问题描述
为什么 5 * 7 的字面求值会失败，而 5 + 7 不会?
Why does the literal evaluation of 5 * 7 fail, while 5 + 7 doesn't?
import ast
print(ast.literal_eval('5 + 7'))
# -> 12
print(ast.literal_eval('5 * 7'))
# ->
Traceback (most recent call last):
...
ValueError: malformed node or string: <_ast.BinOp object at ...>
文档没有解释一下.
我在 SO 上回答这个问题后发现了这个问题:获取字符串的结果.
I found that problem after answering this question on SO: Getting the result of a string.
推荐答案
ast.literal_eval() 在评估数据中接受 + 因为 5+2j(复数)是有效的文字.这同样适用于 -.为了保持代码简单，没有尝试将 + 或 - 作为二元运算符排除在外.
ast.literal_eval() accepts + in the evaluated data because 5+2j (complex number) are valid literals. The same applies to -. To keep the code simple, no attempt is made to exclude + or - as a binary operators.
不允许其他操作符；该函数应该只接受文字，不接受表达式.
No other operators are allowed; the function is supposed to only accept literals, not expressions.
换句话说，5 + 7 工作是一个错误，但如果不破坏对构造复数的支持，就很难修复.实现 将使用限制为数字操作数，一元 + 和 - 或其他二元运算符(因此您不能使用它们来连接列表或产生集差).
In other words, that 5 + 7 works is a bug, but one that is hard to fix without breaking support for constructing complex numbers. The implementation limits the use to operands that are numbers, unary + and -, or other binary operators (so you can't use these to concatenate lists or produce a set difference).
还可以看到几个相关的 Python 错误跟踪器条目:#25335 ast.literal_eval 无法解析带有前导+"的数字", #22525 ast.literal_eval() 没有做文档中的事情说和#4907 ast.literal_eval 没有正确处理复数
Also see several related Python bugtracker entries: #25335 ast.literal_eval fails to parse numbers with leading "+", #22525 ast.literal_eval() doesn't do what the documentation says and #4907 ast.literal_eval does not properly handled complex numbers
从技术上讲，2j 是一个有效的文字；Python 将 5+2j 解析为 int(5) binop(+) complex(0, 2)，然后才产生 complex(5, 2) 来自结果的对象，当实际执行加法时.
Technically speaking, 2j is a valid literal; Python parses 5+2j as int(5) binop(+) complex(0, 2), and only later produces a complex(5, 2) object from the result, when actually executing the addition.
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