new function safer_getattr_raise (#288)

* new function `safer_getattr_raise` similar to `safer_getattr` but with `default` handling like `getattr`

* [force ci]

* force ci

* Apply review suggestion from @icemac

Co-authored-by: Michael Howitz <[email protected]>

---------

Co-authored-by: Michael Howitz <[email protected]>

CHANGES.rst

@@ -7,6 +7,13 @@ Changes
 - Allow to use the package with Python 3.13 -- Caution: No security
   audit has been done so far.
 
+- Provide new function ``RestrictedPython.Guards.safer_getattr_raise``.
+  It is similar to ``safer_getattr`` but handles its parameter
+  ``default`` like ``getattr``, i.e. it raises ``AttributeError``
+  if the attribute lookup fails and this parameter is not provided,
+  fixes `#287 <https://github.com/zopefoundation/RestrictedPython/issues/287>`_.
+
+
 
 7.3 (2024-09-30)
 ----------------

src/RestrictedPython/Guards.py

@@ -240,6 +240,9 @@ def guarded_delattr(object, name):
 safe_builtins['delattr'] = guarded_delattr
 
 
+raise_ = object()
+
+
 def safer_getattr(object, name, default=None, getattr=getattr):
     """Getattr implementation which prevents using format on string objects.
 
@@ -263,12 +266,18 @@ def safer_getattr(object, name, default=None, getattr=getattr):
             '"{name}" is an invalid attribute name because it '
             'starts with "_"'.format(name=name)
         )
-    return getattr(object, name, default)
+    args = (object, name) + (() if default is raise_ else (default,))
+    return getattr(*args)
 
 
 safe_builtins['_getattr_'] = safer_getattr
 
 
+def safer_getattr_raise(object, name, default=raise_):
+    """like ``safer_getattr`` but raising ``AttributeError`` if failing."""
+    return safer_getattr(object, name, default)
+
+
 def guarded_iter_unpack_sequence(it, spec, _getiter_):
     """Protect sequence unpacking of targets in a 'for loop'.
 

tests/test_Guards.py

@@ -329,6 +329,18 @@ def test_Guards__safer_getattr__5():
     ) == str(err.value)
 
 
+def test_Guards__safer_getattr_raise():
+    from types import SimpleNamespace
+
+    from RestrictedPython.Guards import safer_getattr_raise
+
+    o = SimpleNamespace(a="a")
+    assert safer_getattr_raise(o, "a") == "a"
+    assert safer_getattr_raise(o, "b", None) is None
+    with pytest.raises(AttributeError):
+        safer_getattr_raise(o, "b")
+
+
 def test_call_py3_builtins():
     """It should not be allowed to access global builtins in Python3."""
     result = compile_restricted_exec('builtins["getattr"]')