1 files changed, 32 insertions, 9 deletions

diff --git a/lib/testtools/testtools/helpers.py b/lib/testtools/testtools/helpers.py
index 0f489c73f6..dbf66719ed 100644
--- a/lib/testtools/testtools/helpers.py
+++ b/lib/testtools/testtools/helpers.py
@@ -1,64 +1,87 @@
-# Copyright (c) 2010 Jonathan M. Lange. See LICENSE for details.
+# Copyright (c) 2010-2011 testtools developers. See LICENSE for details.
 
 __all__ = [
+    'safe_hasattr',
     'try_import',
     'try_imports',
     ]
 
+import sys
 
-def try_import(name, alternative=None):
-    """Attempt to import `name`.  If it fails, return `alternative`.
+
+def try_import(name, alternative=None, error_callback=None):
+    """Attempt to import ``name``.  If it fails, return ``alternative``.
 
     When supporting multiple versions of Python or optional dependencies, it
     is useful to be able to try to import a module.
 
-    :param name: The name of the object to import, e.g. 'os.path' or
-        'os.path.join'.
+    :param name: The name of the object to import, e.g. ``os.path`` or
+        ``os.path.join``.
     :param alternative: The value to return if no module can be imported.
         Defaults to None.
+    :param error_callback: If non-None, a callable that is passed the ImportError
+        when the module cannot be loaded.
     """
     module_segments = name.split('.')
+    last_error = None
     while module_segments:
         module_name = '.'.join(module_segments)
         try:
             module = __import__(module_name)
         except ImportError:
+            last_error = sys.exc_info()[1]
             module_segments.pop()
             continue
         else:
             break
     else:
+        if last_error is not None and error_callback is not None:
+            error_callback(last_error)
         return alternative
     nonexistent = object()
     for segment in name.split('.')[1:]:
         module = getattr(module, segment, nonexistent)
         if module is nonexistent:
+            if last_error is not None and error_callback is not None:
+                error_callback(last_error)
             return alternative
     return module
 
 
 _RAISE_EXCEPTION = object()
-def try_imports(module_names, alternative=_RAISE_EXCEPTION):
+def try_imports(module_names, alternative=_RAISE_EXCEPTION, error_callback=None):
     """Attempt to import modules.
 
-    Tries to import the first module in `module_names`.  If it can be
+    Tries to import the first module in ``module_names``.  If it can be
     imported, we return it.  If not, we go on to the second module and try
     that.  The process continues until we run out of modules to try.  If none
     of the modules can be imported, either raise an exception or return the
-    provided `alternative` value.
+    provided ``alternative`` value.
 
     :param module_names: A sequence of module names to try to import.
     :param alternative: The value to return if no module can be imported.
         If unspecified, we raise an ImportError.
+    :param error_callback: If None, called with the ImportError for *each*
+        module that fails to load.
     :raises ImportError: If none of the modules can be imported and no
         alternative value was specified.
     """
     module_names = list(module_names)
     for module_name in module_names:
-        module = try_import(module_name)
+        module = try_import(module_name, error_callback=error_callback)
         if module:
             return module
     if alternative is _RAISE_EXCEPTION:
         raise ImportError(
             "Could not import any of: %s" % ', '.join(module_names))
     return alternative
+
+
+def safe_hasattr(obj, attr, _marker=object()):
+    """Does 'obj' have an attribute 'attr'?
+
+    Use this rather than built-in hasattr, as the built-in swallows exceptions
+    in some versions of Python and behaves unpredictably with respect to
+    properties.
+    """
+    return getattr(obj, attr, _marker) is not _marker