Merged revisions 84737 via svnmerge from

svn+ssh://pythondev@svn.python.org/python/branches/py3k

........
  r84737 | antoine.pitrou | 2010-09-12 16:51:20 +0200 (dim., 12 sept. 2010) | 4 lines

  Issue #9837: The read() method of ZipExtFile objects (as returned by
  ZipFile.open()) could return more bytes than requested.
........

Lib/zipfile.py

@@ -565,17 +565,20 @@ class ZipExtFile(io.BufferedIOBase):
         """Read and return up to n bytes.
         If the argument is omitted, None, or negative, data is read and returned until EOF is reached..
         """
-
         buf = ''
-        while n < 0 or n is None or n > len(buf):
-            data = self.read1(n)
+        if n is None:
+            n = -1
+        while True:
+            if n < 0:
+                data = self.read1(n)
+            elif n > len(buf):
+                data = self.read1(n - len(buf))
+            else:
+                return buf
             if len(data) == 0:
                 return buf
-
             buf += data
 
-        return buf
-
     def _update_crc(self, newdata, eof):
         # Update the CRC using the given data.
         if self._expected_crc is None: