bpo-25625: add contextlib.chdir (GH-28271)

Added non parallel-safe :func:`~contextlib.chdir` context manager to change
the current working directory and then restore it on exit. Simple wrapper
around :func:`~os.chdir`.

Signed-off-by: Filipe Laíns <lains@riseup.net>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>

4 files changed, 83 insertions, 3 deletions

diff --git a/Doc/library/contextlib.rst b/Doc/library/contextlib.rst
index bc38a63..ae0ee72 100644
--- a/Doc/library/contextlib.rst
+++ b/Doc/library/contextlib.rst
@@ -353,6 +353,23 @@ Functions and classes provided:
    .. versionadded:: 3.5
 
 
+.. function:: chdir(path)
+
+   Non parallel-safe context manager to change the current working directory.
+   As this changes a global state, the working directory, it is not suitable
+   for use in most threaded or aync contexts. It is also not suitable for most
+   non-linear code execution, like generators, where the program execution is
+   temporarily relinquished -- unless explicitely desired, you should not yield
+   when this context manager is active.
+
+   This is a simple wrapper around :func:`~os.chdir`, it changes the current
+   working directory upon entering and restores the old one on exit.
+
+   This context manager is :ref:`reentrant <reentrant-cms>`.
+
+   .. versionadded:: 3.11
+
+
 .. class:: ContextDecorator()
 
    A base class that enables a context manager to also be used as a decorator.
@@ -900,8 +917,8 @@ but may also be used *inside* a :keyword:`!with` statement that is already
 using the same context manager.
 
 :class:`threading.RLock` is an example of a reentrant context manager, as are
-:func:`suppress` and :func:`redirect_stdout`. Here's a very simple example of
-reentrant use::
+:func:`suppress`, :func:`redirect_stdout`, and :func:`chdir`. Here's a very
+simple example of reentrant use::
 
     >>> from contextlib import redirect_stdout
     >>> from io import StringIO
diff --git a/Lib/contextlib.py b/Lib/contextlib.py
index d90ca5d..ee72258 100644
--- a/Lib/contextlib.py
+++ b/Lib/contextlib.py
@@ -1,5 +1,6 @@
 """Utilities for with-statement contexts.  See PEP 343."""
 import abc
+import os
 import sys
 import _collections_abc
 from collections import deque
@@ -9,7 +10,8 @@ from types import MethodType, GenericAlias
 __all__ = ["asynccontextmanager", "contextmanager", "closing", "nullcontext",
            "AbstractContextManager", "AbstractAsyncContextManager",
            "AsyncExitStack", "ContextDecorator", "ExitStack",
-           "redirect_stdout", "redirect_stderr", "suppress", "aclosing"]
+           "redirect_stdout", "redirect_stderr", "suppress", "aclosing",
+           "chdir"]
 
 
 class AbstractContextManager(abc.ABC):
@@ -762,3 +764,18 @@ class nullcontext(AbstractContextManager, AbstractAsyncContextManager):
 
     async def __aexit__(self, *excinfo):
         pass
+
+
+class chdir(AbstractContextManager):
+    """Non thread-safe context manager to change the current working directory."""
+
+    def __init__(self, path):
+        self.path = path
+        self._old_cwd = []
+
+    def __enter__(self):
+        self._old_cwd.append(os.getcwd())
+        os.chdir(self.path)
+
+    def __exit__(self, *excinfo):
+        os.chdir(self._old_cwd.pop())
diff --git a/Lib/test/test_contextlib.py b/Lib/test/test_contextlib.py
index 7982d9d..bc8e4e4 100644
--- a/Lib/test/test_contextlib.py
+++ b/Lib/test/test_contextlib.py
@@ -1,6 +1,7 @@
 """Unit tests for contextlib.py, and other context managers."""
 
 import io
+import os
 import sys
 import tempfile
 import threading
@@ -1114,5 +1115,47 @@ class TestSuppress(unittest.TestCase):
             1/0
         self.assertTrue(outer_continued)
 
+
+class TestChdir(unittest.TestCase):
+    def test_simple(self):
+        old_cwd = os.getcwd()
+        target = os.path.join(os.path.dirname(__file__), 'data')
+        self.assertNotEqual(old_cwd, target)
+
+        with chdir(target):
+            self.assertEqual(os.getcwd(), target)
+        self.assertEqual(os.getcwd(), old_cwd)
+
+    def test_reentrant(self):
+        old_cwd = os.getcwd()
+        target1 = os.path.join(os.path.dirname(__file__), 'data')
+        target2 = os.path.join(os.path.dirname(__file__), 'ziptestdata')
+        self.assertNotIn(old_cwd, (target1, target2))
+        chdir1, chdir2 = chdir(target1), chdir(target2)
+
+        with chdir1:
+            self.assertEqual(os.getcwd(), target1)
+            with chdir2:
+                self.assertEqual(os.getcwd(), target2)
+                with chdir1:
+                    self.assertEqual(os.getcwd(), target1)
+                self.assertEqual(os.getcwd(), target2)
+            self.assertEqual(os.getcwd(), target1)
+        self.assertEqual(os.getcwd(), old_cwd)
+
+    def test_exception(self):
+        old_cwd = os.getcwd()
+        target = os.path.join(os.path.dirname(__file__), 'data')
+        self.assertNotEqual(old_cwd, target)
+
+        try:
+            with chdir(target):
+                self.assertEqual(os.getcwd(), target)
+                raise RuntimeError("boom")
+        except RuntimeError as re:
+            self.assertEqual(str(re), "boom")
+        self.assertEqual(os.getcwd(), old_cwd)
+
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2021-09-10-12-53-28.bpo-25625.SzcBCw.rst b/Misc/NEWS.d/next/Library/2021-09-10-12-53-28.bpo-25625.SzcBCw.rst
new file mode 100644
index 0000000..c001683
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-09-10-12-53-28.bpo-25625.SzcBCw.rst
@@ -0,0 +1,3 @@
+Added non parallel-safe :func:`~contextlib.chdir` context manager to change
+the current working directory and then restore it on exit. Simple wrapper
+around :func:`~os.chdir`.