python - 找出异常上下文

Question

tlndr:如何在函数中判断它是否是从 except block 调用的(直接/间接)。 python2.7/cpython.

我使用 python 2.7 并尝试为我的自定义异常类提供类似于 py3 的 __context__ 的内容:

class MyErr(Exception):
    def __init__(self, *args):
        Exception.__init__(self, *args)
        self.context = sys.exc_info()[1]
    def __str__(self):
        return repr(self.args) + ' from ' + repr(self.context)

这似乎工作正常:

try:
   1/0
except:
   raise MyErr('bang!')

#>__main__.MyErr: ('bang!',) from ZeroDivisionError('integer division or modulo by zero',)

有时我需要在异常 block 之外引发 MyErr。这也很好:

raise MyErr('just so')

#>__main__.MyErr: ('just so',) from None

但是，如果在此之前已经处理过异常，则它被错误地设置为 MyErr 的上下文:

try:
    print xxx
except Exception as e:
    pass

# ...1000 lines of code....
raise MyErr('look out')

#>__main__.MyErr: ('look out',) from NameError("name 'xxx' is not defined",) <-- BAD

我想原因是sys.exc_info只返回“最后”而不是“当前”异常:

This function returns a tuple of three values that give information about the exception that is currently being handled. <...> Here, “handling an exception” is defined as “executing or having executed an except clause.”

所以，我的问题是:如何判断解释器正在执行except子句(而不是有它在过去执行过).换句话说:有没有办法知道 MyErr.__init__ 中是否有 except 在堆栈上？

我的应用程序不可移植，欢迎任何特定于 CPython 的 hack。

Answer 1

这是用 CPython 2.7.3 测试的:

$ python myerr.py 
MyErr('bang!',) from ZeroDivisionError('integer division or modulo by zero',)
MyErr('nobang!',)

只要魔术异常是在 except 子句的范围内直接创建的，它就可以工作。不过，一些额外的代码可以解除该限制。

import sys
import opcode

SETUP_EXCEPT = opcode.opmap["SETUP_EXCEPT"]
SETUP_FINALLY = opcode.opmap["SETUP_FINALLY"]
END_FINALLY = opcode.opmap["END_FINALLY"]

def try_blocks(co):
    """Generate code positions for try/except/end-of-block."""
    stack = []
    code = co.co_code
    n = len(code)
    i = 0
    while i < n:
        op = ord(code[i])
        if op in (SETUP_EXCEPT, SETUP_FINALLY):
            stack.append((i, i + ord(code[i+1]) + ord(code[i+2])*256))
        elif op == END_FINALLY:
            yield stack.pop() + (i,)
        i += 3 if op >= opcode.HAVE_ARGUMENT else 1

class MyErr(Exception):
    """Magic exception."""

    def __init__(self, *args):
        callee = sys._getframe(1)
        try:
            in_except = any(i[1] <= callee.f_lasti < i[2] for i in try_blocks(callee.f_code))
        finally:
            callee = None

        Exception.__init__(self, *args)
        self.cause = sys.exc_info()[1] if in_except else None

    def __str__(self):
        return "%r from %r" % (self, self.cause) if self.cause else repr(self)

if __name__ == "__main__":
    try:
        try:
            1/0
        except:
            x = MyErr('bang!')
            raise x
    except Exception as exc:
        print exc

    try:
        raise MyErr('nobang!')
    except Exception as exc:
        print exc
    finally:
        pass

请记住，“显式优于隐式”，所以如果你问我，这会更好:

try:
    …
except Exception as exc:
    raise MyErr("msg", cause=exc)