Question

我试图以（相对）安全的方式使用Python（2.7）eval。因此，我定义了：

def safer_eval(string):
    """Safer version of eval() as globals and builtins are inaccessible"""
    return eval(string, {'__builtins__': {}})

正如所料，以下内容不再适用：

print safer_eval("True")
NameError: name 'True' is not defined

但是，我仍然可以评估"None"字符串：

print safer_eval("None")
None

那么，None不是内置的吗？它们至少是__builtin__ ...
为什么它仍然可以评估？
如果必须的话，我怎么能摆脱它？

Answer 1

None是Python中的常量，请参阅Keywords documentation：

版本2.4中已更改：None成为常量，现在被编译器识别为内置对象None的名称。虽然它不是关键字，但您无法为其指定其他对象。

编译器只需在命名时插入对单例None对象的引用：

>>> from dis import dis
>>> dis(compile('None', '', 'eval'))
  1           0 LOAD_CONST               0 (None)
              3 RETURN_VALUE

True和False是Python 2中的内置函数，这也意味着它们可以被屏蔽。

在Python 3中，None，True和False都是now keywords，所有这三个都只是通过命名来实现：

>>> eval('True', {'__builtins__': {}})
True

请参阅Guido van Rossum's blog post on why this was changed。

请注意，即使__builtins__已中断，也有关于eval的无安全，因为它仍然可以通过其他方式引用：

>>> s = '''
... [
...     c for c in ().__class__.__base__.__subclasses__()
...     if c.__name__ == 'catch_warnings'
... ][0]()._module.__builtins__
... '''
>>> eval(s, {'__builtins__': {}})
{'bytearray': <type 'bytearray'>, 'IndexError': <type 'exceptions.IndexError'>, 'all': <built-in function all>, 'help': Type help() for interactive help, or help(object) for help about object., 'vars': <built-in function vars>, 'SyntaxError': <type 'exceptions.SyntaxError'>, 'unicode': <type 'unicode'>, 'UnicodeDecodeError': <type 'exceptions.UnicodeDecodeError'>, 'memoryview': <type 'memoryview'>, 'isinstance': <built-in function isinstance>, 'copyright': Copyright (c) 2001-2015 Python Software Foundation.
All Rights Reserved.

Copyright (c) 2000 BeOpen.com.
All Rights Reserved.

Copyright (c) 1995-2001 Corporation for National Research Initiatives.
All Rights Reserved.

Copyright (c) 1991-1995 Stichting Mathematisch Centrum, Amsterdam.
All Rights Reserved., 'NameError': <type 'exceptions.NameError'>, 'BytesWarning': <type 'exceptions.BytesWarning'>, 'dict': <type 'dict'>, 'input': <built-in function input>, 'oct': <built-in function oct>, 'bin': <built-in function bin>, 'SystemExit': <type 'exceptions.SystemExit'>, 'StandardError': <type 'exceptions.StandardError'>, 'format': <built-in function format>, 'repr': <built-in function repr>, 'sorted': <built-in function sorted>, 'False': False, 'RuntimeWarning': <type 'exceptions.RuntimeWarning'>, 'list': <type 'list'>, 'iter': <built-in function iter>, 'reload': <built-in function reload>, 'Warning': <type 'exceptions.Warning'>, '__package__': None, 'round': <built-in function round>, 'dir': <built-in function dir>, 'cmp': <built-in function cmp>, 'set': <type 'set'>, 'bytes': <type 'str'>, 'reduce': <built-in function reduce>, 'intern': <built-in function intern>, 'issubclass': <built-in function issubclass>, 'Ellipsis': Ellipsis, 'EOFError': <type 'exceptions.EOFError'>, 'locals': <built-in function locals>, 'BufferError': <type 'exceptions.BufferError'>, 'slice': <type 'slice'>, 'FloatingPointError': <type 'exceptions.FloatingPointError'>, 'sum': <built-in function sum>, 'getattr': <built-in function getattr>, 'abs': <built-in function abs>, 'exit': Use exit() or Ctrl-D (i.e. EOF) to exit, 'print': <built-in function print>, 'True': True, 'FutureWarning': <type 'exceptions.FutureWarning'>, 'ImportWarning': <type 'exceptions.ImportWarning'>, 'None': None, 'hash': <built-in function hash>, 'ReferenceError': <type 'exceptions.ReferenceError'>, 'len': <built-in function len>, 'credits':     Thanks to CWI, CNRI, BeOpen.com, Zope Corporation and a cast of thousands
    for supporting Python development.  See www.python.org for more information., 'frozenset': <type 'frozenset'>, '__name__': '__builtin__', 'ord': <built-in function ord>, 'super': <type 'super'>, '_': None, 'TypeError': <type 'exceptions.TypeError'>, 'license': See http://www.python.org/2.7/license.html, 'KeyboardInterrupt': <type 'exceptions.KeyboardInterrupt'>, 'UserWarning': <type 'exceptions.UserWarning'>, 'filter': <built-in function filter>, 'range': <built-in function range>, 'staticmethod': <type 'staticmethod'>, 'SystemError': <type 'exceptions.SystemError'>, 'BaseException': <type 'exceptions.BaseException'>, 'pow': <built-in function pow>, 'RuntimeError': <type 'exceptions.RuntimeError'>, 'float': <type 'float'>, 'MemoryError': <type 'exceptions.MemoryError'>, 'StopIteration': <type 'exceptions.StopIteration'>, 'globals': <built-in function globals>, 'divmod': <built-in function divmod>, 'enumerate': <type 'enumerate'>, 'apply': <built-in function apply>, 'LookupError': <type 'exceptions.LookupError'>, 'open': <built-in function open>, 'quit': Use quit() or Ctrl-D (i.e. EOF) to exit, 'basestring': <type 'basestring'>, 'UnicodeError': <type 'exceptions.UnicodeError'>, 'zip': <built-in function zip>, 'hex': <built-in function hex>, 'long': <type 'long'>, 'next': <built-in function next>, 'ImportError': <type 'exceptions.ImportError'>, 'chr': <built-in function chr>, 'xrange': <type 'xrange'>, 'type': <type 'type'>, '__doc__': "Built-in functions, exceptions, and other objects.\n\nNoteworthy: None is the `nil' object; Ellipsis represents `...' in slices.", 'Exception': <type 'exceptions.Exception'>, 'tuple': <type 'tuple'>, 'UnicodeTranslateError': <type 'exceptions.UnicodeTranslateError'>, 'reversed': <type 'reversed'>, 'UnicodeEncodeError': <type 'exceptions.UnicodeEncodeError'>, 'IOError': <type 'exceptions.IOError'>, 'hasattr': <built-in function hasattr>, 'delattr': <built-in function delattr>, 'setattr': <built-in function setattr>, 'raw_input': <built-in function raw_input>, 'SyntaxWarning': <type 'exceptions.SyntaxWarning'>, 'compile': <built-in function compile>, 'ArithmeticError': <type 'exceptions.ArithmeticError'>, 'str': <type 'str'>, 'property': <type 'property'>, 'GeneratorExit': <type 'exceptions.GeneratorExit'>, 'int': <type 'int'>, '__import__': <built-in function __import__>, 'KeyError': <type 'exceptions.KeyError'>, 'coerce': <built-in function coerce>, 'PendingDeprecationWarning': <type 'exceptions.PendingDeprecationWarning'>, 'file': <type 'file'>, 'EnvironmentError': <type 'exceptions.EnvironmentError'>, 'unichr': <built-in function unichr>, 'id': <built-in function id>, 'OSError': <type 'exceptions.OSError'>, 'DeprecationWarning': <type 'exceptions.DeprecationWarning'>, 'min': <built-in function min>, 'UnicodeWarning': <type 'exceptions.UnicodeWarning'>, 'execfile': <built-in function execfile>, 'any': <built-in function any>, 'complex': <type 'complex'>, 'bool': <type 'bool'>, 'ValueError': <type 'exceptions.ValueError'>, 'NotImplemented': NotImplemented, 'map': <built-in function map>, 'buffer': <type 'buffer'>, 'max': <built-in function max>, 'object': <type 'object'>, 'TabError': <type 'exceptions.TabError'>, 'callable': <built-in function callable>, 'ZeroDivisionError': <type 'exceptions.ZeroDivisionError'>, 'eval': <built-in function eval>, '__debug__': True, 'IndentationError': <type 'exceptions.IndentationError'>, 'AssertionError': <type 'exceptions.AssertionError'>, 'classmethod': <type 'classmethod'>, 'UnboundLocalError': <type 'exceptions.UnboundLocalError'>, 'NotImplementedError': <type 'exceptions.NotImplementedError'>, 'AttributeError': <type 'exceptions.AttributeError'>, 'OverflowError': <type 'exceptions.OverflowError'>}

或者您可以通过创建损坏的代码对象来简单地炸毁解释器。请参阅Eval really is dangerous。

如果您只想加载Python文字语法（列表，元组，词典，字符串，数字等），那么您希望使用ast.literal_eval() function，专门设计为安全。

Answer 2

在Python 2.7中，这是一个非常有效的陈述。

True = False
>>> print True
False

尝试使用None，然后..

SyntaxError: cannot assign to None

可以让你的头旋转。我不确定，但我怀疑True是内置的，而None实际上是常量。您可以将 True更改为其他值，但您无法更改None。

没有真的是内置的吗？

2 个答案: