python - Is None really a built-in? -
i trying use python's (2.7) eval in (relatively) safe manner. hence, defined:
def safer_eval(string): """safer version of eval() globals , builtins inaccessible""" return eval(string, {'__builtins__': {}}) as expected, following not work more:
print safer_eval("true") nameerror: name 'true' not defined however, can still eval "none" string:
print safer_eval("none") none so,
nonenot built-in? @ least both part of__builtin__...why still eval-able?
- how rid of it, if had to?
none constant in python, see keywords documentation:
changed in version 2.4:
nonebecame constant , recognized compiler name built-in objectnone. although not keyword, cannot assign different object it.
the compiler inserts reference singleton none object whenever name it:
>>> dis import dis >>> dis(compile('none', '', 'eval')) 1 0 load_const 0 (none) 3 return_value true , false built-ins in python 2, means can masked.
in python 3, none, true , false now keywords, , 3 materialised merely naming them:
>>> eval('true', {'__builtins__': {}}) true see guido van rossum's blog post on why changed.
note there nothing safe eval, __builtins__ neutered, can still referenced via other means:
>>> s = ''' ... [ ... c 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() interactive help, or help(object) 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. rights reserved. copyright (c) 2000 beopen.com. rights reserved. copyright (c) 1995-2001 corporation national research initiatives. rights reserved. copyright (c) 1991-1995 stichting mathematisch centrum, amsterdam. 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) 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': cwi, cnri, beopen.com, zope corporation , cast of thousands supporting python development. see www.python.org 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) 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, , other objects.\n\nnoteworthy: none `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'>} or can blow interpreter creating broken code object. see eval dangerous.
if want load python literal syntax (lists, tuples, dictionaries, strings, numbers, etc.) want use ast.literal_eval() function, designed safe.
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