Index: Lib/zipfile.py
===================================================================
--- Lib/zipfile.py	(revision 67931)
+++ Lib/zipfile.py	(working copy)
@@ -128,18 +128,30 @@
 _CD64_DIRECTORY_SIZE = 8
 _CD64_OFFSET_START_CENTDIR = 9
 
-def is_zipfile(filename):
-    """Quickly see if file is a ZIP file by checking the magic number."""
+def _check_zipfile(fp):
     try:
-        fpin = open(filename, "rb")
-        endrec = _EndRecData(fpin)
-        fpin.close()
-        if endrec:
-            return True                 # file has correct magic number
+        if _EndRecData(fp):
+            return True         # file has correct magic number
     except IOError:
         pass
     return False
 
+def is_zipfile(filename):
+    """Quickly see if a file is a ZIP file by checking the magic number.
+
+    The filename argument may be a file or file-like object too.
+    """
+    result = False
+    try:
+        if hasattr(filename, "read"):
+            result = _check_zipfile(fp=filename)
+        else:
+            with open(filename, "rb") as fp:
+                result = _check_zipfile(fp)
+    except IOError:
+        pass
+    return result
+
 def _EndRecData64(fpin, offset, endrec):
     """
     Read the ZIP64 end-of-archive records and use that to update endrec
Index: Lib/test/test_zipfile.py
===================================================================
--- Lib/test/test_zipfile.py	(revision 67931)
+++ Lib/test/test_zipfile.py	(working copy)
@@ -634,20 +634,49 @@
     def testIsZipErroneousFile(self):
         # This test checks that the is_zipfile function correctly identifies
         # a file that is not a zip file
-        fp = open(TESTFN, "w")
+
+        # - passing a filename
+        with open(TESTFN, "w") as fp:
+            fp.write("this is not a legal zip file\n")
+        chk = zipfile.is_zipfile(TESTFN)
+        self.assert_(not chk)
+        # - passing a file object
+        with open(TESTFN, "rb") as fp:
+            chk = zipfile.is_zipfile(fp)
+            self.assert_(not chk)
+        # - passing a file-like object
+        fp = StringIO()
         fp.write("this is not a legal zip file\n")
-        fp.close()
-        chk = zipfile.is_zipfile(TESTFN)
-        self.assert_(chk is False)
+        chk = zipfile.is_zipfile(fp)
+        self.assert_(not chk)
+        fp.seek(0,0)
+        chk = zipfile.is_zipfile(fp)
+        self.assert_(not chk)
 
     def testIsZipValidFile(self):
         # This test checks that the is_zipfile function correctly identifies
         # a file that is a zip file
+
+        # - passing a filename
         zipf = zipfile.ZipFile(TESTFN, mode="w")
         zipf.writestr("foo.txt", "O, for a Muse of Fire!")
         zipf.close()
         chk = zipfile.is_zipfile(TESTFN)
-        self.assert_(chk is True)
+        self.assert_(chk)
+        # - passing a file object
+        with open(TESTFN, "rb") as fp:
+            chk = zipfile.is_zipfile(fp)
+            self.assert_(chk)
+            fp.seek(0,0)
+            zip_contents = fp.read()
+        # - passing a file-like object
+        fp = StringIO()
+        fp.write(zip_contents)
+        chk = zipfile.is_zipfile(fp)
+        self.assert_(chk)
+        fp.seek(0,0)
+        chk = zipfile.is_zipfile(fp)
+        self.assert_(chk)
 
     def testNonExistentFileRaisesIOError(self):
         # make sure we don't raise an AttributeError when a partially-constructed
Index: Doc/library/zipfile.rst
===================================================================
--- Doc/library/zipfile.rst	(revision 67931)
+++ Doc/library/zipfile.rst	(working copy)
@@ -66,9 +66,11 @@
 .. function:: is_zipfile(filename)
 
    Returns ``True`` if *filename* is a valid ZIP file based on its magic number,
-   otherwise returns ``False``.  This module does not currently handle ZIP files
-   which have appended comments.
+   otherwise returns ``False``.  *filename* may be a file or file-like object too.
+   This module does not currently handle ZIP files which have appended comments.
 
+   .. versionchanged:: 2.7
+   Support for file and file-like objects.
 
 .. data:: ZIP_STORED