compat: add range and use it in the vfs

[bup.git] / lib / bup / compat.py

diff --git a/lib/bup/compat.py b/lib/bup/compat.py
index 707ccbafea529cb8dfeaf5de0195306929b32913..895a84a3da86ac65c63ba184e94c341cdef15d14 100644 (file)
--- a/lib/bup/compat.py
+++ b/lib/bup/compat.py
@@ -1,13 +1,17 @@
 
-from __future__ import print_function
+from __future__ import absolute_import, print_function
 from traceback import print_exception
 import sys
 
+# Please see CODINGSTYLE for important exception handling guidelines
+# and the rationale behind add_ex_tb(), chain_ex(), etc.
+
 py_maj = sys.version_info[0]
 py3 = py_maj >= 3
 
 if py3:
 
+    range = range
     str_type = str
 
     def add_ex_tb(ex):
@@ -15,10 +19,15 @@ if py3:
         return ex
 
     def chain_ex(ex, context_ex):
+        """Do nothing (already handled by Python 3 infrastructure)."""
         return ex
 
+    def items(x):
+        return x.items()
+
 else:  # Python 2
 
+    range = xrange
     str_type = basestring
 
     def add_ex_tb(ex):
@@ -30,8 +39,11 @@ else:  # Python 2
         return ex
 
     def chain_ex(ex, context_ex):
+        """Chain context_ex to ex as the __context__ (unless it already has
+        one).  Return ex.
+
+        """
         if context_ex:
-            add_ex_tb(context_ex)
             if not getattr(ex, '__context__', None):
                 ex.__context__ = context_ex
         return ex
@@ -52,6 +64,10 @@ else:  # Python 2
             tb = getattr(ex, '__traceback__', None)
             print_exception(type(ex), ex, tb)
 
+    def items(x):
+        return x.iteritems()
+
+
 def wrap_main(main):
     """Run main() and raise a SystemExit with the return value if it
     returns, pass along any SystemExit it raises, convert
@@ -72,11 +88,14 @@ def wrap_main(main):
         sys.exit(1)
 
 
-# Excepting wrap_main() in the traceback, these should produce the same output:
+# Excepting wrap_main() in the traceback, these should produce similar output:
 #   python2 lib/bup/compat.py
 #   python3 lib/bup/compat.py
 # i.e.:
 #   diff -u <(python2 lib/bup/compat.py 2>&1) <(python3 lib/bup/compat.py 2>&1)
+#
+# Though the python3 output for 'second' will include a stacktrace
+# starting from wrap_main, rather than from outer().
 
 if __name__ == '__main__':
 
@@ -87,6 +106,10 @@ if __name__ == '__main__':
         try:
             inner()
         except Exception as ex:
-            raise chain_ex(Exception('second'), ex)
+            add_ex_tb(ex)
+            try:
+                raise Exception('second')
+            except Exception as ex2:
+                raise chain_ex(add_ex_tb(ex2), ex)
 
     wrap_main(outer)