格式错误的字符串ValueError ast.literal_eval(),字符串表示为元组

Question

我正在尝试从文件中读取元组的字符串表示,并将元组添加到列表中.这是相关的代码.

raw_data = userfile.read().split('\n')
for a in raw_data : 
    print a
    btc_history.append(ast.literal_eval(a))

这是输出:

(Decimal('11.66985'), Decimal('0E-8'))
Traceback (most recent call last):


File "./goxnotify.py", line 74, in <module>
    main()
  File "./goxnotify.py", line 68, in main
    local.load_user_file(username,btc_history)
  File "/home/unix-dude/Code/GoxNotify/local_functions.py", line 53, in load_user_file
    btc_history.append(ast.literal_eval(a))
  File "/usr/lib/python2.7/ast.py", line 80, in literal_eval
    return _convert(node_or_string)

  `File "/usr/lib/python2.7/ast.py", line 58, in _convert
   return tuple(map(_convert, node.elts))
  File "/usr/lib/python2.7/ast.py", line 79, in _convert
   raise ValueError('malformed string')
   ValueError: malformed string

Answer 1

ast.literal_eval(位于ast.py)首先解析树ast.parse,然后用相当丑陋的递归函数计算代码,解释解析树元素并用它们的文字等价物替换它们.不幸的是,代码根本不可扩展,因此要添加Decimal到代码中,您需要复制所有代码并重新开始.

对于稍微简单的方法,您可以使用ast.parse模块来解析表达式,然后使用ast.NodeVisitor或ast.NodeTransformer确保没有不需要的语法或不需要的变量访问.然后使用compile和编译eval得到结果.

代码literal_eval与此代码实际使用的代码略有不同eval,但在我看来,理解起来更简单,并且不需要深入研究AST树.它特别只允许一些语法,明确禁止例如lambdas,属性访问(foo.__dict__非常邪恶),或访问任何不被认为安全的名称.它解析你的表达式很好,另外我还添加了Num(浮点数和整数),列表和字典文字.

此外,在2.7和3.3上也一样

import ast
import decimal

source = "(Decimal('11.66985'), Decimal('1e-8'),"\
    "(1,), (1,2,3), 1.2, [1,2,3], {1:2})"

tree = ast.parse(source, mode='eval')

# using the NodeTransformer, you can also modify the nodes in the tree,
# however in this example NodeVisitor could do as we are raising exceptions
# only.
class Transformer(ast.NodeTransformer):
    ALLOWED_NAMES = set(['Decimal', 'None', 'False', 'True'])
    ALLOWED_NODE_TYPES = set([
        'Expression', # a top node for an expression
        'Tuple',      # makes a tuple
        'Call',       # a function call (hint, Decimal())
        'Name',       # an identifier...
        'Load',       # loads a value of a variable with given identifier
        'Str',        # a string literal

        'Num',        # allow numbers too
        'List',       # and list literals
        'Dict',       # and dicts...
    ])

    def visit_Name(self, node):
        if not node.id in self.ALLOWED_NAMES:
            raise RuntimeError("Name access to %s is not allowed" % node.id)

        # traverse to child nodes
        return self.generic_visit(node)

    def generic_visit(self, node):
        nodetype = type(node).__name__
        if nodetype not in self.ALLOWED_NODE_TYPES:
            raise RuntimeError("Invalid expression: %s not allowed" % nodetype)

        return ast.NodeTransformer.generic_visit(self, node)


transformer = Transformer()

# raises RuntimeError on invalid code
transformer.visit(tree)

# compile the ast into a code object
clause = compile(tree, '<AST>', 'eval')

# make the globals contain only the Decimal class,
# and eval the compiled object
result = eval(clause, dict(Decimal=decimal.Decimal))

print(result)

Answer 2

从文档为ast.literal_eval():

安全地评估表达式节点或包含Python表达式的字符串.提供的字符串或节点可能只包含以下Python文字结构:字符串,数字,元组,列表,dicts,布尔值和None.

Decimal不在允许的事项列表中ast.literal_eval().

Answer 3

就我而言，我解决了：

my_string = my_string.replace(':false', ':False').replace(':true', ':True')
ast.literal_eval(my_string)