pathlib ABCs: raise text encoding warnings at correct stack level

Ensure that warnings about unspecified text encodings are emitted from
`ReadablePath.read_text()`, `WritablePath.write_text()` and `magic_open()`
with the correct stack level set.

Author: barneygale

Lib/pathlib/_os.py

@@ -172,12 +172,16 @@ def magic_open(path, mode='r', buffering=-1, encoding=None, errors=None,
     Open the file pointed to by this path and return a file object, as
     the built-in open() function does.
     """
+    text = 'b' not in mode
+    if text:
+        # Call io.text_encoding() here to ensure any warning is raised at an
+        # appropriate stack level.
+        encoding = io.text_encoding(encoding)
     try:
         return io.open(path, mode, buffering, encoding, errors, newline)
     except TypeError:
         pass
     cls = type(path)
-    text = 'b' not in mode
     mode = ''.join(sorted(c for c in mode if c not in 'bt'))
     if text:
         try:

Lib/pathlib/types.py

@@ -12,6 +12,7 @@
 
 from abc import ABC, abstractmethod
 from glob import _PathGlobber
+from io import text_encoding
 from pathlib._os import magic_open, ensure_distinct_paths, ensure_different_files, copyfileobj
 from pathlib import PurePath, Path
 from typing import Optional, Protocol, runtime_checkable
@@ -262,6 +263,9 @@ def read_text(self, encoding=None, errors=None, newline=None):
         """
         Open the file in text mode, read it, and close the file.
         """
+        # Call io.text_encoding() here to ensure any warning is raised at an
+        # appropriate stack level.
+        encoding = text_encoding(encoding)
         with magic_open(self, mode='r', encoding=encoding, errors=errors, newline=newline) as f:
             return f.read()
 
@@ -391,6 +395,9 @@ def write_text(self, data, encoding=None, errors=None, newline=None):
         """
         Open the file in text mode, write to it, and close the file.
         """
+        # Call io.text_encoding() here to ensure any warning is raised at an
+        # appropriate stack level.
+        encoding = text_encoding(encoding)
         if not isinstance(data, str):
             raise TypeError('data must be str, not %s' %
                             data.__class__.__name__)

Lib/test/test_pathlib/test_read.py

@@ -4,6 +4,7 @@
 
 import collections.abc
 import io
+import sys
 import unittest
 
 from .support import is_pypi
@@ -35,6 +36,17 @@ def test_open_r(self):
             self.assertIsInstance(f, io.TextIOBase)
             self.assertEqual(f.read(), 'this is file A\n')
 
+    @unittest.skipIf(
+        not getattr(sys.flags, 'warn_default_encoding', 0),
+        "Requires warn_default_encoding",
+    )
+    def test_open_r_encoding_warning(self):
+        p = self.root / 'fileA'
+        with self.assertWarns(EncodingWarning) as wc:
+            with magic_open(p, 'r'):
+                pass
+        self.assertEqual(wc.filename, __file__)
+
     def test_open_rb(self):
         p = self.root / 'fileA'
         with magic_open(p, 'rb') as f:
@@ -55,6 +67,16 @@ def test_read_text(self):
         self.assertEqual(q.read_text(encoding='latin-1'), 'äbcdefg')
         self.assertEqual(q.read_text(encoding='utf-8', errors='ignore'), 'bcdefg')
 
+    @unittest.skipIf(
+        not getattr(sys.flags, 'warn_default_encoding', 0),
+        "Requires warn_default_encoding",
+    )
+    def test_read_text_encoding_warning(self):
+        p = self.root / 'fileA'
+        with self.assertWarns(EncodingWarning) as wc:
+            p.read_text()
+        self.assertEqual(wc.filename, __file__)
+
     def test_read_text_with_newlines(self):
         p = self.root / 'abc'
         self.ground.create_file(p, b'abcde\r\nfghlk\n\rmnopq')

Lib/test/test_pathlib/test_write.py

@@ -4,6 +4,7 @@
 
 import io
 import os
+import sys
 import unittest
 
 from .support import is_pypi
@@ -35,6 +36,17 @@ def test_open_w(self):
             f.write('this is file A\n')
         self.assertEqual(self.ground.readtext(p), 'this is file A\n')
 
+    @unittest.skipIf(
+        not getattr(sys.flags, 'warn_default_encoding', 0),
+        "Requires warn_default_encoding",
+    )
+    def test_open_w_encoding_warning(self):
+        p = self.root / 'fileA'
+        with self.assertWarns(EncodingWarning) as wc:
+            with magic_open(p, 'w'):
+                pass
+        self.assertEqual(wc.filename, __file__)
+
     def test_open_wb(self):
         p = self.root / 'fileA'
         with magic_open(p, 'wb') as f:
@@ -61,6 +73,16 @@ def test_write_text(self):
         self.assertRaises(TypeError, p.write_text, b'somebytes')
         self.assertEqual(self.ground.readbytes(p), b'\xe4bcdefg')
 
+    @unittest.skipIf(
+        not getattr(sys.flags, 'warn_default_encoding', 0),
+        "Requires warn_default_encoding",
+    )
+    def test_write_text_encoding_warning(self):
+        p = self.root / 'fileA'
+        with self.assertWarns(EncodingWarning) as wc:
+            p.write_text('abcdefg')
+        self.assertEqual(wc.filename, __file__)
+
     def test_write_text_with_newlines(self):
         # Check that `\n` character change nothing
         p = self.root / 'fileA'