单行异常处理

Question

在Python中，可以使用一行程序以简单直观的方式设置具有特殊条件（如默认值或条件）的值。

result = 0 or "Does not exist."  # "Does not exist."

result = "Found user!" if user in user_list else "User not found."

是否可以编写一个捕捉异常的类似语句？

from json import loads

result = loads('{"value": true}') or "Oh no, explosions occurred!"
# {'value': True}

result = loads(None) or "Oh no, explosions occurred!"
# "Oh no, explosions occurred!" is desired, but a TypeError is raised.

Answer 1

可以在一行中使用exec：

parse_float = lambda x, y=exec("def f(s):\n try:\n  return float(s)\n except:  return None"): f(x)

Answer 2

在python中不可能执行单行异常处理语句。我们可以编写一个函数来实现这一点。

def safe_execute(default, exception, function, *args):
    try:
        return function(*args)
    except exception:
        return default

示例用法：

from json import loads
safe_execute("Oh no, explosions occurred!", TypeError, loads, None)
# Returns "Oh no, explosions occurred!"
safe_execute("Huh?", TypeError, int, "10")
#Returns 10

支持多个参数

from operator import div
safe_execute(
    "Divsion by zero is invalid.",
    ZeroDivisionError,
    div, 1, 0
)
# Returns "Divsion by zero is invalid."

safe_execute(
    "Divsion by zero is invalid.",
    ZeroDivisionError,
    div, 1, 1
)
# Returns 1.

错误捕获过程仍可能被中断：

from time import sleep
safe_execute(
    "Panic!",
    Exception,
    sleep, 8
)
# Ctrl-c will raise a KeyboardInterrupt

from sys import exit
safe_execute("Failed to exit!", Exception, exit)
# Exits the Python interpreter

如果此行为是不需要的，请使用BaseException：

from time import sleep
safe_execute("interrupted",
             BaseException,
             sleep, 8)
#Pressing Ctrl-c will return "interrupted"
from sys import exit
safe_execute("Naughty little program!",
             BaseException,
             exit)
#Returns "Naughty little program!"