bpo-38530: Offer suggestions on NameError (GH-25397)

When printing NameError raised by the interpreter, PyErr_Display
will offer suggestions of simmilar variable names in the function that the exception
was raised from:

    >>> schwarzschild_black_hole = None
    >>> schwarschild_black_hole
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'schwarschild_black_hole' is not defined. Did you mean: schwarzschild_black_hole?

Lib/test/test_exceptions.py

@@ -1413,6 +1413,129 @@ class ExceptionTests(unittest.TestCase):
 
             gc_collect()
 
+global_for_suggestions = None
+
+class NameErrorTests(unittest.TestCase):
+    def test_name_error_has_name(self):
+        try:
+            bluch
+        except NameError as exc:
+            self.assertEqual("bluch", exc.name)
+
+    def test_name_error_suggestions(self):
+        def Substitution():
+            noise = more_noise = a = bc = None
+            blech = None
+            print(bluch)
+
+        def Elimination():
+            noise = more_noise = a = bc = None
+            blch = None
+            print(bluch)
+
+        def Addition():
+            noise = more_noise = a = bc = None
+            bluchin = None
+            print(bluch)
+
+        def SubstitutionOverElimination():
+            blach = None
+            bluc = None
+            print(bluch)
+
+        def SubstitutionOverAddition():
+            blach = None
+            bluchi = None
+            print(bluch)
+
+        def EliminationOverAddition():
+            blucha = None
+            bluc = None
+            print(bluch)
+
+        for func, suggestion in [(Substitution, "blech?"),
+                                (Elimination, "blch?"),
+                                (Addition, "bluchin?"),
+                                (EliminationOverAddition, "blucha?"),
+                                (SubstitutionOverElimination, "blach?"),
+                                (SubstitutionOverAddition, "blach?")]:
+            err = None
+            try:
+                func()
+            except NameError as exc:
+                with support.captured_stderr() as err:
+                    sys.__excepthook__(*sys.exc_info())
+            self.assertIn(suggestion, err.getvalue())
+
+    def test_name_error_suggestions_from_globals(self):
+        def func():
+            print(global_for_suggestio)
+        try:
+            func()
+        except NameError as exc:
+            with support.captured_stderr() as err:
+                sys.__excepthook__(*sys.exc_info())
+        self.assertIn("global_for_suggestions?", err.getvalue())
+
+    def test_name_error_suggestions_do_not_trigger_for_long_names(self):
+        def f():
+            somethingverywronghehehehehehe = None
+            print(somethingverywronghe)
+
+        try:
+            f()
+        except NameError as exc:
+            with support.captured_stderr() as err:
+                sys.__excepthook__(*sys.exc_info())
+
+        self.assertNotIn("somethingverywronghehe", err.getvalue())
+
+    def test_name_error_suggestions_do_not_trigger_for_big_dicts(self):
+        def f():
+            # Mutating locals() is unreliable, so we need to do it by hand
+            a1 = a2 = a3 = a4 = a5 = a6 = a7 = a8 = a9 = a10 = a11 = a12 = a13 = \
+            a14 = a15 = a16 = a17 = a18 = a19 = a20 = a21 = a22 = a23 = a24 = a25 = \
+            a26 = a27 = a28 = a29 = a30 = a31 = a32 = a33 = a34 = a35 = a36 = a37 = \
+            a38 = a39 = a40 = a41 = a42 = a43 = a44 = a45 = a46 = a47 = a48 = a49 = \
+            a50 = a51 = a52 = a53 = a54 = a55 = a56 = a57 = a58 = a59 = a60 = a61 = \
+            a62 = a63 = a64 = a65 = a66 = a67 = a68 = a69 = a70 = a71 = a72 = a73 = \
+            a74 = a75 = a76 = a77 = a78 = a79 = a80 = a81 = a82 = a83 = a84 = a85 = \
+            a86 = a87 = a88 = a89 = a90 = a91 = a92 = a93 = a94 = a95 = a96 = a97 = \
+            a98 = a99 = a100 = a101 = a102 = a103 = None
+            print(a0)
+
+        try:
+            f()
+        except NameError as exc:
+            with support.captured_stderr() as err:
+                sys.__excepthook__(*sys.exc_info())
+
+        self.assertNotIn("a10", err.getvalue())
+
+    def test_name_error_with_custom_exceptions(self):
+        def f():
+            blech = None
+            raise NameError()
+
+        try:
+            f()
+        except NameError as exc:
+            with support.captured_stderr() as err:
+                sys.__excepthook__(*sys.exc_info())
+
+        self.assertNotIn("blech", err.getvalue())
+
+        def f():
+            blech = None
+            raise NameError
+
+        try:
+            f()
+        except NameError as exc:
+            with support.captured_stderr() as err:
+                sys.__excepthook__(*sys.exc_info())
+
+        self.assertNotIn("blech", err.getvalue())
 
 class AttributeErrorTests(unittest.TestCase):
     def test_attributes(self):