如何在Python中将字符串转换为可运行的（未评估）功能

Question

我有一个代码从脚本中创建一个代表var的字符串：

"((((x) * (320)) + ((y) * (270))) + ((z) * (400))) + ((w) * (500))"

我的目标是 store （不评估）它作为函数：

f = ((((x) * (320)) + ((y) * (270))) + ((z) * (400))) + ((w) * (500))

其中x，y，z和w是我的python脚本中的实际变量。

因此，可以通过更改一个或多个变量来动态更改它。

有图书馆这样做吗？

Answer 1

其他答案是错误的。很可能在Python中懒惰地评估这一点。

>>> create_function=lambda s:lambda:eval(s)
>>> x=2
>>> y=3
>>> create_function('x+y')()
5
>>> z=create_function('x*y')
>>> z()
6

Answer 2

import re
from typing import List, Any
from dataclasses import dataclass, field

@dataclass
class Database:
    variables: List[str] = field(default_factory=lambda: [])
    expression: str = ''

def store_expression(database: Database, plain_expression: str) -> None:
    variable_regexes = r'[a-z]'
    # you can get the variables as str and store them
    database.variables = re.findall(variable_regexes, plain_expression)
    
    # Then you can replace the equation with {} and store it too
    database.expression = re.sub(r'[a-z]', '{}', plain_expression)

def eval_expression(database: Database) -> Any:
    return eval(
        database.expression.format(
            *database.variables
        )
    )

s = "((((x) * (320)) + ((y) * (270))) + ((z) * (400))) + ((w) * (500))"

in_memory_db = Database()
store_expression(in_memory_db, s)

x = 1
y = 2
z = 3
w = 4

result = eval_expression(in_memory_db)
print(result)