Issue #26039: zipfile.ZipFile.open() can now be used to write data into a ZIP

file, as well as for extracting data.  Patch by Thomas Kluyver.

Doc/library/zipfile.rst

@@ -207,15 +207,15 @@ ZipFile Objects
 .. index::
    single: universal newlines; zipfile.ZipFile.open method
 
-.. method:: ZipFile.open(name, mode='r', pwd=None)
+.. method:: ZipFile.open(name, mode='r', pwd=None, force_zip64=False)
 
-   Extract a member from the archive as a file-like object (ZipExtFile). *name*
-   is the name of the file in the archive, or a :class:`ZipInfo` object. The
+   Access a member of the archive as a file-like object.  *name*
+   is the name of the file in the archive, or a :class:`ZipInfo` object.  The
    *mode* parameter, if included, must be one of the following: ``'r'`` (the
-   default), ``'U'``, or ``'rU'``. Choosing ``'U'`` or  ``'rU'`` will enable
-   :term:`universal newlines` support in the read-only object.  *pwd* is the
-   password used for encrypted files.  Calling  :meth:`.open` on a closed
-   ZipFile will raise a  :exc:`RuntimeError`.
+   default), ``'U'``, ``'rU'`` or ``'w'``.  Choosing ``'U'`` or  ``'rU'`` will
+   enable :term:`universal newlines` support in the read-only object.  *pwd* is
+   the password used to decrypt encrypted ZIP files.  Calling  :meth:`.open` on
+   a closed ZipFile will raise a  :exc:`RuntimeError`.
 
    :meth:`~ZipFile.open` is also a context manager and therefore supports the
    :keyword:`with` statement::
@@ -224,17 +224,23 @@ ZipFile Objects
           with myzip.open('eggs.txt') as myfile:
               print(myfile.read())
 
-   .. note::
-
-      The file-like object is read-only and provides the following methods:
-      :meth:`~io.BufferedIOBase.read`, :meth:`~io.IOBase.readline`,
-      :meth:`~io.IOBase.readlines`, :meth:`__iter__`,
-      :meth:`~iterator.__next__`.
+   With *mode* ``'r'``, ``'U'`` or ``'rU'``, the file-like object
+   (``ZipExtFile``) is read-only and provides the following methods:
+   :meth:`~io.BufferedIOBase.read`, :meth:`~io.IOBase.readline`,
+   :meth:`~io.IOBase.readlines`, :meth:`__iter__`,
+   :meth:`~iterator.__next__`.  These objects can operate independently of
+   the ZipFile.
 
-   .. note::
+   With ``mode='w'``, a writable file handle is returned, which supports the
+   :meth:`~io.BufferedIOBase.write` method.  While a writable file handle is open,
+   attempting to read or write other files in the ZIP file will raise a
+   :exc:`RuntimeError`.
 
-      Objects returned by :meth:`.open` can operate independently of the
-      ZipFile.
+   When writing a file, if the file size is not known in advance but may exceed
+   2 GiB, pass ``force_zip64=True`` to ensure that the header format is
+   capable of supporting large files.  If the file size is known in advance,
+   construct a :class:`ZipInfo` object with :attr:`~ZipInfo.file_size` set, and
+   use that as the *name* parameter.
 
    .. note::
 
@@ -246,6 +252,10 @@ ZipFile Objects
       The ``'U'`` or  ``'rU'`` mode.  Use :class:`io.TextIOWrapper` for reading
       compressed text files in :term:`universal newlines` mode.
 
+   .. versionchanged:: 3.6
+      :meth:`open` can now be used to write files into the archive with the
+      ``mode='w'`` option.
+
 .. method:: ZipFile.extract(member, path=None, pwd=None)
 
    Extract a member from the archive to the current working directory; *member*

Doc/whatsnew/3.6.rst

@@ -350,6 +350,10 @@ A new :meth:`ZipInfo.is_dir() <zipfile.ZipInfo.is_dir>` method can be used
 to check if the :class:`~zipfile.ZipInfo` instance represents a directory.
 (Contributed by Thomas Kluyver in :issue:`26039`.)
 
+The :meth:`ZipFile.open() <zipfile.ZipFile.open>` method can now be used to
+write data into a ZIP file, as well as for extracting data.
+(Contributed by Thomas Kluyver in :issue:`26039`.)
+
 
 zlib
 ----

Lib/test/test_zipfile.py

@@ -61,6 +61,9 @@ class AbstractTestsWithSourceFile:
             zipfp.write(TESTFN, "another.name")
             zipfp.write(TESTFN, TESTFN)
             zipfp.writestr("strfile", self.data)
+            with zipfp.open('written-open-w', mode='w') as f:
+                for line in self.line_gen:
+                    f.write(line)
 
     def zip_test(self, f, compression):
         self.make_test_archive(f, compression)
@@ -76,7 +79,7 @@ class AbstractTestsWithSourceFile:
             zipfp.printdir(file=fp)
             directory = fp.getvalue()
             lines = directory.splitlines()
-            self.assertEqual(len(lines), 4) # Number of files + header
+            self.assertEqual(len(lines), 5) # Number of files + header
 
             self.assertIn('File Name', lines[0])
             self.assertIn('Modified', lines[0])
@@ -90,23 +93,25 @@ class AbstractTestsWithSourceFile:
 
             # Check the namelist
             names = zipfp.namelist()
-            self.assertEqual(len(names), 3)
+            self.assertEqual(len(names), 4)
             self.assertIn(TESTFN, names)
             self.assertIn("another.name", names)
             self.assertIn("strfile", names)
+            self.assertIn("written-open-w", names)
 
             # Check infolist
             infos = zipfp.infolist()
             names = [i.filename for i in infos]
-            self.assertEqual(len(names), 3)
+            self.assertEqual(len(names), 4)
             self.assertIn(TESTFN, names)
             self.assertIn("another.name", names)
             self.assertIn("strfile", names)
+            self.assertIn("written-open-w", names)
             for i in infos:
                 self.assertEqual(i.file_size, len(self.data))
 
             # check getinfo
-            for nm in (TESTFN, "another.name", "strfile"):
+            for nm in (TESTFN, "another.name", "strfile", "written-open-w"):
                 info = zipfp.getinfo(nm)
                 self.assertEqual(info.filename, nm)
                 self.assertEqual(info.file_size, len(self.data))
@@ -372,14 +377,18 @@ class StoredTestsWithSourceFile(AbstractTestsWithSourceFile,
     test_low_compression = None
 
     def zip_test_writestr_permissions(self, f, compression):
-        # Make sure that writestr creates files with mode 0600,
-        # when it is passed a name rather than a ZipInfo instance.
+        # Make sure that writestr and open(... mode='w') create files with
+        # mode 0600, when they are passed a name rather than a ZipInfo
+        # instance.
 
         self.make_test_archive(f, compression)
         with zipfile.ZipFile(f, "r") as zipfp:
             zinfo = zipfp.getinfo('strfile')
             self.assertEqual(zinfo.external_attr, 0o600 << 16)
 
+            zinfo2 = zipfp.getinfo('written-open-w')
+            self.assertEqual(zinfo2.external_attr, 0o600 << 16)
+
     def test_writestr_permissions(self):
         for f in get_files(self):
             self.zip_test_writestr_permissions(f, zipfile.ZIP_STORED)
@@ -451,6 +460,10 @@ class StoredTestsWithSourceFile(AbstractTestsWithSourceFile,
         with zipfile.ZipFile(TESTFN2, mode="r") as zipfp:
             self.assertRaises(RuntimeError, zipfp.write, TESTFN)
 
+        with zipfile.ZipFile(TESTFN2, mode="r") as zipfp:
+            with self.assertRaises(RuntimeError):
+                zipfp.open(TESTFN, mode='w')
+
     def test_add_file_before_1980(self):
         # Set atime and mtime to 1970-01-01
         os.utime(TESTFN, (0, 0))
@@ -1428,6 +1441,35 @@ class OtherTests(unittest.TestCase):
             # testzip returns the name of the first corrupt file, or None
             self.assertIsNone(zipf.testzip())
 
+    def test_open_conflicting_handles(self):
+        # It's only possible to open one writable file handle at a time
+        msg1 = b"It's fun to charter an accountant!"
+        msg2 = b"And sail the wide accountant sea"
+        msg3 = b"To find, explore the funds offshore"
+        with zipfile.ZipFile(TESTFN2, 'w', zipfile.ZIP_STORED) as zipf:
+            with zipf.open('foo', mode='w') as w2:
+                w2.write(msg1)
+            with zipf.open('bar', mode='w') as w1:
+                with self.assertRaises(RuntimeError):
+                    zipf.open('handle', mode='w')
+                with self.assertRaises(RuntimeError):
+                    zipf.open('foo', mode='r')
+                with self.assertRaises(RuntimeError):
+                    zipf.writestr('str', 'abcde')
+                with self.assertRaises(RuntimeError):
+                    zipf.write(__file__, 'file')
+                with self.assertRaises(RuntimeError):
+                    zipf.close()
+                w1.write(msg2)
+            with zipf.open('baz', mode='w') as w2:
+                w2.write(msg3)
+
+        with zipfile.ZipFile(TESTFN2, 'r') as zipf:
+            self.assertEqual(zipf.read('foo'), msg1)
+            self.assertEqual(zipf.read('bar'), msg2)
+            self.assertEqual(zipf.read('baz'), msg3)
+            self.assertEqual(zipf.namelist(), ['foo', 'bar', 'baz'])
+
     def tearDown(self):
         unlink(TESTFN)
         unlink(TESTFN2)
@@ -1761,6 +1803,22 @@ class UnseekableTests(unittest.TestCase):
                     with zipf.open('twos') as zopen:
                         self.assertEqual(zopen.read(), b'222')
 
+    def test_open_write(self):
+        for wrapper in (lambda f: f), Tellable, Unseekable:
+            with self.subTest(wrapper=wrapper):
+                f = io.BytesIO()
+                f.write(b'abc')
+                bf = io.BufferedWriter(f)
+                with zipfile.ZipFile(wrapper(bf), 'w', zipfile.ZIP_STORED) as zipf:
+                    with zipf.open('ones', 'w') as zopen:
+                        zopen.write(b'111')
+                    with zipf.open('twos', 'w') as zopen:
+                        zopen.write(b'222')
+                self.assertEqual(f.getvalue()[:5], b'abcPK')
+                with zipfile.ZipFile(f) as zipf:
+                    self.assertEqual(zipf.read('ones'), b'111')
+                    self.assertEqual(zipf.read('twos'), b'222')
+
 
 @requires_zlib
 class TestsWithMultipleOpens(unittest.TestCase):
@@ -1870,6 +1928,19 @@ class TestsWithMultipleOpens(unittest.TestCase):
         with open(os.devnull) as f:
             self.assertLess(f.fileno(), 100)
 
+    def test_write_while_reading(self):
+        with zipfile.ZipFile(TESTFN2, 'w', zipfile.ZIP_DEFLATED) as zipf:
+            zipf.writestr('ones', self.data1)
+        with zipfile.ZipFile(TESTFN2, 'a', zipfile.ZIP_DEFLATED) as zipf:
+            with zipf.open('ones', 'r') as r1:
+                data1 = r1.read(500)
+                with zipf.open('twos', 'w') as w1:
+                    w1.write(self.data2)
+                data1 += r1.read()
+        self.assertEqual(data1, self.data1)
+        with zipfile.ZipFile(TESTFN2) as zipf:
+            self.assertEqual(zipf.read('twos'), self.data2)
+
     def tearDown(self):
         unlink(TESTFN2)
 

Lib/zipfile.py

@@ -686,14 +686,19 @@ def _get_decompressor(compress_type):
 
 
 class _SharedFile:
-    def __init__(self, file, pos, close, lock):
+    def __init__(self, file, pos, close, lock, writing):
         self._file = file
         self._pos = pos
         self._close = close
         self._lock = lock
+        self._writing = writing
 
     def read(self, n=-1):
         with self._lock:
+            if self._writing():
+                raise RuntimeError("Can't read from the ZIP file while there "
+                        "is an open writing handle on it. "
+                        "Close the writing handle before trying to read.")
             self._file.seek(self._pos)
             data = self._file.read(n)
             self._pos = self._file.tell()
@@ -993,6 +998,76 @@ class ZipExtFile(io.BufferedIOBase):
             super().close()
 
 
+class _ZipWriteFile(io.BufferedIOBase):
+    def __init__(self, zf, zinfo, zip64):
+        self._zinfo = zinfo
+        self._zip64 = zip64
+        self._zipfile = zf
+        self._compressor = _get_compressor(zinfo.compress_type)
+        self._file_size = 0
+        self._compress_size = 0
+        self._crc = 0
+
+    @property
+    def _fileobj(self):
+        return self._zipfile.fp
+
+    def writable(self):
+        return True
+
+    def write(self, data):
+        nbytes = len(data)
+        self._file_size += nbytes
+        self._crc = crc32(data, self._crc)
+        if self._compressor:
+            data = self._compressor.compress(data)
+            self._compress_size += len(data)
+        self._fileobj.write(data)
+        return nbytes
+
+    def close(self):
+        super().close()
+        # Flush any data from the compressor, and update header info
+        if self._compressor:
+            buf = self._compressor.flush()
+            self._compress_size += len(buf)
+            self._fileobj.write(buf)
+            self._zinfo.compress_size = self._compress_size
+        else:
+            self._zinfo.compress_size = self._file_size
+        self._zinfo.CRC = self._crc
+        self._zinfo.file_size = self._file_size
+
+        # Write updated header info
+        if self._zinfo.flag_bits & 0x08:
+            # Write CRC and file sizes after the file data
+            fmt = '<LQQ' if self._zip64 else '<LLL'
+            self._fileobj.write(struct.pack(fmt, self._zinfo.CRC,
+                self._zinfo.compress_size, self._zinfo.file_size))
+            self._zipfile.start_dir = self._fileobj.tell()
+        else:
+            if not self._zip64:
+                if self._file_size > ZIP64_LIMIT:
+                    raise RuntimeError('File size unexpectedly exceeded ZIP64 '
+                                       'limit')
+                if self._compress_size > ZIP64_LIMIT:
+                    raise RuntimeError('Compressed size unexpectedly exceeded '
+                                       'ZIP64 limit')
+            # Seek backwards and write file header (which will now include
+            # correct CRC and file sizes)
+
+            # Preserve current position in file
+            self._zipfile.start_dir = self._fileobj.tell()
+            self._fileobj.seek(self._zinfo.header_offset)
+            self._fileobj.write(self._zinfo.FileHeader(self._zip64))
+            self._fileobj.seek(self._zipfile.start_dir)
+
+        self._zipfile._writing = False
+
+        # Successfully written: Add file to our caches
+        self._zipfile.filelist.append(self._zinfo)
+        self._zipfile.NameToInfo[self._zinfo.filename] = self._zinfo
+
 class ZipFile:
     """ Class with methods to open, read, write, close, list zip files.
 
@@ -1055,6 +1130,7 @@ class ZipFile:
         self._fileRefCnt = 1
         self._lock = threading.RLock()
         self._seekable = True
+        self._writing = False
 
         try:
             if mode == 'r':
@@ -1267,30 +1343,59 @@ class ZipFile:
         with self.open(name, "r", pwd) as fp:
             return fp.read()
 
-    def open(self, name, mode="r", pwd=None):
-        """Return file-like object for 'name'."""
-        if mode not in ("r", "U", "rU"):
-            raise RuntimeError('open() requires mode "r", "U", or "rU"')
+    def open(self, name, mode="r", pwd=None, force_zip64=False):
+        """Return file-like object for 'name'.
+
+        name is a string for the file name within the ZIP file, or a ZipInfo
+        object.
+
+        mode should be 'r' to read a file already in the ZIP file, or 'w' to
+        write to a file newly added to the archive.
+
+        pwd is the password to decrypt files (only used for reading).
+
+        When writing, if the file size is not known in advance but may exceed
+        2 GiB, pass force_zip64 to use the ZIP64 format, which can handle large
+        files.  If the size is known in advance, it is best to pass a ZipInfo
+        instance for name, with zinfo.file_size set.
+        """
+        if mode not in {"r", "w", "U", "rU"}:
+            raise RuntimeError('open() requires mode "r", "w", "U", or "rU"')
         if 'U' in mode:
             import warnings
             warnings.warn("'U' mode is deprecated",
                           DeprecationWarning, 2)
         if pwd and not isinstance(pwd, bytes):
             raise TypeError("pwd: expected bytes, got %s" % type(pwd))
+        if pwd and (mode == "w"):
+            raise ValueError("pwd is only supported for reading files")
         if not self.fp:
             raise RuntimeError(
-                "Attempt to read ZIP archive that was already closed")
+                "Attempt to use ZIP archive that was already closed")
 
         # Make sure we have an info object
         if isinstance(name, ZipInfo):
             # 'name' is already an info object
             zinfo = name
+        elif mode == 'w':
+            zinfo = ZipInfo(name)
+            zinfo.compress_type = self.compression
         else:
             # Get info object for name
             zinfo = self.getinfo(name)
 
+        if mode == 'w':
+            return self._open_to_write(zinfo, force_zip64=force_zip64)
+
+        if self._writing:
+            raise RuntimeError("Can't read from the ZIP file while there "
+                    "is an open writing handle on it. "
+                    "Close the writing handle before trying to read.")
+
+        # Open for reading:
         self._fileRefCnt += 1
-        zef_file = _SharedFile(self.fp, zinfo.header_offset, self._fpclose, self._lock)
+        zef_file = _SharedFile(self.fp, zinfo.header_offset,
+                               self._fpclose, self._lock, lambda: self._writing)
         try:
             # Skip the file header:
             fheader = zef_file.read(sizeFileHeader)
@@ -1355,6 +1460,49 @@ class ZipFile:
             zef_file.close()
             raise
 
+    def _open_to_write(self, zinfo, force_zip64=False):
+        if force_zip64 and not self._allowZip64:
+            raise ValueError(
+                "force_zip64 is True, but allowZip64 was False when opening "
+                "the ZIP file."
+            )
+        if self._writing:
+            raise RuntimeError("Can't write to the ZIP file while there is "
+                               "another write handle open on it. "
+                               "Close the first handle before opening another.")
+
+        # Sizes and CRC are overwritten with correct data after processing the file
+        if not hasattr(zinfo, 'file_size'):
+            zinfo.file_size = 0
+        zinfo.compress_size = 0
+        zinfo.CRC = 0
+
+        zinfo.flag_bits = 0x00
+        if zinfo.compress_type == ZIP_LZMA:
+            # Compressed data includes an end-of-stream (EOS) marker
+            zinfo.flag_bits |= 0x02
+        if not self._seekable:
+            zinfo.flag_bits |= 0x08
+
+        if not zinfo.external_attr:
+            zinfo.external_attr = 0o600 << 16  # permissions: ?rw-------
+
+        # Compressed size can be larger than uncompressed size
+        zip64 = self._allowZip64 and \
+                (force_zip64 or zinfo.file_size * 1.05 > ZIP64_LIMIT)
+
+        if self._seekable:
+            self.fp.seek(self.start_dir)
+        zinfo.header_offset = self.fp.tell()
+
+        self._writecheck(zinfo)
+        self._didModify = True
+
+        self.fp.write(zinfo.FileHeader(zip64))
+
+        self._writing = True
+        return _ZipWriteFile(self, zinfo, zip64)
+
     def extract(self, member, path=None, pwd=None):
         """Extract a member from the archive to the current working directory,
            using its full name. Its file information is extracted as accurately
@@ -1464,6 +1612,10 @@ class ZipFile:
         if not self.fp:
             raise RuntimeError(
                 "Attempt to write to ZIP archive that was already closed")
+        if self._writing:
+            raise RuntimeError(
+                "Can't write to ZIP archive while an open writing handle exists"
+            )
 
         zinfo = ZipInfo.from_file(filename, arcname)
 
@@ -1476,75 +1628,25 @@ class ZipFile:
             else:
                 zinfo.compress_type = self.compression
 
-        with self._lock:
-            if self._seekable:
-                self.fp.seek(self.start_dir)
-            zinfo.header_offset = self.fp.tell()    # Start of header bytes
-            if zinfo.compress_type == ZIP_LZMA:
+        if zinfo.is_dir():
+            with self._lock:
+                if self._seekable:
+                    self.fp.seek(self.start_dir)
+                zinfo.header_offset = self.fp.tell()  # Start of header bytes
+                if zinfo.compress_type == ZIP_LZMA:
                 # Compressed data includes an end-of-stream (EOS) marker
-                zinfo.flag_bits |= 0x02
+                    zinfo.flag_bits |= 0x02
 
-            self._writecheck(zinfo)
-            self._didModify = True
+                self._writecheck(zinfo)
+                self._didModify = True
 
-            if zinfo.is_dir():
                 self.filelist.append(zinfo)
                 self.NameToInfo[zinfo.filename] = zinfo
                 self.fp.write(zinfo.FileHeader(False))
                 self.start_dir = self.fp.tell()
-                return
-
-            cmpr = _get_compressor(zinfo.compress_type)
-            if not self._seekable:
-                zinfo.flag_bits |= 0x08
-            with open(filename, "rb") as fp:
-                # Must overwrite CRC and sizes with correct data later
-                zinfo.CRC = CRC = 0
-                zinfo.compress_size = compress_size = 0
-                # Compressed size can be larger than uncompressed size
-                zip64 = self._allowZip64 and \
-                    zinfo.file_size * 1.05 > ZIP64_LIMIT
-                self.fp.write(zinfo.FileHeader(zip64))
-                file_size = 0
-                while 1:
-                    buf = fp.read(1024 * 8)
-                    if not buf:
-                        break
-                    file_size = file_size + len(buf)
-                    CRC = crc32(buf, CRC)
-                    if cmpr:
-                        buf = cmpr.compress(buf)
-                        compress_size = compress_size + len(buf)
-                    self.fp.write(buf)
-            if cmpr:
-                buf = cmpr.flush()
-                compress_size = compress_size + len(buf)
-                self.fp.write(buf)
-                zinfo.compress_size = compress_size
-            else:
-                zinfo.compress_size = file_size
-            zinfo.CRC = CRC
-            zinfo.file_size = file_size
-            if zinfo.flag_bits & 0x08:
-                # Write CRC and file sizes after the file data
-                fmt = '<LQQ' if zip64 else '<LLL'
-                self.fp.write(struct.pack(fmt, zinfo.CRC, zinfo.compress_size,
-                                          zinfo.file_size))
-                self.start_dir = self.fp.tell()
-            else:
-                if not zip64 and self._allowZip64:
-                    if file_size > ZIP64_LIMIT:
-                        raise RuntimeError('File size has increased during compressing')
-                    if compress_size > ZIP64_LIMIT:
-                        raise RuntimeError('Compressed size larger than uncompressed size')
-                # Seek backwards and write file header (which will now include
-                # correct CRC and file sizes)
-                self.start_dir = self.fp.tell() # Preserve current position in file
-                self.fp.seek(zinfo.header_offset)
-                self.fp.write(zinfo.FileHeader(zip64))
-                self.fp.seek(self.start_dir)
-            self.filelist.append(zinfo)
-            self.NameToInfo[zinfo.filename] = zinfo
+        else:
+            with open(filename, "rb") as src, self.open(zinfo, 'w') as dest:
+                shutil.copyfileobj(src, dest, 1024*8)
 
     def writestr(self, zinfo_or_arcname, data, compress_type=None):
         """Write a file into the archive.  The contents is 'data', which
@@ -1569,45 +1671,18 @@ class ZipFile:
         if not self.fp:
             raise RuntimeError(
                 "Attempt to write to ZIP archive that was already closed")
+        if self._writing:
+            raise RuntimeError(
+                "Can't write to ZIP archive while an open writing handle exists."
+            )
+
+        if compress_type is not None:
+            zinfo.compress_type = compress_type
 
         zinfo.file_size = len(data)            # Uncompressed size
         with self._lock:
-            if self._seekable:
-                self.fp.seek(self.start_dir)
-            zinfo.header_offset = self.fp.tell()    # Start of header data
-            if compress_type is not None:
-                zinfo.compress_type = compress_type
-            zinfo.header_offset = self.fp.tell()    # Start of header data
-            if compress_type is not None:
-                zinfo.compress_type = compress_type
-            if zinfo.compress_type == ZIP_LZMA:
-                # Compressed data includes an end-of-stream (EOS) marker
-                zinfo.flag_bits |= 0x02
-
-            self._writecheck(zinfo)
-            self._didModify = True
-            zinfo.CRC = crc32(data)       # CRC-32 checksum
-            co = _get_compressor(zinfo.compress_type)
-            if co:
-                data = co.compress(data) + co.flush()
-                zinfo.compress_size = len(data)    # Compressed size
-            else:
-                zinfo.compress_size = zinfo.file_size
-            zip64 = zinfo.file_size > ZIP64_LIMIT or \
-                zinfo.compress_size > ZIP64_LIMIT
-            if zip64 and not self._allowZip64:
-                raise LargeZipFile("Filesize would require ZIP64 extensions")
-            self.fp.write(zinfo.FileHeader(zip64))
-            self.fp.write(data)
-            if zinfo.flag_bits & 0x08:
-                # Write CRC and file sizes after the file data
-                fmt = '<LQQ' if zip64 else '<LLL'
-                self.fp.write(struct.pack(fmt, zinfo.CRC, zinfo.compress_size,
-                                          zinfo.file_size))
-            self.fp.flush()
-            self.start_dir = self.fp.tell()
-            self.filelist.append(zinfo)
-            self.NameToInfo[zinfo.filename] = zinfo
+            with self.open(zinfo, mode='w') as dest:
+                dest.write(data)
 
     def __del__(self):
         """Call the "close()" method in case the user forgot."""
@@ -1619,6 +1694,11 @@ class ZipFile:
         if self.fp is None:
             return
 
+        if self._writing:
+            raise RuntimeError("Can't close the ZIP file while there is "
+                               "an open writing handle on it. "
+                               "Close the writing handle before closing the zip.")
+
         try:
             if self.mode in ('w', 'x', 'a') and self._didModify: # write ending records
                 with self._lock:

Misc/NEWS

@@ -277,6 +277,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #26039: zipfile.ZipFile.open() can now be used to write data into a ZIP
+  file, as well as for extracting data.  Patch by Thomas Kluyver.
+
 - Issue #26892: Honor debuglevel flag in urllib.request.HTTPHandler. Patch
   contributed by Chi Hsuan Yen.