always true'):
+            compile('assert(x, "msg")', '<testcase>', 'exec')
+        with warnings.catch_warnings():
+            warnings.filterwarnings('error', category=SyntaxWarning)
+            with self.assertRaisesRegex(SyntaxError, 'assertion is always true'):
+                compile('assert(x, "msg")', '<testcase>', 'exec')
+            compile('assert x, "msg"', '<testcase>', 'exec')
+
 
     ### compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt | funcdef | classdef
     # Tested below
@@ -1217,11 +1226,33 @@ def test_comparison(self):
         if 1 > 1: pass
         if 1 <= 1: pass
         if 1 >= 1: pass
-        if 1 is 1: pass
-        if 1 is not 1: pass
+        if x is x: pass
+        if x is not x: pass
         if 1 in (): pass
         if 1 not in (): pass
-        if 1 < 1 > 1 == 1 >= 1 <= 1 != 1 in 1 not in 1 is 1 is not 1: pass
+        if 1 < 1 > 1 == 1 >= 1 <= 1 != 1 in 1 not in x is x is not x: pass
+
+    def test_comparison_is_literal(self):
+        def check(test, msg='"is" with a literal'):
+            with self.assertWarnsRegex(SyntaxWarning, msg):
+                compile(test, '<testcase>', 'exec')
+            with warnings.catch_warnings():
+                warnings.filterwarnings('error', category=SyntaxWarning)
+                with self.assertRaisesRegex(SyntaxError, msg):
+                    compile(test, '<testcase>', 'exec')
+
+        check('x is 1')
+        check('x is "thing"')
+        check('1 is x')
+        check('x is y is 1')
+        check('x is not 1', '"is not" with a literal')
+
+        with warnings.catch_warnings():
+            warnings.filterwarnings('error', category=SyntaxWarning)
+            compile('x is None', '<testcase>', 'exec')
+            compile('x is False', '<testcase>', 'exec')
+            compile('x is True', '<testcase>', 'exec')
+            compile('x is ...', '<testcase>', 'exec')
 
     def test_binary_mask_ops(self):
         x = 1 & 1
@@ -1511,9 +1542,11 @@ def test_paren_evaluation(self):
         self.assertEqual(16 // (4 // 2), 8)
         self.assertEqual((16 // 4) // 2, 2)
         self.assertEqual(16 // 4 // 2, 2)
-        self.assertTrue(False is (2 is 3))
-        self.assertFalse((False is 2) is 3)
-        self.assertFalse(False is 2 is 3)
+        x = 2
+        y = 3
+        self.assertTrue(False is (x is y))
+        self.assertFalse((False is x) is y)
+        self.assertFalse(False is x is y)
 
     def test_matrix_mul(self):
         # This is not intended to be a comprehensive test, rather just to be few

Misc/NEWS.d/next/Core and Builtins/2018-09-30-11-19-55.bpo-34850.CbgDwb.rst

@@ -0,0 +1,4 @@
+The compiler will emit a syntax warning when the ``is`` or ``is not``
+operator is used with a literal (except singletons ``None``, ``False``,
+``True`` and ``...``). If :exc:`SyntaxWarning` is raised as error, it will
+be replaced with a :exc:`SyntaxError`.

Python/compile.c

@@ -174,6 +174,7 @@ static int compiler_addop(struct compiler *, int);
 static int compiler_addop_i(struct compiler *, int, Py_ssize_t);
 static int compiler_addop_j(struct compiler *, int, basicblock *, int);
 static int compiler_error(struct compiler *, const char *);
+static int compiler_warn(struct compiler *, const char *);
 static int compiler_nameop(struct compiler *, identifier, expr_context_ty);
 
 static PyCodeObject *compiler_mod(struct compiler *, mod_ty);
@@ -2155,6 +2156,41 @@ compiler_class(struct compiler *c, stmt_ty s)
     return 1;
 }
 
+static int
+check_is_arg(expr_ty e)
+{
+    if (e->kind != Constant_kind) {
+        return 1;
+    }
+    PyObject *value = e->v.Constant.value;
+    return (value == Py_None
+         || value == Py_False
+         || value == Py_True
+         || value == Py_Ellipsis);
+}
+
+static int
+check_compare(struct compiler *c, expr_ty e)
+{
+    Py_ssize_t i, n;
+    int left = check_is_arg(e->v.Compare.left);
+    n = asdl_seq_LEN(e->v.Compare.ops);
+    for (i = 0; i < n; i++) {
+        cmpop_ty op = (cmpop_ty)asdl_seq_GET(e->v.Compare.ops, i);
+        int right = check_is_arg((expr_ty)asdl_seq_GET(e->v.Compare.comparators, i));
+        if (op == Is || op == IsNot) {
+            if (!right || !left) {
+                const char *msg = (op == Is)
+                        ? "\"is\" with a literal. Did you mean \"==\""
+                        : "\"is not\" with a literal. Did you mean \"!=\"";
+                return compiler_warn(c, msg);
+            }
+        }
+        left = right;
+    }
+    return 1;
+}
+
 static int
 cmpop(cmpop_ty op)
 {
@@ -2234,6 +2270,9 @@ compiler_jump_if(struct compiler *c, expr_ty e, basicblock *next, int cond)
         return 1;
     }
     case Compare_kind: {
+        if (!check_compare(c, e)) {
+            return 0;
+        }
         Py_ssize_t i, n = asdl_seq_LEN(e->v.Compare.ops) - 1;
         if (n > 0) {
             basicblock *cleanup = compiler_new_block(c);
@@ -2970,7 +3009,6 @@ compiler_assert(struct compiler *c, stmt_ty s)
 {
     static PyObject *assertion_error = NULL;
     basicblock *end;
-    PyObject* msg;
 
     if (c->c_optimize)
         return 1;
@@ -2980,18 +3018,13 @@ compiler_assert(struct compiler *c, stmt_ty s)
             return 0;
     }
     if (s->v.Assert.test->kind == Tuple_kind &&
-        asdl_seq_LEN(s->v.Assert.test->v.Tuple.elts) > 0) {
-        msg = PyUnicode_FromString("assertion is always true, "
-                                   "perhaps remove parentheses?");
-        if (msg == NULL)
-            return 0;
-        if (PyErr_WarnExplicitObject(PyExc_SyntaxWarning, msg,
-                                     c->c_filename, c->u->u_lineno,
-                                     NULL, NULL) == -1) {
-            Py_DECREF(msg);
+        asdl_seq_LEN(s->v.Assert.test->v.Tuple.elts) > 0)
+    {
+        if (!compiler_warn(c, "assertion is always true, "
+                              "perhaps remove parentheses?"))
+        {
             return 0;
         }
-        Py_DECREF(msg);
     }
     end = compiler_new_block(c);
     if (end == NULL)
@@ -3546,6 +3579,9 @@ compiler_compare(struct compiler *c, expr_ty e)
 {
     Py_ssize_t i, n;
 
+    if (!check_compare(c, e)) {
+        return 0;
+    }
     VISIT(c, expr, e->v.Compare.left);
     assert(asdl_seq_LEN(e->v.Compare.ops) > 0);
     n = asdl_seq_LEN(e->v.Compare.ops) - 1;
@@ -4792,6 +4828,32 @@ compiler_error(struct compiler *c, const char *errstr)
     return 0;
 }
 
+/* Emits a SyntaxWarning and returns 1 on success.
+   If a SyntaxWarning is raised as error, replaces it with a SyntaxError
+   and returns 0.
+*/
+
+static int
+compiler_warn(struct compiler *c, const char *errstr)
+{
+    PyObject *msg = PyUnicode_FromString(errstr);
+    if (msg == NULL) {
+        return 0;
+    }
+    if (PyErr_WarnExplicitObject(PyExc_SyntaxWarning, msg, c->c_filename,
+                                 c->u->u_lineno, NULL, NULL) < 0)
+    {
+        Py_DECREF(msg);
+        if (PyErr_ExceptionMatches(PyExc_SyntaxWarning)) {
+            PyErr_Clear();
+            return compiler_error(c, errstr);
+        }
+        return 0;
+    }
+    Py_DECREF(msg);
+    return 1;
+}
+
 static int
 compiler_handle_subscr(struct compiler *c, const char *kind,
                        expr_context_ty ctx)