diff -r a4405b799e1b Lib/imghdr.py
--- a/Lib/imghdr.py	Thu Jul 07 12:59:31 2011 +0100
+++ b/Lib/imghdr.py	Thu Jul 07 15:51:19 2011 -0400
@@ -1,4 +1,5 @@
 """Recognize image file formats based on their first few bytes."""
+import io
 
 __all__ = ["what"]
 
@@ -6,17 +7,44 @@
 # Recognize image headers #
 #-------------------------#
 
+# The initial number of bytes to read from a file or stream. We assume this
+# number of bytes is sufficient to be able to tell what type of image the file
+# contains.
+BLOCK_SIZE = 32
+
+
 def what(file, h=None):
+    """Wrapper around the :func:`_what` function for backward compatibility."""
     if h is None:
         if isinstance(file, str):
-            f = open(file, 'rb')
-            h = f.read(32)
+            with open(file, 'rb') as f:
+                return _what(f)
         else:
-            location = file.tell()
-            h = file.read(32)
-            file.seek(location)
-            f = None
+            return _what(file)
     else:
+        return _what(h)
+
+
+def _what(data):
+    """Returns the type of image specified by `data`, which can be either an
+    open file-like object, or a sequence of bytes.
+
+    If `data` is an open file, the file will remain open and its position as
+    returned by :func:`file.tell` will be reset to the value it had when this
+    function was entered (unless a custom test affects the file pointer).
+
+    If `data` is a sequence of bytes, this function assumes that the characters
+    are the bytes read from the file whose type will be guessed.
+
+    """
+    if (isinstance(data, io.RawIOBase) or isinstance(data, io.BufferedIOBase)
+        or isinstance(data, io.TextIOBase)):
+        location = data.tell()
+        h = data.read(BLOCK_SIZE)
+        data.seek(location)
+        f = data
+    else:
+        h = data
         f = None
     try:
         for tf in tests:
diff -r a4405b799e1b Lib/test/test_imghdr.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Lib/test/test_imghdr.py	Thu Jul 07 15:51:19 2011 -0400
@@ -0,0 +1,71 @@
+"""Provides unit tests for the imghdr module."""
+from imghdr import _what
+from io import BytesIO
+from io import StringIO
+from os import unlink
+from test.support import run_unittest
+from unittest import TestCase
+
+# The initial 32 bytes of an example image of each type.
+IMAGES = {
+    'pbm': (b'P4\n# CREATOR: GIMP PNM Filter Ve'),
+    'pgm': (b'P5\n# CREATOR: GIMP PNM Filter Ve'),
+    'ppm': (b'P6\n# CREATOR: GIMP PNM Filter Ve'),
+    'xbm': (b'#define image_width 1\n#define im'),
+    'rast': (b'\x59\xA6\x6A\x95\x00\x00\x00\x00'
+             b'\x00\x00\x00\x00\x00\x00\x00\x00'
+             b'\x00\x00\x00\x00\x00\x00\x00\x00'
+             b'\x00\x00\x00\x00\x00\x00\x00\x00'),
+    'rgb': (b'\x01\xda\x00\x01\x00\x03\x00\x01'
+            b'\x00\x01\x00\x03\x00\x00\x00\x00'
+            b'\x00\x00\x00\xff\x00\x00\x00\x00'
+            b'\x00\x00\x00\x00\x00\x00\x00\x00'),
+    'gif': (b'GIF87a\x01\x00'
+            b'\x01\x00\x80\x00\x00\xff\xff\xff'
+            b'\xff\xff\xff,\x00\x00\x00\x00'
+            b'\x01\x00\x01\x00\x00\x02\x02D'),
+    'tiff': (b'II*\x00\x0c\x00\x00\x00'
+             b'\xff\xff\xff\x00\x11\x00\xfe\x00'
+             b'\x04\x00\x01\x00\x00\x00\x00\x00'
+             b'\x00\x00\x00\x01\x03\x00\x01\x00'),
+    'jpeg': (b'\xff\xd8\xff\xe0\x00\x10JF'
+             b'IF\x00\x01\x01\x01\x00H'
+             b'\x00H\x00\x00\xff\xfe\x00\x13'
+             b'Created '),
+    'bmp': (b'BM:\x00\x00\x00\x00\x00'
+            b'\x00\x006\x00\x00\x00(\x00'
+            b'\x00\x00\x01\x00\x00\x00\x01\x00'
+            b'\x00\x00\x01\x00\x18\x00\x00\x00'),
+    'png': (b'\x89PNG\r\n\x1a\n'
+            b'\x00\x00\x00\rIHDR'
+            b'\x00\x00\x00\x01\x00\x00\x00\x01'
+            b'\x08\x02\x00\x00\x00\x90wS')
+    }
+
+
+class ImghdrTest(TestCase):
+    """Test case for the imghdr module."""
+
+    def test__what_bytes(self):
+        """Test case for determining image type from a sequence of bytes."""
+        for imagetype, image in IMAGES.items():
+            self.assertEqual(imagetype, _what(image))
+
+    def test__what_file(self):
+        """Test case for determining image type from an open file."""
+        for imagetype, image in IMAGES.items():
+            with BytesIO(image) as f:
+                self.assertEqual(imagetype, _what(f))
+
+    def test__what_unknown(self):
+        """Test case for an unknown type of image."""
+        with BytesIO(b'garbage') as f:
+            self.assertIsNone(_what(f))
+
+
+def test_main():
+    """Runs all the tests in this module."""
+    run_unittest(ImghdrTest)
+
+if __name__ == "__main__":
+    test_main()