gh-84481: Add ZipFile.data_offset attribute

Doc/library/zipfile.rst

@@ -538,6 +538,14 @@ The following data attributes are also available:
    it should be no longer than 65535 bytes.  Comments longer than this will be
    truncated.
 
+.. attribute:: ZipFile.data_offset
+
+   The offset to the start of ZIP data from the beginning of the file. When the
+   :class:`ZipFile` is opened in either mode ``'w'`` or ``'x'`` and the
+   underlying file does not support ``tell()``, the value will be ``None``
+   instead.
+
+   .. versionadded:: 3.14
 
 .. _path-objects:
 

Lib/test/test_zipfile/test_core.py

@@ -3312,6 +3312,54 @@ def test_execute_zip64(self):
         self.assertIn(b'number in executable: 5', output)
 
 
+class TestDataOffsetPrependedZip(unittest.TestCase):
+    """Test .data_offset on reading zip files with an executable prepended."""
+
+    def setUp(self):
+        self.exe_zip = findfile('exe_with_zip', subdir='archivetestdata')
+        self.exe_zip64 = findfile('exe_with_z64', subdir='archivetestdata')
+
+    def _test_data_offset(self, name):
+        with zipfile.ZipFile(name) as zipfp:
+            self.assertEqual(zipfp.data_offset, 713)
+
+    def test_data_offset_with_exe_prepended(self):
+        self._test_data_offset(self.exe_zip)
+
+    def test_data_offset_with_exe_prepended_zip64(self):
+        self._test_data_offset(self.exe_zip64)
+
+class TestDataOffsetZipWrite(unittest.TestCase):
+    """Test .data_offset for ZipFile opened in write mode."""
+
+    def setUp(self):
+        os.mkdir(TESTFNDIR)
+        self.addCleanup(rmtree, TESTFNDIR)
+        self.test_path = os.path.join(TESTFNDIR, 'testoffset.zip')
+
+    def test_data_offset_write_no_prefix(self):
+        with io.BytesIO() as fp:
+            with zipfile.ZipFile(fp, "w") as zipfp:
+                self.assertEqual(zipfp.data_offset, 0)
+
+    def test_data_offset_write_with_prefix(self):
+        with io.BytesIO() as fp:
+            fp.write(b"this is a prefix")
+            with zipfile.ZipFile(fp, "w") as zipfp:
+                self.assertEqual(zipfp.data_offset, 16)
+
+    def test_data_offset_write_no_tell(self):
+        # The initializer in ZipFile checks if tell raises AttributeError or
+        # OSError when creating a file in write mode when deducing the offset
+        # of the beginning of zip data
+        class NoTellBytesIO(io.BytesIO):
+            def tell(self):
+                raise OSError("Unimplemented!")
+        with NoTellBytesIO() as fp:
+            with zipfile.ZipFile(fp, "w") as zipfp:
+                self.assertIs(zipfp.data_offset, None)
+
+
 class EncodedMetadataTests(unittest.TestCase):
     file_names = ['\u4e00', '\u4e8c', '\u4e09']  # Han 'one', 'two', 'three'
     file_content = [

Lib/zipfile/__init__.py

@@ -1413,10 +1413,12 @@ def __init__(self, file, mode="r", compression=ZIP_STORED, allowZip64=True,
                 self._didModify = True
                 try:
                     self.start_dir = self.fp.tell()
+                    self._data_offset = self.start_dir
                 except (AttributeError, OSError):
                     self.fp = _Tellable(self.fp)
                     self.start_dir = 0
                     self._seekable = False
+                    self._data_offset = None
                 else:
                     # Some file-like objects can provide tell() but not seek()
                     try:
@@ -1486,6 +1488,10 @@ def _RealGetContents(self):
             # If Zip64 extension structures are present, account for them
             concat -= (sizeEndCentDir64 + sizeEndCentDir64Locator)
 
+        # store the offset to the beginning of data for the
+        # .data_offset property
+        self._data_offset = concat
+
         if self.debug > 2:
             inferred = concat + offset_cd
             print("given, inferred, offset", offset_cd, inferred, concat)
@@ -1551,6 +1557,12 @@ def _RealGetContents(self):
             zinfo._end_offset = end_offset
             end_offset = zinfo.header_offset
 
+    @property
+    def data_offset(self):
+        """The offset to the start of zip data in the file or None if
+        unavailable."""
+        return self._data_offset
+
     def namelist(self):
         """Return a list of file names in the archive."""
         return [data.filename for data in self.filelist]

Misc/NEWS.d/next/Library/2025-04-06-09-55-43.gh-issue-84481.cX4yTn.rst

@@ -0,0 +1,5 @@
+Add the :attr:`zipfile.ZipFile.data_offset` attribute, which stores the
+offset to the beginning of ZIP data in a file when available. When the
+:class:`zipfile.ZipFile` is opened in either mode ``'w'`` or ``'x'`` and the
+underlying file does not support ``tell()``, the value will be ``None``
+instead.