try语句的可选else子句的预期用途是什么?


当前回答

假设您的编程逻辑取决于字典中是否有具有给定键的条目。你可以使用if…来测试dict.get(key)的结果。其他的……或者你可以这样做:

try:
    val = dic[key]
except KeyError:
    do_some_stuff()
else:
    do_some_stuff_with_val(val)

其他回答

else:块令人困惑并且(几乎)毫无用处。它也是for和while语句的一部分。

实际上,即使在if语句中,else:也可能被滥用,造成非常难以发现的错误。

考虑这一点。

   if a < 10:
       # condition stated explicitly
   elif a > 10 and b < 10:
       # condition confusing but at least explicit
   else:
       # Exactly what is true here?
       # Can be hard to reason out what condition is true

三思而后行。这通常是个问题。除非在if语句中,否则避免使用它,即使在if语句中,也要考虑记录else-条件,使其显式。

一种用法:测试一些应该引发异常的代码。

try:
    this_should_raise_TypeError()
except TypeError:
    pass
except:
    assert False, "Raised the wrong exception type"
else:
    assert False, "Didn't raise any exception"

(在实践中,这段代码应该被抽象为更通用的测试。)

Python try-else try语句的可选else子句的预期用途是什么?

其预期用途是,如果没有预期要处理的异常,则为运行更多代码提供上下文。

这个上下文避免意外处理您没有预料到的错误。

但是理解导致else子句运行的确切条件非常重要,因为return、continue和break可以中断到else的控制流。

总之

如果没有异常,并且没有被return、continue或break语句中断,else语句将运行。

其他答案忽略了最后一部分。

从文档中可以看出:

类型的控件流出时,执行可选的else子句 try子句结束

(粗体)。脚注写道:

*目前,控制“从末端流出”,除非在 异常或return、continue或break语句的执行。

它至少需要一个except子句(参见语法)。所以它实际上不是“try-else”,而是“try-except-else(-finally)”,其中else(和finally)是可选的。

Python教程详细说明了预期的用法:

The try ... except statement has an optional else clause, which, when present, must follow all except clauses. It is useful for code that must be executed if the try clause does not raise an exception. For example: for arg in sys.argv[1:]: try: f = open(arg, 'r') except IOError: print 'cannot open', arg else: print arg, 'has', len(f.readlines()), 'lines' f.close() The use of the else clause is better than adding additional code to the try clause because it avoids accidentally catching an exception that wasn’t raised by the code being protected by the try ... except statement.

示例区分else和try块后面的代码

如果处理错误,else块将不会运行。例如:

def handle_error():
    try:
        raise RuntimeError('oops!')
    except RuntimeError as error:
        print('handled a RuntimeError, no big deal.')
    else:
        print('if this prints, we had no error!') # won't print!
    print('And now we have left the try block!')  # will print!

现在,

>>> handle_error()
handled a RuntimeError, no big deal.
And now we have left the try block!

PEP 380中有一个很好的try-else示例。基本上,它归结为在算法的不同部分做不同的异常处理。

大概是这样的:

try:
    do_init_stuff()
except:
    handle_init_suff_execption()
else:
    try:
        do_middle_stuff()
    except:
        handle_middle_stuff_exception()

这允许您在异常发生的地方编写异常处理代码。

我已经找到了…Else:在运行数据库查询并将这些查询的结果记录到相同风格/类型的单独数据库的情况下,构造非常有用。假设我有很多工作线程,它们都处理提交给队列的数据库查询

#in a long running loop
try:
    query = queue.get()
    conn = connect_to_db(<main db>)
    curs = conn.cursor()
    try:
        curs.execute("<some query on user input that may fail even if sanitized">)
    except DBError:
        logconn = connect_to_db(<logging db>)
        logcurs = logconn.cursor()
        logcurs.execute("<update in DB log with record of failed query")
        logcurs.close()
        logconn.close()
    else:

        #we can't put this in main try block because an error connecting
        #to the logging DB would be indistinguishable from an error in 
        #the mainquery 

        #We can't put this after the whole try: except: finally: block
        #because then we don't know if the query was successful or not

        logconn = connect_to_db(<logging db>)
        logcurs = logconn.cursor()
        logcurs.execute("<update in DB log with record of successful query")
        logcurs.close()
        logconn.close()
        #do something in response to successful query
except DBError:
    #This DBError is because of a problem with the logging database, but 
    #we can't let that crash the whole thread over what might be a
    #temporary network glitch
finally:
    curs.close()
    conn.close()
    #other cleanup if necessary like telling the queue the task is finished

当然,如果您能够区分可能抛出的异常,则不必使用这种方法,但是如果代码对成功的代码段的响应可能会抛出与成功的代码段相同的异常,并且您不能让第二个可能的异常消失,或者在成功时立即返回(在我的例子中,这会杀死线程),那么这种方法就会派上用场。