diff -r 8e7a738c3840 Lib/zipfile.py
--- a/Lib/zipfile.py	Sat Nov 16 10:47:17 2013 -0500
+++ b/Lib/zipfile.py	Sat Nov 16 20:41:45 2013 +0200
@@ -13,6 +13,7 @@
 import shutil
 import struct
 import binascii
+import collections
 
 
 try:
@@ -160,6 +161,8 @@
 _CD64_DIRECTORY_SIZE = 8
 _CD64_OFFSET_START_CENTDIR = 9
 
+_EXTRA_FIELD_STRUCT = struct.Struct('<HH')
+
 def _check_zipfile(fp):
     try:
         if _EndRecData(fp):
@@ -288,6 +291,37 @@
     return None
 
 
+class ExtraMap(collections.MutableMapping):
+    def __init__(self, zinfo):
+        self._zinfo = zinfo
+
+    def __getitem__(self, key):
+        for xid, data in reversed(self._zinfo._extra_list):
+            if xid == key:
+                return data
+        raise KeyError(key)
+
+    def __setitem__(self, key, value):
+        self._zinfo._extra_list = [(xid, data)
+                                   for xid, data in self._zinfo._extra_list
+                                   if xid != key] + [(key, value)]
+
+    def __delitem__(self, key):
+        self._zinfo._extra_list = [(xid, data)
+                                   for xid, data in self._zinfo._extra_list
+                                   if xid != key]
+
+    def __iter__(self):
+        for xid, data in self._zinfo._extra_list:
+            yield xid
+
+    def items(self):
+        return iter(self._zinfo._extra_list)
+
+    def __len__(self):
+        return len(self._zinfo._extra_list)
+
+
 class ZipInfo (object):
     """Class with attributes describing each file in the ZIP archive."""
 
@@ -297,7 +331,8 @@
         'date_time',
         'compress_type',
         'comment',
-        'extra',
+        '_extra_bytes',
+        '_extra_items',
         'create_system',
         'create_version',
         'extract_version',
@@ -336,7 +371,8 @@
         # Standard values:
         self.compress_type = ZIP_STORED # Type of compression for the file
         self.comment = b""              # Comment for each file
-        self.extra = b""                # ZIP extra data
+        self._extra_bytes = b""           # ZIP extra data
+        self._extra_items = None
         if sys.platform == 'win32':
             self.create_system = 0          # System which created ZIP archive
         else:
@@ -368,15 +404,11 @@
             compress_size = self.compress_size
             file_size = self.file_size
 
-        extra = self.extra
-
         min_version = 0
         if zip64 is None:
             zip64 = file_size > ZIP64_LIMIT or compress_size > ZIP64_LIMIT
         if zip64:
-            fmt = '<HHQQ'
-            extra = extra + struct.pack(fmt,
-                                        1, struct.calcsize(fmt)-4, file_size, compress_size)
+            self.extra_map[1] = struct.pack('<QQ', file_size, compress_size)
         if file_size > ZIP64_LIMIT or compress_size > ZIP64_LIMIT:
             if not zip64:
                 raise LargeZipFile("Filesize would require ZIP64 extensions")
@@ -398,8 +430,8 @@
                              self.extract_version, self.reserved, flag_bits,
                              self.compress_type, dostime, dosdate, CRC,
                              compress_size, file_size,
-                             len(filename), len(extra))
-        return header + filename + extra
+                             len(filename), len(self.extra))
+        return header + filename + self.extra
 
     def _encodeFilenameFlags(self):
         try:
@@ -407,41 +439,62 @@
         except UnicodeEncodeError:
             return self.filename.encode('utf-8'), self.flag_bits | 0x800
 
+    @property
+    def extra(self):
+        if self._extra_bytes is None:
+            self._extra_bytes = b''.join(
+                    _EXTRA_FIELD_STRUCT.pack(xid, len(data)) + data
+                    for xid, data in self._extra_items)
+        return self._extra_bytes
+
+    @extra.setter
+    def extra(self, value):
+        self._extra_bytes = value
+        self._extra_items = None
+
+    @property
+    def _extra_list(self):
+        if self._extra_items is None:
+            extra = self._extra_bytes
+            items = []
+            append = items.append
+            unpack = _EXTRA_FIELD_STRUCT.unpack
+            i = 0
+            while i < len(extra):
+                xid, xlen = unpack(extra[i: i + 4])
+                append((xid, extra[i + 4: i + 4 + xlen]))
+                i += 4 + xlen
+            self._extra_items = items
+        return self._extra_items
+
+    @_extra_list.setter
+    def _extra_list(self, value):
+        self._extra_bytes = None
+        self._extra_items = value
+
+    @property
+    def extra_map(self):
+        return ExtraMap(self)
+
+    @extra_map.setter
+    def extra_map(self, value):
+        self._extra_list = tuple(value.items())
+
     def _decodeExtra(self):
         # Try to decode the extra field.
-        extra = self.extra
-        unpack = struct.unpack
-        while extra:
-            tp, ln = unpack('<HH', extra[:4])
+        for tp, data in self._extra_list:
             if tp == 1:
-                if ln >= 24:
-                    counts = unpack('<QQQ', extra[4:28])
-                elif ln == 16:
-                    counts = unpack('<QQ', extra[4:20])
-                elif ln == 8:
-                    counts = unpack('<Q', extra[4:12])
-                elif ln == 0:
-                    counts = ()
-                else:
-                    raise RuntimeError("Corrupt extra field %s"%(ln,))
-
-                idx = 0
-
-                # ZIP64 extension (large files and/or large archives)
-                if self.file_size in (0xffffffffffffffff, 0xffffffff):
-                    self.file_size = counts[idx]
-                    idx += 1
-
-                if self.compress_size == 0xFFFFFFFF:
-                    self.compress_size = counts[idx]
-                    idx += 1
-
-                if self.header_offset == 0xffffffff:
-                    old = self.header_offset
-                    self.header_offset = counts[idx]
-                    idx+=1
-
-            extra = extra[ln+4:]
+                try:
+                    counts = struct.iter_unpack('<Q', data)
+                    # ZIP64 extension (large files and/or large archives)
+                    if self.file_size in (0xFFFFFFFFFFFFFFFF, 0xFFFFFFFF):
+                        self.file_size, = next(counts)
+                    if self.compress_size == 0xFFFFFFFF:
+                        self.compress_size, = next(counts)
+                    if self.header_offset == 0xFFFFFFFF:
+                        self.header_offset, = next(counts)
+                except (struct.error, StopIteration):
+                    raise RuntimeError("Corrupt extra field %s" % len(data))
 
 
 class _ZipDecrypter:
@@ -1480,14 +1533,10 @@
                     else:
                         header_offset = zinfo.header_offset
 
-                    extra_data = zinfo.extra
                     min_version = 0
                     if extra:
                         # Append a ZIP64 field to the extra's
-                        extra_data = struct.pack(
-                            '<HH' + 'Q'*len(extra),
-                            1, 8*len(extra), *extra) + extra_data
-
+                        zinfo.extra_map[1] = struct.pack('Q' * len(extra), *extra)
                         min_version = ZIP64_VERSION
 
                     if zinfo.compress_type == ZIP_BZIP2:
@@ -1504,7 +1553,7 @@
                                               zinfo.create_system, extract_version, zinfo.reserved,
                                               flag_bits, zinfo.compress_type, dostime, dosdate,
                                               zinfo.CRC, compress_size, file_size,
-                                              len(filename), len(extra_data), len(zinfo.comment),
+                                              len(filename), len(zinfo.extra), len(zinfo.comment),
                                               0, zinfo.internal_attr, zinfo.external_attr,
                                               header_offset)
                     except DeprecationWarning:
@@ -1512,13 +1561,13 @@
                                zinfo.create_system, extract_version, zinfo.reserved,
                                zinfo.flag_bits, zinfo.compress_type, dostime, dosdate,
                                zinfo.CRC, compress_size, file_size,
-                               len(zinfo.filename), len(extra_data), len(zinfo.comment),
+                               len(zinfo.filename), len(zinfo.extra), len(zinfo.comment),
                                0, zinfo.internal_attr, zinfo.external_attr,
                                header_offset), file=sys.stderr)
                         raise
                     self.fp.write(centdir)
                     self.fp.write(filename)
-                    self.fp.write(extra_data)
+                    self.fp.write(zinfo.extra)
                     self.fp.write(zinfo.comment)
 
                 pos2 = self.fp.tell()