BF(PY2): CommandError - make __str__ return bytes via assure_bytes, _…

…_unicode__ to return str

Should be reverted in master where there is no PY2 compatibility

datalad/support/exceptions.py

@@ -12,6 +12,7 @@
 import re
 from os import linesep
 
+from six import PY2
 
 class CommandError(RuntimeError):
     """Thrown if a command call fails.
@@ -25,16 +26,24 @@ def __init__(self, cmd="", msg="", code=None, stdout="", stderr=""):
         self.stdout = stdout
         self.stderr = stderr
 
-    def __str__(self):
-        from datalad.utils import assure_unicode
+    def _str(self):
+        from ..utils import assure_unicode
         to_str = "%s: " % self.__class__.__name__
         if self.cmd:
             to_str += "command '%s'" % (self.cmd,)
         if self.code:
             to_str += " failed with exitcode %d" % self.code
-        to_str += "\n%s" % assure_unicode(self.msg)
+        to_str += "\n" + assure_unicode(self.msg)
         return to_str
 
+    if PY2:
+        def __str__(self):
+            from ..utils import assure_bytes
+            return assure_bytes(self._str())
+        __unicode__ = _str
+    else:
+        __str__ = _str
+
 
 class MissingExternalDependency(RuntimeError):
     """External dependency is missing error"""