try语句的可选else子句的预期用途是什么?
当前回答
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!
其他回答
错误和异常#处理异常- docs.python.org
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.
可以在finally子句中以通用的方式使用此构造来处理异常,而在没有异常时执行其他操作:
class TooManyRetries(RuntimeError):
pass
n_tries = 0
max_retries = 2
while True:
try:
n_tries += 1
if n_tries >= max_retries:
raise TooManyRetries
fail_prone_operation()
except Exception1 as ex:
# handle1
except Exception2 as ex:
# handle2
except Exception3 as ex:
# handle3
except TooManyRetries as ex:
raise
else: # No exception
n_tries = 0
finally:
common_restore_state()
continue
我能想到的一种使用场景是不可预测的异常,如果您再次尝试,可以避免这种异常。例如,当try块中的操作涉及随机数时:
while True:
try:
r = random.random()
some_operation_that_fails_for_specific_r(r)
except Exception:
continue
else:
break
但是如果可以预测异常,则应该始终在异常之前选择验证。然而,并不是所有的事情都可以预测,所以这种代码模式有它的一席之地。
也许它的用法是:
#debug = []
def debuglog(text, obj=None):
" Simple little logger. "
try:
debug # does global exist?
except NameError:
pass # if not, don't even bother displaying
except:
print('Unknown cause. Debug debuglog().')
else:
# debug does exist.
# Now test if you want to log this debug message
# from caller "obj"
try:
if obj in debug:
print(text) # stdout
except TypeError:
print('The global "debug" flag should be an iterable.')
except:
print('Unknown cause. Debug debuglog().')
def myfunc():
debuglog('Made it to myfunc()', myfunc)
debug = [myfunc,]
myfunc()
也许这对你也有帮助。
我发现其他有用的处理可能不正确的配置文件:
try:
value, unit = cfg['lock'].split()
except ValueError:
msg = 'lock monitoring config must consist of two words separated by white space'
self.log('warn', msg)
else:
# get on with lock monitoring if config is ok
读取锁配置的异常禁用锁监控,ValueErrors记录有用的警告消息。
推荐文章
- 证书验证失败:无法获得本地颁发者证书
- 当使用pip3安装包时,“Python中的ssl模块不可用”
- 无法切换Python与pyenv
- Python if not == vs if !=
- 如何从scikit-learn决策树中提取决策规则?
- 为什么在Mac OS X v10.9 (Mavericks)的终端中apt-get功能不起作用?
- 将旋转的xtick标签与各自的xtick对齐
- 为什么元组可以包含可变项?
- 如何合并字典的字典?
- 如何创建类属性?
- 不区分大小写的“in”
- 在Python中获取迭代器中的元素个数
- 解析日期字符串并更改格式
- 使用try和。Python中的if
- 如何在Python中获得所有直接子目录