第752页 | Learning Python | 阅读 ‧ 电子书库

Unified try Example

Here’s a demonstration of the merged try statement form at work. The following file, mergedexc.py, codes four common scenarios, with print statements that describe the meaning of each:

sep = '-' * 32 + '\n'
print(sep + 'EXCEPTION RAISED AND CAUGHT')
try:
    x = 'spam'[99]
except IndexError:
    print('except run')
finally:
    print('finally run')
print('after run')


print(sep + 'NO EXCEPTION RAISED')
try:
    x = 'spam'[3]
except IndexError:
    print('except run')
finally:
    print('finally run')
print('after run')


print(sep + 'NO EXCEPTION RAISED, WITH ELSE')
try:
    x = 'spam'[3]
except IndexError:
    print('except run')
else:
    print('else run')
finally:
    print('finally run')
print('after run')


print(sep + 'EXCEPTION RAISED BUT NOT CAUGHT')
try:
    x = 1 / 0
except IndexError:
    print('except run')
finally:
    print('finally run')
print('after run')

When this code is run, the following output is produced in Python 3.0 (actually, its behavior and output are the same in 2.6, because the print calls each print a single item). Trace through the code to see how exception handling produces the output of each of the four tests here:

c:\misc> C:\Python30\python mergedexc.py
--------------------------------
EXCEPTION RAISED AND CAUGHT
except run
finally run
after run
--------------------------------
NO EXCEPTION RAISED
finally run
after run
--------------------------------
NO EXCEPTION RAISED, WITH ELSE
else run
finally run
after run
--------------------------------
EXCEPTION RAISED BUT NOT CAUGHT
finally run
Traceback (most recent call last):
  File "mergedexc.py", line 36, in <module>
    x = 1 / 0
ZeroDivisionError: int division or modulo by zero

This example uses built-in operations in the main action to trigger exceptions (or not), and it relies on the fact that Python always checks for errors as code is running. The next section shows how to raise exceptions manually instead.