Allow list comprehensions in RestrictionCapableEval.eval(). (#97)

Fixes #95.

docs/CHANGES.rst

@@ -7,6 +7,9 @@ Changes
 - Warn when using another Python implementation than CPython as it is not safe to use RestrictedPython with other versions than CPyton.
   See https://bitbucket.org/pypy/pypy/issues/2653 for PyPy.
 
+- Allow to use list comprehensions in the default implementation of
+  ``RestrictionCapableEval.eval()``.
+
 4.0b2 (2017-09-15)
 ------------------
 
@@ -62,7 +65,7 @@ Changes
 
 - Mostly complete rewrite based on Python AST module.
   [loechel (Alexander Loechel), icemac (Michael Howitz), stephan-hof (Stephan Hofmockel), tlotze (Thomas Lotze)]
-  
+
 - Support Python versions 3.4 up to 3.6.
 
 - switch to pytest

src/RestrictedPython/Eval.py

@@ -35,6 +35,11 @@ def default_guarded_getitem(ob, index):
     return ob[index]
 
 
+def default_guarded_getiter(ob):
+    # No restrictions.
+    return ob
+
+
 class RestrictionCapableEval(object):
     """A base class for restricted code."""
 
@@ -99,7 +104,8 @@ def eval(self, mapping):
 
         global_scope = {
             '_getattr_': default_guarded_getattr,
-            '_getitem_': default_guarded_getitem
+            '_getitem_': default_guarded_getitem,
+            '_getiter_': default_guarded_getiter,
         }
 
         global_scope.update(self.globals)

tests/test_eval.py

@@ -95,3 +95,10 @@ def test_Eval__RestictionCapableEval__eval_1():
     ob.globals['c'] = 8
     result = ob.eval(dict(a=1, b=2, c=4))
     assert result == 11
+
+
+def test_Eval__RestictionCapableEval__eval__2():
+    """It allows to use list comprehensions."""
+    ob = RestrictionCapableEval("[item for item in (1, 2)]")
+    result = ob.eval({})
+    assert result == [1, 2]