class Foo(object):
    a = 3
    def bar(self, x): return x + a

>>> eval("open('/tmp/tst', 'w')", {}, {})
<open file '/tmp/tst', mode 'w' at 0x7fe8703ce198>

>>> eval("open('/tmp/tst', 'w')", {'__builtins__':[]}, {})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1, in <module>
NameError: name 'open' is not defined