Python Nested Attempt / Exception

Question

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

What instruction should I write instead return to final commands, so that anyone exceptreturns to the top-level instructions following an external attempt? And an acceptable structure?

Edit: Here is an example of toys (I know I can do this with ifs, this is just an example, suppose you need to write it with 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

Answer 1

At the very least, you should be able to reduce this structure to only 2 nested levels by repeating 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

, , , . , , . , , . , try/except :

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