eval(expression[, globals[, locals]])

eval()函数执行一个python表达式字符串并返表达式执行后的结果:

>>> x = 1
>>> eval('x + 1')
2

expression参数为一个表达式字符串,globalslocals呢而挑选的参数,globals得是一个字典对象,locals可是为任意映射对象,分别作代码执行的大局与部分命名空间。

globalslocals参数缺失的时刻,表达式会动用时条件之大局和片命名空间值:

>>> x = 1
>>> g = {'x' : 2}
>>> eval('x + 1', g)
3
>>> eval('x + 1')
2

eval()函数在数据类型转换中酷有因此,可以将字符串转换成为字典、列表等:

>>> exp = '[1,2,3,4]'
>>> eval(exp)
[1, 2, 3, 4]
>>> exp = '{"a" : 1}'
>>> eval(exp)
{'a': 1}

eval()也可行使模块:

>>> import os
>>> eval('os.getcwd()')
'/home/user'

自,eval不克一直对模块进行操作,如果不要是动eval进行import:

>>> eval('__import__("os").getcwd()')
'/home/user'

当时貌似用于因客户要求动态的调用不同的模块。

旁的用做计算器啥的啊未以讲话下,不过eval函数也不克滥用,比如使赢得用户之输入并求值eval(raw_input()),

这种气象下,用户输入open(__file__).read()可一直拿原本文件读出来了。

若果安全的行使eval()函数,可以应用globals和locals两独参数来安白名单,当参数缺失之时节,表达式会利用时环境的全局和有些命名空间值,即globals()和locals()中寓的模块和函数:

>>> import os
>>> 'os' in globals()
True
>>> eval('os.getcwd()')
'/home/user'

将globals和locals两单参数设置为空:

>>> import os
>>> eval('os.getcwd()', {}, {})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1, in <module>
NameError: name 'os' is not defined

但利用内置函数却足以绕了白名单:

>>> eval('abs(10)', {}, {})
10
>>> eval('__import__("os").getcwd()', {}, {})
'/home/user'

足看下globals():

>>> globals()
{'__builtins__': <module '__builtin__' (built-in)>, '__name__': '__main__', '__doc__': None, '__package__': None}

__builtins__安装在python的放函数,下面的写法是一律的:

>>> abs(10)
10
>>> __builtins__.abs(10)
10

将__builtins__安为空则可避免内置函数的滥用:

>>> eval('abs(10)', {'__builtins__' : None}, {})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1, in <module>
NameError: name 'abs' is not defined 

 看状态就生是平安了,但是还是得绕了的:

>>> ().__class__.__bases__[0].__subclasses__()
[<type 'type'>, <type 'weakref'>, <type 'weakcallableproxy'>, <type 'weakproxy'>, <type 'int'>, <type 'basestring'>, <type 'bytearray'>, <type 'list'>, <type 'NoneType'>, <type 'NotImplementedType'>, <type 'traceback'>, <type 'super'>, <type 'xrange'>, <type 'dict'>, <type 'set'>, <type 'slice'>, <type 'staticmethod'>, <type 'complex'>, <type 'float'>, <type 'buffer'>, <type 'long'>, <type 'frozenset'>, <type 'property'>, <type 'memoryview'>, <type 'tuple'>, <type 'enumerate'>, <type 'reversed'>, <type 'code'>, <type 'frame'>, <type 'builtin_function_or_method'>, <type 'instancemethod'>, <type 'function'>, <type 'classobj'>, <type 'dictproxy'>, <type 'generator'>, <type 'getset_descriptor'>, <type 'wrapper_descriptor'>, <type 'instance'>, <type 'ellipsis'>, <type 'member_descriptor'>, <type 'file'>, <type 'PyCapsule'>, <type 'cell'>, <type 'callable-iterator'>, <type 'iterator'>, <type 'sys.long_info'>, <type 'sys.float_info'>, <type 'EncodingMap'>, <type 'fieldnameiterator'>, <type 'formatteriterator'>, <type 'sys.version_info'>, <type 'sys.flags'>, <type 'sys.getwindowsversion'>, <type 'exceptions.BaseException'>, <type 'module'>, <type 'imp.NullImporter'>, <type 'zipimport.zipimporter'>, <type 'nt.stat_result'>, <type 'nt.statvfs_result'>, <class 'warnings.WarningMessage'>, <class 'warnings.catch_warnings'>, <class '_weakrefset._IterationGuard'>, <class '_weakrefset.WeakSet'>, <class '_abcoll.Hashable'>, <type 'classmethod'>, <class '_abcoll.Iterable'>, <class '_abcoll.Sized'>, <class '_abcoll.Container'>, <class '_abcoll.Callable'>, <class 'site._Printer'>, <class 'site._Helper'>, <type '_sre.SRE_Pattern'>, <type '_sre.SRE_Match'>, <type '_sre.SRE_Scanner'>, <class 'site.Quitter'>, <class 'codecs.IncrementalEncoder'>, <class 'codecs.IncrementalDecoder'>, <type 'operator.itemgetter'>, <type 'operator.attrgetter'>, <type 'operator.methodcaller'>, <type 'functools.partial'>, <type 'MultibyteCodec'>, <type 'MultibyteIncrementalEncoder'>, <type 'MultibyteIncrementalDecoder'>, <type 'MultibyteStreamReader'>, <type 'MultibyteStreamWriter'>, <type 'time.struct_time'>, <type '_ssl._SSLContext'>, <type '_ssl._SSLSocket'>, <type 'cStringIO.StringO'>, <type 'cStringIO.StringI'>, <class 'socket._closedsocket'>, <type '_socket.socket'>, <type 'method_descriptor'>, <class 'socket._socketobject'>, <class 'socket._fileobject'>, <type '_thread._localdummy'>, <type 'thread._local'>, <type 'thread.lock'>, <type 'collections.deque'>, <type 'deque_iterator'>, <type 'deque_reverse_iterator'>, <type 'itertools.combinations'>, <type 'itertools.combinations_with_replacement'>, <type 'itertools.cycle'>, <type 'itertools.dropwhile'>, <type 'itertools.takewhile'>, <type 'itertools.islice'>, <type 'itertools.starmap'>, <type 'itertools.imap'>, <type 'itertools.chain'>, <type 'itertools.compress'>, <type 'itertools.ifilter'>, <type 'itertools.ifilterfalse'>, <type 'itertools.count'>, <type 'itertools.izip'>, <type 'itertools.izip_longest'>, <type 'itertools.permutations'>, <type 'itertools.product'>, <type 'itertools.repeat'>, <type 'itertools.groupby'>, <type 'itertools.tee_dataobject'>, <type 'itertools.tee'>, <type 'itertools._grouper'>, <class 'threading._Verbose'>, <class 'string.Template'>, <class 'string.Formatter'>, <type 'CArgObject'>, <type '_ctypes.CThunkObject'>, <type '_ctypes._CData'>, <type '_ctypes.CField'>, <type '_ctypes.DictRemover'>, <type 'Struct'>, <class 'ctypes.CDLL'>, <class 'ctypes.LibraryLoader'>, <type 'cPickle.Unpickler'>, <type 'cPickle.Pickler'>, <class 'idlelib.rpc.SocketIO'>, <class 'idlelib.rpc.RemoteObject'>, <class 'idlelib.rpc.RemoteProxy'>, <class 'idlelib.rpc.RPCProxy'>, <class 'idlelib.rpc.MethodProxy'>, <type '_io._IOBase'>, <type '_io.IncrementalNewlineDecoder'>, <class 'subprocess.Popen'>, <class 'webbrowser.BaseBrowser'>, <class 'idlelib.tabbedpages.Page'>, <class 'idlelib.EditorWindow.HelpDialog'>, <type '_hashlib.HASH'>, <type '_random.Random'>, <class 'idlelib.EditorWindow.EditorWindow'>, <class 'idlelib.EditorWindow.IndentSearcher'>, <class 'idlelib.run.Executive'>]

透过tuple的class找到她的基类,也便是object,然后再次找到object的各种子类,从中可以视众多模块。

利用Quitter退出解释器:

>>> eval("[x for x in ().__class__.__bases__[0].__subclasses__() if x.__name__== 'Quitter'][0](0)()", {'__builtins__':None})
user:~$ 

configobj,urllib,urllib2,setuptools等模块中都发生os模块的停放:

>>> import configobj
>>> 'os' in configobj.__dict__
True
>>> import urllib2
>>> 'os' in urllib2.__dict__
True
>>> import setuptools
>>> 'os' in setuptools.__dict__
True

运zipimport通过egg文件导入这些模块就好动用os模块了:

eval("[x for x in ().__class__.__bases__[0].__subclasses__() if x.__name__ == 'zipimporter'][0]('/path/to/configobj-5.0.5-py2.7.egg').load_module('configobj').os.getcwd()", {'__builtins__':None})

上述方可看,eval()函数的纰漏还是广大之,如果只是用来开类型转换,可以采取ast.literal_eval
代替eval:

>>> import ast
>>> ast.literal_eval('[1, 2, 3]')
[1, 2, 3]
>>> ast.literal_eval('abs(10)')

Traceback (most recent call last):
  File "<pyshell#12>", line 1, in <module>
    ast.literal_eval('abs(10)')
  File "C:\Python27\lib\ast.py", line 80, in literal_eval
    return _convert(node_or_string)
  File "C:\Python27\lib\ast.py", line 79, in _convert
    raise ValueError('malformed string')
ValueError: malformed string
>>> eval('abs(10)')
10

 exec(str [, globals[, locals]])

 execfile(filename [, globals[, locals]])

仿佛之,exec函数执行一个分包python代码的字符串,execfile则执行一个文件,后少单参数和eval类似。

>>> a = [1, 2, 3, 4]
>>> exec('for i in a: print i')

activate_this = '/path/to/env/bin/activate_this.py'
execfile(activate_this, dict(__file__=activate_this))

给eval或者exec函数传递字符串时,解析器首先会把字符串编译成字节码,为避免吃资源,可以下compile函数将字符串预编译:

compile(str, filename, kind)

str是要预编译的字符串,filename为字符串所当的公文,kind参数为single时表示一致长长的语句,exec代表一如既往组报告句,eval代表一个表达式

>>> s = 'for i in [1, 2, 3]: print i'
>>> c = compile(s, '', 'exec')
>>> exec(c)
1
2
3
>>> s2 = '1+1'
>>> c2 = compile(s2, '', 'eval')
>>> eval(c2)
2

eval执行exec类型的预编译字节码时,会回到None:

>>> s2 = '1+1'
>>> c2 = compile(s2, '', 'exec')
>>> print eval(c2)
None

 以前看了一个案例,使用eval转换不规则的json数据:

>>> blog = "{url : 'www.example.com'}"

方url缺少引号,使用eval和json.loads()均会报错。

>>> eval(blog)
Traceback (most recent call last):
  File "<pyshell#27>", line 1, in <module>
    eval(blog)
  File "<string>", line 1, in <module>
NameError: name 'url' is not defined

下eval时,eval会将url视为变量名并打算在globals和locals中搜索url的值,添加一下globals参数:

>>> eval(blog, {'url' : 'url'})
{'url': 'www.example.com'}

所以,现在而举行的凡就是起一个类dict的投射对象,其中value的价需要跟key值相同,可以采取type函数来树:

>>> ValueFromKey = type('ValueFromKey', (dict,), dict(__getitem__ = lambda self,k : k))
>>> eval(blog, ValueFromKey())
{'url': 'www.example.com'}

在新建的type对象基类元祖中含下dict,然后于靶的dict中定义一下__getitem__措施,使value值与key值一致即可。

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