add support for serializing to file-like objects (e.g. StringIO) to PIL plugin

skimage/io/_plugins/pil_plugin.py

@@ -59,17 +59,21 @@ def _palette_is_grayscale(pil_image):
     return np.allclose(np.diff(valid_palette), 0)
 
 
-def imsave(fname, arr):
+def imsave(fname, arr, format_str=None):
     """Save an image to disk.
 
     Parameters
     ----------
-    fname : str
+    fname : str or file-like object
         Name of destination file.
     arr : ndarray of uint8 or float
         Array (image) to save.  Arrays of data-type uint8 should have
         values in [0, 255], whereas floating-point arrays must be
         in [0, 1].
+    format_str: str
+        Format to save as, this is required if using a file-like object;
+        this is optional if fname is a string and the format can be
+        derived from the extension.
 
     Notes
     -----
@@ -101,7 +105,11 @@ def imsave(fname, arr):
         arr = arr.astype(np.uint8)
 
     img = Image.fromstring(mode, (arr.shape[1], arr.shape[0]), arr.tostring())
-    img.save(fname)
+
+    if isinstance(fname, basestring):
+        img.save(fname, format=format_str)
+    elif callable(getattr(fname, 'write', None)):
+        img.save(fname, format=format_str)
 
 
 def imshow(arr):