support for saving to file-like object in imsave

skimage/io/_plugins/pil_plugin.py

@@ -71,9 +71,8 @@ def imsave(fname, arr, format_str=None):
         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.
+        Format to save as, this is defaulted to PNG if using a file-like
+        object; this will be derived from the extension if fname is a string
 
     Notes
     -----
@@ -104,6 +103,10 @@ def imsave(fname, arr, format_str=None):
         # Force all integers to bytes
         arr = arr.astype(np.uint8)
 
+    # default to PNG if file-like object
+    if not isinstance(fname, basestring) and format_str is None:
+        format_str = "PNG"
+
     img = Image.fromstring(mode, (arr.shape[1], arr.shape[0]), arr.tostring())
     img.save(fname, format=format_str)
 

skimage/io/tests/test_io.py

@@ -2,6 +2,7 @@ import os
 
 from numpy.testing import *
 import numpy as np
+from StringIO import StringIO
 
 import skimage.io as io
 from skimage import data_dir
@@ -27,6 +28,17 @@ def test_imread_url():
     image = io.imread(image_url)
     assert image.shape == (512, 512)
 
+def test_imsave_filelike():
+    image = np.array([[0, 0], [0, 0]], dtype=float)
+    s = StringIO()
+
+    # save to file-like object
+    io.imsave(s, image)
+
+    # read from file-like object
+    s.seek(0)
+    image = io.imread(s)
+    assert image.shape == (2, 2)
 
 if __name__ == "__main__":
     run_module_suite()