Python - 有条件地捕获异常

Question

是否可以在 python 中有条件地捕获异常？我希望能够编写一个函数，以便调用者可以决定谁处理异常。

基本上，我想要这样的东西：

def my_func(my_arg, handle_exceptions):
    try:
        do_something(my_arg)
    except Exception as e if handle_exceptions:
        print "my_func is handling the exception"

我知道我可以编写一些杂乱无章的代码来做我想做的事，但如果有的话，我想要一个 Python 式的答案。
谢谢。

Answer 1

如果不想处理，可以重新引发异常：

def my_func(my_arg, handle_exceptions):
    try:
        do_something(my_arg)
    except Exception, e:
        if not handle_exceptions:
            # preserve prior stack trace
            raise

            # Or, if you dont care about the stack prior to this point
            #raise Exception(e)

            # similarly, you can just re-raise e.  The stack trace will start here though.
            #raise e
        else:
            print "my_func is handling the exception"

---

另一种选择是创建您自己的子类Exception 的异常（或特定异常，如urllib2.HTTPError），然后只捕获/抛出（raise）您的自定义异常：

class MyException(Exception):
    def __init__(self, message):
        self.message = message

class MyExceptionTwo(Exception):
    def __init__(self, message):
        self.message = message
    def __repr__(self):
        return "Hi, I'm MyExceptionTwo.  My error message is: %s" % self.message

def something():
    if not tuesday:
        raise MyException("Error: it's not Tuesday.")
    else:
        raise MyExceptionTwo("Error: it's Tuesday.")

def my_func(my_arg):
    try:
        something()
    except MyException, e:
        print e.message
    # Will pass MyExceptionTwo up the call chain

def my_other_func():
    try:
        my_func(your_arg)
    except MyExceptionTwo, e:
        print str(e)
    # No need to catch MyException here since we know my_func() handles it
    # but we can hadle MyExceptionTwo here

Answer 2

这个问题没有足够的答案;-)

这里还有一本记录簿。只需创建一个虚拟异常：

class NeverMatch(Exception):
    'An exception class that is never raised by any code anywhere'

然后，使用条件表达式来决定是否匹配真正的异常或占位符异常（永远不会引发）：

try:
    do_something(my_arg)
except (Exception if handle_exceptions else NeverMatch) as e:
    print 'I am handling it'

Answer 3

你可以使用：

def my_func(my_arg, handle_exceptions):
  try:
    do_something(my_arg);
  except Exception as e:
    if not handle_exceptions: raise
    print "my_func is handling the exception";

Answer 4

异常类型可以是变量。

def my_func(my_arg, handle_exceptions):
  if handle_exceptions:
    exc_type = Exception
  else:
    exc_type = None

  try:
    do_something(my_arg);
  except exc_type, e:
    print "my_func is handling the exception";

混淆 Python（“Pythonic”？）版本：

def my_func(my_arg, handle_exceptions):
  try:
    do_something(my_arg);
  except (handle_exceptions and Exception), e:
    print "my_func is handling the exception";

实际上，不带括号也可以工作，但只要我们被混淆，我们就不要用鲜为人知的规则混淆人们，比如除了语句的优先级。

Answer 5

你总是可以抓住它并像这样有条件地重新加注：

def my_func(my_arg, handle_exceptions):
  try:
    do_something(my_arg)
  except Exception:
    if handle_exceptions:
      print "my_func is handling the exception"
      #handle it
    else: 
      print "my_func is NOT handling the exception"
      raise

Answer 6

您有两个基本选择：

1. 将handle_exceptions 视为布尔值，如果False 则重新加注
2. 将handle_exceptions 视为要处理的异常

沿着布尔路径，您有两个基本选择：

def my_func(my_arg, handle_exceptions):
    try:
        do_something(my_arg)
    except Exception, e:
        if not handle_exceptions:
            raise
        print "my_func is handling the exception"

或

def my_func(my_arg, handle_exceptions):
    if handle_exceptions:
        exceptions = ValueError, IndexError # or whatever
    else:
        exceptions = NoExceptions # None in 2.x, or custom Exception class in 3.x
    try:
        do_something(my_arg)
    except exceptions, e:
        print "my_func is handling the exception"

沿着“将handle_exceptions 视为要处理的例外”路线，您可以这样做：

class NoExceptions(Exception):
    'Dummy exception, never raised'

def my_func(my_arg, handle_exceptions=NoExceptions):
    try:
        do_something(my_arg)
    except handle_exceptions, e:
        print "my_func is handling the exception"

你会这样称呼它：

my_func(some_arg, ValueError)  # to handle ValueErrors

或

my_func(some_arg)  # to not handle any exeptions

这具有调用者能够指定处理哪些异常的优点/缺点。如果您确实采取了最后一条路线，您可能还想指定一个异常处理程序，可能是这样的：

def my_func(my_arg, handle_exceptions=NoExceptions, handler=None):
    try:
        do_something(my_arg)
    except handle_exceptions, e:
        if handler is not None:
            handler(e)
        else:
            log_this_exception()

Answer 7

是的。在有意义的情况下，我更喜欢积极的条件：

def my_func(my_arg, handle_exceptions):
  try:
    do_something(my_arg);
  except Exception, e:
    if handle_exceptions:
        print "my_func is handling the exception"
    else:
        raise

Answer 8

改进其他答案的一个漂亮的技术是将条件异常处理逻辑包装在可以重用的上下文管理器中，如下所示：

from contextlib import contextmanager 

@contextmanager
def ignore_errors_if(exception_type, skip_condition): 
    try: 
        yield 
    except exception_type, excn: 
        if skip_condition: 
            logging.debug("SKIPPING EXCEPTION %s" % excn)  # etc... 
            pass 
        else: 
            raise excn 

然后，在你的代码中，你可以说：

def some_function(): 
    # ... 
    with ignore_errors_if(Exception, should_ignore_errors): 
        result = some_funciton_that_might_raise() 
    # Deal with result, although know that it might not be set...