bpo-33185: Fix regression in pydoc CLI sys.path handling (GH-6419)

The pydoc CLI assumed -m pydoc would add the empty string
to sys.path, and hence got confused when it switched to
adding the full initial working directory instead.

This refactors the pydoc CLI path manipulation to be
more testable, and ensures it won't accidentally
remove the standard library directory containing
pydoc itself from sys.path.

Lib/test/test_pydoc.py

@@ -11,6 +11,7 @@ import pkgutil
 import re
 import stat
 import string
+import tempfile
 import test.support
 import time
 import types
@@ -1084,6 +1085,71 @@ class PydocWithMetaClasses(unittest.TestCase):
         self.assertIn('class Enum', helptext)
 
 
+class TestInternalUtilities(unittest.TestCase):
+
+    def setUp(self):
+        tmpdir = tempfile.TemporaryDirectory()
+        self.argv0dir = tmpdir.name
+        self.argv0 = os.path.join(tmpdir.name, "nonexistent")
+        self.addCleanup(tmpdir.cleanup)
+        self.abs_curdir = abs_curdir = os.getcwd()
+        self.curdir_spellings = ["", os.curdir, abs_curdir]
+
+    def _get_revised_path(self, given_path, argv0=None):
+        # Checking that pydoc.cli() actually calls pydoc._get_revised_path()
+        # is handled via code review (at least for now).
+        if argv0 is None:
+            argv0 = self.argv0
+        return pydoc._get_revised_path(given_path, argv0)
+
+    def _get_starting_path(self):
+        # Get a copy of sys.path without the current directory
+        clean_path = sys.path.copy()
+        for spelling in self.curdir_spellings:
+            for __ in range(clean_path.count(spelling)):
+                clean_path.remove(spelling)
+        return clean_path
+
+    def test_sys_path_adjustment_adds_missing_curdir(self):
+        clean_path = self._get_starting_path()
+        expected_path = [self.abs_curdir] + clean_path
+        self.assertEqual(self._get_revised_path(clean_path), expected_path)
+
+    def test_sys_path_adjustment_removes_argv0_dir(self):
+        clean_path = self._get_starting_path()
+        expected_path = [self.abs_curdir] + clean_path
+        leading_argv0dir = [self.argv0dir] + clean_path
+        self.assertEqual(self._get_revised_path(leading_argv0dir), expected_path)
+        trailing_argv0dir = clean_path + [self.argv0dir]
+        self.assertEqual(self._get_revised_path(trailing_argv0dir), expected_path)
+
+
+    def test_sys_path_adjustment_protects_pydoc_dir(self):
+        def _get_revised_path(given_path):
+            return self._get_revised_path(given_path, argv0=pydoc.__file__)
+        clean_path = self._get_starting_path()
+        leading_argv0dir = [self.argv0dir] + clean_path
+        expected_path = [self.abs_curdir] + leading_argv0dir
+        self.assertEqual(_get_revised_path(leading_argv0dir), expected_path)
+        trailing_argv0dir = clean_path + [self.argv0dir]
+        expected_path = [self.abs_curdir] + trailing_argv0dir
+        self.assertEqual(_get_revised_path(trailing_argv0dir), expected_path)
+
+    def test_sys_path_adjustment_when_curdir_already_included(self):
+        clean_path = self._get_starting_path()
+        for spelling in self.curdir_spellings:
+            with self.subTest(curdir_spelling=spelling):
+                # If curdir is already present, no alterations are made at all
+                leading_curdir = [spelling] + clean_path
+                self.assertIsNone(self._get_revised_path(leading_curdir))
+                trailing_curdir = clean_path + [spelling]
+                self.assertIsNone(self._get_revised_path(trailing_curdir))
+                leading_argv0dir = [self.argv0dir] + leading_curdir
+                self.assertIsNone(self._get_revised_path(leading_argv0dir))
+                trailing_argv0dir = trailing_curdir + [self.argv0dir]
+                self.assertIsNone(self._get_revised_path(trailing_argv0dir))
+
+
 @reap_threads
 def test_main():
     try:
@@ -1094,6 +1160,7 @@ def test_main():
                                   PydocUrlHandlerTest,
                                   TestHelper,
                                   PydocWithMetaClasses,
+                                  TestInternalUtilities,
                                   )
     finally:
         reap_children()