gh-152845: No more forbid metadata_encoding in mode 'a'

Allow `metadata_encoding` parameter in all modes, allowing proper
decoding with a customized codec in 'a' mode.  Ignore this parameter
for 'w'/'x' modes.

Author: danny0838

Lib/test/test_zipfile/test_core.py

@@ -5735,20 +5735,26 @@ def test_read_with_metadata_encoding(self):
         # Read the ZIP archive with correct metadata_encoding
         with zipfile.ZipFile(TESTFN, "r", metadata_encoding='shift_jis') as zipfp:
             self._test_read(zipfp, self.file_names, self.file_content)
+        with zipfile.ZipFile(TESTFN, "a", metadata_encoding='shift_jis') as zipfp:
+            self._test_read(zipfp, self.file_names, self.file_content)
 
     def test_read_without_metadata_encoding(self):
         # Read the ZIP archive without metadata_encoding
         expected_names = [name.encode('shift_jis').decode('cp437')
                           for name in self.file_names[:2]] + self.file_names[2:]
         with zipfile.ZipFile(TESTFN, "r") as zipfp:
             self._test_read(zipfp, expected_names, self.file_content)
+        with zipfile.ZipFile(TESTFN, "a") as zipfp:
+            self._test_read(zipfp, expected_names, self.file_content)
 
     def test_read_with_incorrect_metadata_encoding(self):
         # Read the ZIP archive with incorrect metadata_encoding
         expected_names = [name.encode('shift_jis').decode('koi8-u')
                           for name in self.file_names[:2]] + self.file_names[2:]
         with zipfile.ZipFile(TESTFN, "r", metadata_encoding='koi8-u') as zipfp:
             self._test_read(zipfp, expected_names, self.file_content)
+        with zipfile.ZipFile(TESTFN, "a", metadata_encoding='koi8-u') as zipfp:
+            self._test_read(zipfp, expected_names, self.file_content)
 
     def test_read_with_unsuitable_metadata_encoding(self):
         # Read the ZIP archive with metadata_encoding unsuitable for
@@ -5757,6 +5763,10 @@ def test_read_with_unsuitable_metadata_encoding(self):
             zipfile.ZipFile(TESTFN, "r", metadata_encoding='ascii')
         with self.assertRaises(UnicodeDecodeError):
             zipfile.ZipFile(TESTFN, "r", metadata_encoding='utf-8')
+        with self.assertRaises(UnicodeDecodeError):
+            zipfile.ZipFile(TESTFN, "a", metadata_encoding='ascii')
+        with self.assertRaises(UnicodeDecodeError):
+            zipfile.ZipFile(TESTFN, "a", metadata_encoding='utf-8')
 
     def test_read_after_append(self):
         newname = '\u56db'  # Han 'four'
@@ -5774,9 +5784,13 @@ def test_read_after_append(self):
 
         with zipfile.ZipFile(TESTFN, "r") as zipfp:
             self._test_read(zipfp, mojibake_expected_names, expected_content)
+        with zipfile.ZipFile(TESTFN, "a") as zipfp:
+            self._test_read(zipfp, mojibake_expected_names, expected_content)
 
         with zipfile.ZipFile(TESTFN, "r", metadata_encoding='shift_jis') as zipfp:
             self._test_read(zipfp, expected_names, expected_content)
+        with zipfile.ZipFile(TESTFN, "a", metadata_encoding='shift_jis') as zipfp:
+            self._test_read(zipfp, expected_names, expected_content)
 
     def test_append_keep_efs_flag(self):
         """Files loaded from an archive should keep original EFS flags when
@@ -5824,11 +5838,18 @@ def test_write_enforce_efs_flag(self):
             self._test_read(zipfp, names, contents, comments, efs_flags)
 
     def test_write_with_metadata_encoding(self):
-        ZF = zipfile.ZipFile
+        """metadata_encoding should not affect the encoding of new files."""
+        names = ['\u4e00', 'file2']
+        contents = ['\u4e00'.encode('utf-8'), '\u4e8c'.encode('utf-8')]
+        efs_flags = [True, False]
+
         for mode in ("w", "x", "a"):
-            with self.assertRaisesRegex(ValueError,
-                                        "^metadata_encoding is only"):
-                ZF("nonesuch.zip", mode, metadata_encoding="shift_jis")
+            unlink(TESTFN)
+            with zipfile.ZipFile(TESTFN, mode, metadata_encoding='shift_jis') as zipfp:
+                for i, name in enumerate(names):
+                    zipfp.writestr(name, contents[i])
+            with zipfile.ZipFile(TESTFN, 'r') as zipfp:
+                self._test_read(zipfp, names, contents, None, efs_flags)
 
     def test_add_comment(self):
         with zipfile.ZipFile(TESTFN, "r") as zipfp:

Lib/zipfile/__init__.py

@@ -1926,11 +1926,6 @@ def __init__(self, file, mode="r", compression=ZIP_STORED, allowZip64=True,
         self._strict_timestamps = strict_timestamps
         self.metadata_encoding = metadata_encoding
 
-        # Check that we don't try to write with nonconforming codecs
-        if self.metadata_encoding and mode != 'r':
-            raise ValueError(
-                "metadata_encoding is only supported for reading files")
-
         # Check if we were passed a file-like object
         if isinstance(file, os.PathLike):
             file = os.fspath(file)