python 如何解释字符串中的变量属性

Question

Suppose I have a string like this

"Style.BRIGHT + 'BRIGHT' + '\\n' + Style.DIM + 'DIM' + '\\n' + Style.NORMAL + 'NORMAL'"

I try to print it directly

from colorama import Style

s = "Style.BRIGHT + 'BRIGHT' + '\\n' + Style.DIM + 'DIM' + '\\n' + Style.NORMAL + 'NORMAL'"
print(s)

Output:

Style.BRIGHT + 'BRIGHT' + '\n' + Style.DIM + 'DIM' + '\n' + Style.NORMAL + 'NORMAL'

This is not the result I want, I want it to display the result in the terminal like this.

So I tried eval and it worked.

from colorama import Style

s = "Style.BRIGHT + 'BRIGHT' + '\\n' + Style.DIM + 'DIM' + '\\n' + Style.NORMAL + 'NORMAL'"
print(eval(s))

Output:

BRIGHT
DIM
NORMAL

Although it succeeded, it seems to be unsafe, I also tried adding the global variable {"Style": Style} , so it might be relatively safe.

But maybe I'll extract properties in other variables, like this xy , where x and y can be any value. So it feels like eval doesn't apply.

The answer of @jfs in this question seems to be what I want, and I modified the program.

from colorama import Style

import ast
import operator as op
# supported operators
operators = {ast.Add: op.add}


def eval_expr(expr):

    return eval_(ast.parse(expr, mode='eval').body)


def eval_(node):
    if isinstance(node, ast.Constant):  # <number>
        return node.n
    elif isinstance(node, ast.BinOp):  # <left> <operator> <right>
        return operators[type(node.op)](eval_(node.left), eval_(node.right))
    elif isinstance(node, ast.Attribute):
        return ast.unparse(node)
    else:
        raise TypeError(node)


s = "Style.BRIGHT + 'BRIGHT' + '\\n' + Style.DIM + 'DIM' + '\\n' + Style.NORMAL + 'NORMAL'"
print(eval_expr(s))

Output:

Style.BRIGHTBRIGHT
Style.DIMDIM
Style.NORMALNORMAL

In the program the string Style.BRIGHT is converted into an ast.Attribute object, I don't know how to get its value( '\x1b[1m' ).

In [1]: from colorama import Style

In [2]: Style.BRIGHT
Out[2]: '\x1b[1m'

If I use eval(ast.unparse(node)) it succeeds, but if so why not just use eval directly...

So my question is:

• How can I extract the primitive value corresponding to the ast.Attribute object?

Or is there any other way to interpret the string.

---

The demo above is just one case, it can be applied to other cases. For example, I want to define a macro, which is a string that adds and sums some properties of a class, which will be applied to other programs or processes. Demo:

class Base:

    a = 1
    b = 2


def get(cls):
    s = []
    for k, v in cls.__dict__.items():
        if k.startswith("_"):
            continue
        s.append(f"{cls.__name__}.{k}")
    return " + ".join(s)


s = get(Base)
print(s)

# Base.a + Base.b

Maybe my case is a bit unsatisfactory, but this problem may appear in a future scenario, maybe it's just me wanting to know how to deal with it.

Answer 1

NB. It would be hundred times better to use the correct object and not a string in the first place, starting with correct input is almost always more explicit and efficient than trying to fix a broken input.

If you have a string with arbitrarily complex operations, you will need to understand its grammar, which is exactly what ast is doing, or blindly (and potentially unsafely) evaluate it with eval .

That said, if you want to simplify your process in the particular case you showed , you need to do 3 things. 1- replace the Styler definitions with the string formatting codes they represent, 2- remove the quotes and +, 3- unescape the escaped newlines.

2 and 3 are quite easy to achieve, so I'll focus only on interpreting the codes here. You can use a regex to find the Styler definitions and re.sub to replace them safely with the actual formatting code.

from colorama import Style
import re

s = "Style.BRIGHT + 'BRIGHT' + '\\n' + Style.DIM + 'DIM' + '\\n' + Style.NORMAL + 'NORMAL'"

s2 = re.sub('Style\.[A-Z]+', lambda m: getattr(Style, m.group(0).split('.')[1]), s)

Output:

"\x1b[1m + 'BRIGHT' + '\\n' + \x1b[2m + 'DIM' + '\\n' + \x1b[22m + 'NORMAL'"