Use shared Python 2/3 compatibility module (#50)

* Incorporate more Python 2/3 compatibility

There is some other Python 2/3 compatibility code being used in
`dask-image`, which needed to be consolidated in the share Python
compatibility module. This adds that shared code.

* Test additional Python 2/3 compatibility code

* Use Python compatibility code in `ndfilters`

Update `ndfilters` utility code to make use of the share Python 2/3
compatibility code.

* Use Python compatibility code in `ndfourier`

Update `ndfourier` utility code to make use of the share Python 2/3
compatibility code.

* Use Python compatibility code in `ndmorph`

Update `ndmorph` utility code to make use of the share Python 2/3
compatibility code.

dask_image/_pycompat.py

@@ -6,6 +6,13 @@
     irange = range
 
 try:
-    from itertools import izip
+    from itertools import imap, izip
 except ImportError:
-    izip = zip
+    imap, izip = map, zip
+
+try:
+    unicode = unicode
+except NameError:
+    unicode = str
+
+strlike = (bytes, unicode)

dask_image/ndfilters/_utils.py

@@ -8,23 +8,7 @@
 
 import numpy
 
-
-try:
-    from itertools import imap, izip
-except ImportError:
-    imap, izip = map, zip
-
-try:
-    irange = xrange
-except NameError:
-    irange = range
-
-try:
-    unicode
-except NameError:
-    unicode = str
-
-strlike = (bytes, unicode)
+from .._pycompat import imap, irange, izip, strlike, unicode
 
 
 def _get_docstring(func):

dask_image/ndfourier/_utils.py

@@ -9,16 +9,7 @@
 import dask.array
 
 from . import _compat
-
-try:
-    from itertools import imap
-except ImportError:
-    imap = map
-
-try:
-    irange = xrange
-except NameError:
-    irange = range
+from .._pycompat import imap, irange
 
 
 def _get_freq_grid(shape, chunks, dtype=float):

dask_image/ndmorph/_ops.py

@@ -6,12 +6,7 @@
 import dask.array
 
 from . import _utils
-
-
-try:
-    irange = xrange
-except NameError:
-    irange = range
+from .._pycompat import irange
 
 
 def _where(condition, x, y):

dask_image/ndmorph/_utils.py

@@ -11,23 +11,7 @@
 
 import dask.array
 
-
-try:
-    from itertools import imap, izip
-except ImportError:
-    imap, izip = map, zip
-
-try:
-    irange = xrange
-except NameError:
-    irange = range
-
-try:
-    unicode
-except NameError:
-    unicode = str
-
-strlike = (bytes, unicode)
+from .._pycompat import imap, irange, izip, strlike, unicode
 
 
 def _get_docstring(func):

tests/test_dask_image/test__pycompat.py

@@ -15,9 +15,31 @@ def test_irange():
     assert list(r) == [0, 1, 2, 3, 4]
 
 
+def test_imap():
+    r = dask_image._pycompat.imap(lambda e : e ** 2, [0, 1, 2, 3])
+
+    assert not isinstance(r, list)
+
+    assert list(r) == [0, 1, 4, 9]
+
+
 def test_izip():
     r = dask_image._pycompat.izip([1, 2], [3, 4, 5])
 
     assert not isinstance(r, list)
 
     assert list(r) == [(1, 3), (2, 4)]
+
+
+def test_strlike():
+    b = b"Hello World!"
+    u = u"Hello World!"
+
+    assert isinstance(b, dask_image._pycompat.strlike)
+    assert isinstance(u, dask_image._pycompat.strlike)
+
+
+def test_unicode():
+    u = u"Hello World!"
+
+    assert isinstance(u, dask_image._pycompat.unicode)