None真的是内置的吗？

Question

我正在尝试以（相对）安全的方式使用 Python 的 (2.7) eval。因此，我定义：

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

如预期的那样，以下不再起作用：

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

所以，None不是内置的吗？它们至少都是 __builtin__ ...
为什么它仍然可以求值？
如果必须，我将如何摆脱它？

Answer 1

在 Python 2.7 中，这是一个完全有效的声明。

True = False
>>> print True
False

虽然尝试用 None 来做，但是..

SyntaxError: cannot assign to None

会让你头晕目眩。我不确定，但我怀疑 True 是一个内置函数，而 None 实际上是一个常量。您可以将 True 更改为不同的值，但不能更改 None.

Answer 2

None 是 Python 中的常量，参见 Keywords documentation:

Changed in version 2.4: None became a constant and is now recognized by the compiler as a name for the built-in object None. Although it is not a keyword, you cannot assign a different object to it.

无论何时命名，编译器都会简单地插入对单例 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Python3中，None、True、False都是now keywords，这三者仅仅通过命名就具体化了：

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

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

请注意，关于 eval 没有什么安全的，即使 __builtins__ 绝育了，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破坏解释器。参见 Eval really is dangerous。

如果您只想加载 Python 文字语法（列表、元组、字典、字符串、数字等），那么您想使用 ast.literal_eval() function，专门设计用于安全。

None真的是内置的吗？

Is None really a built-in?

python

built-in

nonetype