try语句的可选else子句的预期用途是什么?
当前回答
我将添加另一个用例,在处理DB会话时看起来很直接:
# getting a DB connection
conn = db.engine.connect()
# and binding to a DB session
session = db.get_session(bind=conn)
try:
# we build the query to DB
q = session.query(MyTable).filter(MyTable.col1 == 'query_val')
# i.e retrieve one row
data_set = q.one_or_none()
# return results
return [{'col1': data_set.col1, 'col2': data_set.col2, ...}]
except:
# here we make sure to rollback the transaction,
# handy when we update stuff into DB
session.rollback()
raise
else:
# when no errors then we can commit DB changes
session.commit()
finally:
# and finally we can close the session
session.close()
其他回答
try:
statements # statements that can raise exceptions
except:
statements # statements that will be executed to handle exceptions
else:
statements # statements that will be executed if there is no exception
例子:
try:
age=int(input('Enter your age: '))
except:
print ('You have entered an invalid value.')
else:
if age <= 21:
print('You are not allowed to enter, you are too young.')
else:
print('Welcome, you are old enough.')
输出:
>>>
Enter your age: a
You have entered an invalid value.
>>> RESTART
>>>
Enter your age: 25
Welcome, you are old enough.
>>> RESTART
>>>
Enter your age: 13
You are not allowed to enter, you are too young.
>>>
复制自:https://geek-university.com/python/the-try-except-else-statements/
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)
下面是我喜欢使用这种模式的另一个地方:
while data in items:
try
data = json.loads(data)
except ValueError as e:
log error
else:
# work on the `data`
我发现它非常有用,当你有清理工作要做,即使有例外:
try:
data = something_that_can_go_wrong()
except Exception as e: # yes, I know that's a bad way to do it...
handle_exception(e)
else:
do_stuff(data)
finally:
clean_up()
Try-except-else非常适合结合EAFP模式和duck-typing:
try:
cs = x.cleanupSet
except AttributeError:
pass
else:
for v in cs:
v.cleanup()
您可能认为naïve代码是好的:
try:
for v in x.cleanupSet:
v.clenaup()
except AttributeError:
pass
这是在代码中意外隐藏严重错误的好方法。我在那里输入了清理,但是让我知道的AttributeError正在被吞噬。更糟糕的是,如果我写对了,但清理方法偶尔被传递一个具有错误命名属性的用户类型,导致它在中途无声地失败并留下一个未关闭的文件,会怎样?祝你在调试时好运。
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