python中try命令,嵌套在Python中的try / except

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 commands to make that any except returns to the top-level instructions following the outer try? And is it an acceptable structure?

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).

# 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

解决方案

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:

# 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:

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