Python 中的嵌套 try/except
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nested try/except in Python
提问by tic
try:
commands
try:
commands
try:
commands
try:
commands
except:
commands
return to final commands
except:
commands
return to final commands
except:
commands
return to final commands
except:
commands
final commands
Which instruction have I to write in place of return to final commandsto make that any exceptreturns to the top-level instructions following the outer try? And is it an acceptable structure?
这说明有我在的地方写return to final commands作任何except跟随外尝试返回到顶级的指令？它是一个可以接受的结构吗？
Edit: Here is a toy example (I know I can do it using ifs, it's only an example; suppose you have to write it using try/except).
编辑：这是一个玩具示例（我知道我可以使用ifs 来完成，这只是一个示例；假设您必须使用try/编写它except）。
# calculate arcsin(log(sqrt(x)-4))
x = choose one yourself
try
x1=sqrt(x)
return to final message
try
x1=log(x1-4)
return to final message
try
x2=arcsin(x1)
return to final message
except
message="Impossible to calculate arcsin: maybe an argument outside [-1,+1]?"
except
message="Impossible to calculate logarithm: maybe a not positive argument?"
except
message="impossible to calculate square root: maybe a negative argument?"
final message:
print message
采纳答案by Evan
At the very least you should be able to reduce this structure to only 2 nested levels by reraising the exception to avoid the rest of the block:
至少，您应该能够通过重新引发异常以避免块的其余部分来将此结构减少到仅 2 个嵌套级别：
# calculate arcsin(log(sqrt(x)-4))
x = ?
message = None
try:
try:
x1 = sqrt(x)
except Exception:
message = "can't take sqrt"
raise
try:
x1 = log(x1-4)
except Exception:
message = "can't compute log"
raise
try:
x2 = arcsin(x1)
except Exception:
message = "Can't calculate arcsin"
raise
except Exception:
print message
Really, this is not the way to do it, at least in this example. The problem is that you are trying to use exceptions like return error codes. What you should be doing is putting the error message into the exception. Also, normally the outer try/except would be in a higher level function:
真的，这不是这样做的方法，至少在这个例子中是这样。问题是您正在尝试使用诸如返回错误代码之类的异常。您应该做的是将错误消息放入异常中。此外，通常外部 try/except 将位于更高级别的函数中：
def func():
try:
y = calculate_asin_log_sqrt(x)
# stuff that depends on y goes here
except MyError as e:
print e.message
# Stuff that happens whether or not the calculation fails goes here
def calculate_asin_log_sqrt(x):
try:
x1 = sqrt(x)
except Exception:
raise MyError("Can't calculate sqrt")
try:
x1 = log(x1-4)
except Exception:
raise MyError("Can't calculate log")
try:
x2 = arcsin(x1)
except Exception:
raise MyError("Can't calculate arcsin")
return x2
