diff -r 8e7a738c3840 Lib/gzip.py
--- a/Lib/gzip.py	Sat Nov 16 10:47:17 2013 -0500
+++ b/Lib/gzip.py	Sat Nov 16 21:17:50 2013 +0200
@@ -9,6 +9,7 @@
 import zlib
 import builtins
 import io
+import collections
 
 __all__ = ["GzipFile", "open", "compress", "decompress"]
 
@@ -16,6 +17,8 @@
 
 READ, WRITE = 1, 2
 
+_EXTRA_FIELD_STRUCT = struct.Struct('<2sH')
+
 def open(filename, mode="rb", compresslevel=9,
          encoding=None, errors=None, newline=None):
     """Open a gzip-compressed file in binary or text mode.
@@ -121,6 +124,37 @@
         return getattr(self.file, name)
 
 
+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 GzipFile(io.BufferedIOBase):
     """The GzipFile class simulates most of the methods of a file object with
     the exception of the readinto() and truncate() methods.
@@ -134,7 +168,7 @@
     max_read_chunk = 10 * 1024 * 1024   # 10Mb
 
     def __init__(self, filename=None, mode=None,
-                 compresslevel=9, fileobj=None, mtime=None):
+                 compresslevel=9, fileobj=None, mtime=None, *, extra=None):
         """Constructor for the GzipFile class.
 
         At least one of fileobj and filename must be given a
@@ -186,6 +220,9 @@
         if mode is None:
             mode = getattr(fileobj, 'mode', 'rb')
 
+        self._extra_bytes = b''
+        self._extra_items = None
+
         if mode.startswith('r'):
             self.mode = READ
             # Set flag indicating start of a new member
@@ -203,6 +240,14 @@
 
         elif mode.startswith(('w', 'a', 'x')):
             self.mode = WRITE
+            if extra is None:
+                self.extra_bytes = b''
+            elif isinstance(extra, (bytes, bytearray)):
+                self.extra_bytes = extra
+            else:
+                if hasattr(extra, 'items'):
+                    extra = extra.items()
+                self._extra_list = tuple(extra)
             self._init_write(filename)
             self.compress = zlib.compressobj(compresslevel,
                                              zlib.DEFLATED,
@@ -264,13 +309,18 @@
         flags = 0
         if fname:
             flags = FNAME
-        self.fileobj.write(chr(flags).encode('latin-1'))
+        if self.extra_bytes:
+            flags |= FEXTRA
+        self.fileobj.write(struct.pack('B', flags))
         mtime = self.mtime
         if mtime is None:
             mtime = time.time()
         write32u(self.fileobj, int(mtime))
         self.fileobj.write(b'\002')
         self.fileobj.write(b'\377')
+        if self.extra_bytes:
+            self.fileobj.write(struct.unpack('<H', len(self.extra_bytes)))
+            self.fileobj.write(self.extra_bytes)
         if fname:
             self.fileobj.write(fname + b'\000')
 
@@ -301,9 +351,11 @@
             raise OSError('Unknown compression method')
 
         if flag & FEXTRA:
-            # Read & discard the extra field, if present
+            # Read the extra field, if present
             extra_len, = struct.unpack("<H", self._read_exact(2))
-            self._read_exact(extra_len)
+            self.extra_bytes = self._read_exact(extra_len)
+        else:
+            self.extra_bytes = b''
         if flag & FNAME:
             # Read and discard a null-terminated string containing the filename
             while True:
@@ -325,6 +377,47 @@
             self._add_read_data(uncompress)
         return True
 
+    @property
+    def extra_bytes(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_bytes.setter
+    def extra_bytes(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 = list(value.items())
+
     def write(self,data):
         self._check_closed()
         if self.mode != WRITE: