ENH: Add tests for full coverage.

The extra dtype tests are not exhaustive, but, instead, were added to test specific code paths in `_convert`.

skimage/util/tests/test_dtype.py

@@ -1,7 +1,9 @@
 import numpy as np
-from numpy.testing import assert_equal
+from numpy.testing import assert_equal, assert_raises
 from skimage import img_as_int, img_as_float, \
                     img_as_uint, img_as_ubyte
+from skimage.util.dtype import _convert
+
 
 dtype_range = {np.uint8: (0, 255),
                np.uint16: (0, 65535),
@@ -34,6 +36,37 @@ def test_range():
                   "From %s to %s" % (np.dtype(dtype), np.dtype(dt)), \
                   y, omin, omax
 
+
+def test_range_extra_dtypes():
+    """Test code paths that are not skipped by `test_range`"""
+
+    # Add non-standard data types that are allowed by the `_convert` function.
+    dtype_range_extra = dtype_range.copy()
+    dtype_range_extra.update({np.int32: (-2147483648, 2147483647),
+                              np.uint32: (0, 4294967295)})
+
+    dtype_pairs = [(np.uint8, np.uint32),
+                   (np.int8, np.uint32),
+                   (np.int8, np.int32),
+                   (np.int32, np.int8),
+                   (np.float64, np.float32),
+                   (np.int32, np.float32)]
+
+    for dtype_in, dt in dtype_pairs:
+        imin, imax = dtype_range_extra[dtype_in]
+        x = np.linspace(imin, imax, 10).astype(dtype_in)
+        y = _convert(x, dt)
+        omin, omax = dtype_range_extra[dt]
+        yield _verify_range, \
+              "From %s to %s" % (np.dtype(dtype_in), np.dtype(dt)), \
+              y, omin, omax
+
+
+def test_unsupported_dtype():
+    x = np.arange(10).astype(np.uint64)
+    assert_raises(ValueError, img_as_int, x)
+
+
 if __name__ == '__main__':
     np.testing.run_module_suite()