Refactor modify sys_path for execution as python module

4 files changed, 98 insertions, 10 deletions

diff --git a/ChangeLog b/ChangeLog
index 6af40d64b..d1d4aea6e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -25,6 +25,11 @@ Release date: TBA
 
   Close #3263
 
+* Fix issue when executing with ``python -m pylint``
+
+  Closes #4161
+
+
 What's New in Pylint 2.7.2?
 ===========================
 Release date: 2021-02-28
diff --git a/pylint/__init__.py b/pylint/__init__.py
index 1fd6ffd16..20bb554d2 100644
--- a/pylint/__init__.py
+++ b/pylint/__init__.py
@@ -8,6 +8,7 @@
 # Licensed under the GPL: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html
 # For details: https://github.com/PyCQA/pylint/blob/master/COPYING
 
+import os
 import sys
 
 from pylint.__pkginfo__ import version as __version__
@@ -44,4 +45,26 @@ def run_symilar():
     SimilarRun(sys.argv[1:])
 
 
+def modify_sys_path() -> None:
+    """Modify sys path for execution as Python module.
+
+    Strip out the current working directory from sys.path.
+    Having the working directory in `sys.path` means that `pylint` might
+    inadvertently import user code from modules having the same name as
+    stdlib or pylint's own modules.
+    CPython issue: https://bugs.python.org/issue33053
+
+    - Remove the first entry. This will always be either "" or the working directory
+    - Remove the working directory from the second and third entries. This can
+      occur if PYTHONPATH includes a ":" at the beginning or the end.
+      https://github.com/PyCQA/pylint/issues/3636
+    - Don't remove the working directory from the rest. It will be included
+      if pylint is installed in an editable configuration (as the last item).
+      https://github.com/PyCQA/pylint/issues/4161
+    """
+    sys.path = [
+        p for i, p in enumerate(sys.path) if i > 0 and not (i < 3 and p == os.getcwd())
+    ]
+
+
 __all__ = ["__version__"]
diff --git a/pylint/__main__.py b/pylint/__main__.py
index 89bd19924..4d7653718 100644
--- a/pylint/__main__.py
+++ b/pylint/__main__.py
@@ -3,16 +3,7 @@
 # Licensed under the GPL: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html
 # For details: https://github.com/PyCQA/pylint/blob/master/COPYING
 
-import os
-import sys
-
 import pylint
 
-# Strip out the current working directory from sys.path.
-# Having the working directory in `sys.path` means that `pylint` might
-# inadvertently import user code from modules having the same name as
-# stdlib or pylint's own modules.
-# CPython issue: https://bugs.python.org/issue33053
-sys.path = [p for p in sys.path if p not in ("", os.getcwd())]
-
+pylint.modify_sys_path()
 pylint.run_pylint()
diff --git a/tests/test_self.py b/tests/test_self.py
index 5b55f6cca..42ef732ec 100644
--- a/tests/test_self.py
+++ b/tests/test_self.py
@@ -42,12 +42,16 @@ import subprocess
 import sys
 import textwrap
 import warnings
+from copy import copy
 from io import StringIO
 from os.path import abspath, dirname, join
+from typing import Generator
 from unittest import mock
+from unittest.mock import patch
 
 import pytest
 
+from pylint import modify_sys_path
 from pylint.constants import MAIN_CHECKER_NAME, MSG_TYPES_STATUS
 from pylint.lint import Run
 from pylint.reporters import JSONReporter
@@ -700,6 +704,71 @@ class TestRunTC:
         )
 
     @staticmethod
+    def test_modify_sys_path() -> None:
+        @contextlib.contextmanager
+        def test_sys_path() -> Generator[None, None, None]:
+            original_path = sys.path
+            try:
+                yield
+            finally:
+                sys.path = original_path
+
+        with test_sys_path(), patch("os.getcwd") as mock_getcwd:
+            cwd = "/tmp/pytest-of-root/pytest-0/test_do_not_import_files_from_0"
+            mock_getcwd.return_value = cwd
+
+            paths = [
+                cwd,
+                cwd,
+                "/usr/local/lib/python39.zip",
+                "/usr/local/lib/python3.9",
+                "/usr/local/lib/python3.9/lib-dynload",
+                "/usr/local/lib/python3.9/site-packages",
+            ]
+            sys.path = copy(paths)
+            modify_sys_path()
+            assert sys.path == paths[2:]
+
+            paths = [
+                cwd,
+                "/custom_pythonpath",
+                cwd,
+                "/usr/local/lib/python39.zip",
+                "/usr/local/lib/python3.9",
+                "/usr/local/lib/python3.9/lib-dynload",
+                "/usr/local/lib/python3.9/site-packages",
+            ]
+            sys.path = copy(paths)
+            modify_sys_path()
+            assert sys.path == [paths[1]] + paths[3:]
+
+            paths = [
+                "",
+                cwd,
+                "/custom_pythonpath",
+                "/usr/local/lib/python39.zip",
+                "/usr/local/lib/python3.9",
+                "/usr/local/lib/python3.9/lib-dynload",
+                "/usr/local/lib/python3.9/site-packages",
+            ]
+            sys.path = copy(paths)
+            modify_sys_path()
+            assert sys.path == paths[2:]
+
+            paths = [
+                "",
+                cwd,
+                "/usr/local/lib/python39.zip",
+                "/usr/local/lib/python3.9",
+                "/usr/local/lib/python3.9/lib-dynload",
+                "/usr/local/lib/python3.9/site-packages",
+                cwd,
+            ]
+            sys.path = copy(paths)
+            modify_sys_path()
+            assert sys.path == paths[2:]
+
+    @staticmethod
     def test_do_not_import_files_from_local_directory(tmpdir):
         p_astroid = tmpdir / "astroid.py"
         p_astroid.write("'Docstring'\nimport completely_unknown\n")