????? 7. ???????? ????? Python ??? ??? 9. ??

????

• 8.1 ??????

• 8.2 ??

• 8.3 ??????

• 8.4 ?????

• 8.5 ??????????

• 8.6 ???????????

 
8. ???????

??????????н???????????????????????????????????????Щ?????У???????????????Щ??Python?У?????????????????????????? syntax errors and exceptions????

 
8.1 ??????

???????????????????????????Python?????????????????

>>> while True print 'Hello world'
  File "<stdin>", line 1, in ?
    while True print 'Hello world'
                   ^
SyntaxError: invalid syntax

?????????????????У????????????緢??????λ??????????С??????????????????????????????????λ?á?????е?????????????print???????????????????e???:????????????????????к?????????????????????????????????λ?á?

 
8.2 ??

??????????????????????????????????????п????????????????????м??????????????????????????????????????????????????Python?????п??????????????????????????????????????????????

>>> 10 * (1/0)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ZeroDivisionError: integer division or modulo by zero
>>> 4 + spam*3
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
NameError: name 'spam' is not defined
>>> '2' + 2
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: cannot concatenate 'str' and 'int' objects

??????????????????????????????????в?????????????????????????????????????????????е?????????????? ZeroDivisionError ???????????? NameError??????????? TypeError ??????????????????????????????????????????????????е???????????????????????????????????????????????????????????????????????????????????б??????????

????к???????????????????????????????ζ???????????????????????

????????????????????????г?????????λ?á???????????г?????????У???????????????????????????????

Python ??ο???? ?г??????????????????塣

 
8.3 ??????

????????????????????????μ????????????????????????????????????????????????????????????????????????ж??????? Control-C ????????????????????????????????????????????ж????????? KeyboardInterrupt ????

>>> while True:
...     try:
...         x = int(raw_input("Please enter a number: "))
...         break
...     except ValueError:
...         print "Oops! That was no valid number.  Try again..."
...

try ??????·????????

• ???????? try ???????try ?? except???????????????

• ??????????????except ??? ??try???????????????????

• ?????try?????й????з??????????????????????????????????????????except????????????????????????ж????except???????try??????????????????????try??????????

• ??????????????????except????????????????????????????????try????С???????????????????????????????????δ??????????????????У????????????

???try ???????????? except ????????????????????????????????????????С? ??????????????????try????з???????????????try????У?????????з???????????????????except???????????????г????????????????磺

... except (RuntimeError, TypeError, NameError):
...     pass

??????except?????????????????????????????????á????????????????????????????????ε???????????????????????????????????????д??????????????????????????????????????????????

import string, sys

try:
    f = open('myfile.txt')
    s = f.readline()
    i = int(string.strip(s))
except IOError, (errno, strerror):
    print "I/O error(%s): %s" % (errno, strerror)
except ValueError:
    print "Could not convert data to an integer."
except:
    print "Unexpected error:", sys.exc_info()[0]
    raise

try ... except ???????????? else ????? ?????????????????except????????try?????????????????????Щ????????????????????磺

for arg in sys.argv[1:]:
    try:
        f = open(arg, 'r')
    except IOError:
        print 'cannot open', arg
    else:
        print arg, 'has', len(f.readlines()), 'lines'
        f.close()

??? else ?????? try ????и???????????????????????? try ... except ??????????????????????????Щ?????????????

???????????????????????????????????????????????????????????????????????????????

?????????б??????????? except ???????????????????????????????????????洢?? instance.args ??????С??????????????????????? __getitem__ ?? __str__ ??????????????????????????????????? .args??

>>> try:
...    raise Exception('spam', 'eggs')
... except Exception, inst:
...    print type(inst)     # the exception instance
...    print inst.args      # arguments stored in .args
...    print inst           # __str__ allows args to printed directly
...    x, y = inst          # __getitem__ allows args to be unpacked directly
...    print 'x =', x
...    print 'y =', y
...
<type 'instance'>
('spam', 'eggs')
('spam', 'eggs')
x = spam
y = eggs

????δ?????????????????????????????????????????????????????“???”???????????

??????????????????????????try????е?????????????У?????????????????????????????????????????????磺

>>> def this_fails():
...     x = 1/0
... 
>>> try:
...     this_fails()
... except ZeroDivisionError, detail:
...     print 'Handling run-time error:', detail
... 
Handling run-time error: integer division or modulo

 
8.4 ?????

???????????????????????????? raise ????????????????磺

>>> raise NameError, 'HiThere'
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
NameError: HiThere

???????????????????????????????????????????????

????????????????????????????raise ???????????????????????????

>>> try:
...     raise NameError, 'HiThere'
... except NameError:
...     print 'An exception flew by!'
...     raise
...
An exception flew by!
Traceback (most recent call last):
  File "<stdin>", line 2, in ?
NameError: HiThere

 
8.5 ??????????

??????п???????????μ???????????????????????????????????????? Exception ???????????磺

>>> class MyError(Exception):
...     def __init__(self, value):
...         self.value = value
...     def __str__(self):
...         return repr(self.value)
... 
>>> try:
...     raise MyError(2*2)
... except MyError, e:
...     print 'My exception occurred, value:', e.value
... 
My exception occurred, value: 4
>>> raise MyError, 'oops!'
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
__main__.MyError: 'oops!'

?????п???????κ????????п??????????????????????????????????м?????????????????????????????????????′?????????????????????????????????????????????????鶨?????????????????????????????????????????????

class Error(Exception):
    """Base class for exceptions in this module."""
    pass

class InputError(Error):
    """Exception raised for errors in the input.

    Attributes:
        expression -- input expression in which the error occurred
        message -- explanation of the error
    """

    def __init__(self, expression, message):
        self.expression = expression
        self.message = message

class TransitionError(Error):
    """Raised when an operation attempts a state transition that's not
    allowed.

    Attributes:
        previous -- state at beginning of transition
        next -- attempted new state
        message -- explanation of why the specific transition is not allowed
    """

    def __init__(self, previous, next, message):
        self.previous = previous
        self.next = next
        self.message = message

????????????????????????????“Error”??β??

?????????ж????????????????????????????????????????п???????????????????????????μ??? 9?￡?“??”??

 
8.6 ???????????

try ??仹????????????????????????????κ?????????????е????????磺

>>> try:
...     raise KeyboardInterrupt
... finally:
...     print 'Goodbye, world!'
... 
Goodbye, world!
Traceback (most recent call last):
  File "<stdin>", line 2, in ?
KeyboardInterrupt

????try?????????з??????? finally ????????????С??????????????finally?????????????????????? try ??侭?? break ?? return ????????????finally ???

??finally ????е????????????????????????????????????????????Щ???????????????á?

?? try ????п?????????? except ??????? finally ??????????????檔

????? 7. ???????? ????? Python ??? ??? 9. ??

Release 2.3, documentation updated on July 29, 2003.