Python: 如何对集合中的集合使用ast.literal_eval()？

Question

集合不可散列，因为它们是可变的。但是有没有办法在 {1, 2, {3, 4}} 上仍然使用 literal_eval？我只想知道外部结构是一个集合，我不关心内部类型，但集合中的集合是可能的输入。

更新：

输入作为字符串从文件中读取。

Answer 1

问题 1 的答案:

Are sets within sets are possible input?

您可以使用 frozenset 创建嵌套集合对象：

Return a new set or frozenset object whose elements are taken from iterable. The elements of a set must be hashable. To represent sets of sets, the inner sets must be frozenset objects. If iterable is not specified, a new empty set is returned.

例如：

>>> frozen_set = frozenset({3, 4})

>>> my_set = {1, 2, frozen_set}
>>> my_set
{frozenset({3, 4}), 2, 1}

问题 2 的答案:

Is there a way to still use literal_eval on {1, 2, {3, 4}}?

不，没有办法，因为没有 ast.literal_eval for frozenset 可迭代。有一个 proposal in 2008 by Raymond Hettinger for frozenset literal. Guido even pronounced his agreement but then changed his mind. Check this mail 以获得更多见解。

但您可以使用 eval 代替：

>>> my_str = '{frozenset({3, 4}), 2, 1}'

>>> eval(my_str)
{1, 2, frozenset({3, 4})}

在使用eval之前，请同时阅读：Why should exec() and eval() be avoided?

Answer 2

您可以破解 ast.literal_eval 使其 return 在看到集合时成为冻结集合。方法如下：

搜索 Python 安装的库所在的位置
它包含包含函数 literal_eval

ast.py

在您自己的模块中复制该函数（使用不同的名称）并将其更改为从 ast 模块
在处理Set的行中，用frozenset

set

然后您可以使用它来安全地解析包含集合的文字集。对于我的 Python 3.5，我使用了：

def frozen_literal_eval(node_or_string):
    """
    Safely evaluate an expression node or a string containing a Python
    expression.  The string or node provided may only consist of the following
    Python literal structures: strings, bytes, numbers, tuples, lists, dicts,
    sets, booleans, and None.

    SPECIAL: This version uses frozensets instead of sets
    """
    # SPECIAL: import names from ast module
    from ast import parse, Expression, Str, Bytes, Num, Tuple, List, Set, Dict
    from ast import NameConstant, UnaryOp, UAdd, USub, BinOp, Add, Sub
    # END SPECIAL
    if isinstance(node_or_string, str):
        node_or_string = parse(node_or_string, mode='eval')
    if isinstance(node_or_string, Expression):
        node_or_string = node_or_string.body
    def _convert(node):
        if isinstance(node, (Str, Bytes)):
            return node.s
        elif isinstance(node, Num):
            return node.n
        elif isinstance(node, Tuple):
            return tuple(map(_convert, node.elts))
        elif isinstance(node, List):
            return list(map(_convert, node.elts))
        elif isinstance(node, Set):
            #SPECIAL: returns a frozenset
            return frozenset(map(_convert, node.elts))
            # END SPECIAL
        elif isinstance(node, Dict):
            return dict((_convert(k), _convert(v)) for k, v
                        in zip(node.keys, node.values))
        elif isinstance(node, NameConstant):
            return node.value
        elif isinstance(node, UnaryOp) and \
             isinstance(node.op, (UAdd, USub)) and \
             isinstance(node.operand, (Num, UnaryOp, BinOp)):
            operand = _convert(node.operand)
            if isinstance(node.op, UAdd):
                return + operand
            else:
                return - operand
        elif isinstance(node, BinOp) and \
             isinstance(node.op, (Add, Sub)) and \
             isinstance(node.right, (Num, UnaryOp, BinOp)) and \
             isinstance(node.left, (Num, UnaryOp, BinOp)):
            left = _convert(node.left)
            right = _convert(node.right)
            if isinstance(node.op, Add):
                return left + right
            else:
                return left - right
        raise ValueError('malformed node or string: ' + repr(node))
    return _convert(node_or_string)

我可以使用：

>>> s = '{ 1, 2, {3, 4}}'
>>> frozen_literal_eval(s)
frozenset({1, 2, frozenset({3, 4})})

Python: 如何对集合中的集合使用ast.literal_eval()？

Python: How to use ast.literal_eval() for sets in set?

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