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写代码的时候,出现错误必不可免,即使代码没有问题,也可能遇到别的问题。
看下面这段代码:
| import math |
| |
| while True: |
| text = raw_input('> ') |
| if text[0] == 'q': |
| break |
| x = float(text) |
| y = math.log10(x) |
| print "log10({0}) = {1}".format(x, y) |
这段代码接收命令行的输入,当输入为数字时,计算它的对数并输出,直到输入值为 q
为止。
乍看没什么问题,然而当我们输入 0 或者负数时:
| import math |
| |
| while True: |
| text = raw_input('> ') |
| if text[0] == 'q': |
| break |
| x = float(text) |
| y = math.log10(x) |
| print "log10({0}) = {1}".format(x, y) |
| > -1 |
| |
| |
| |
| ValueError Traceback (most recent call last) |
| |
| in () |
| 6 break |
| 7 x = float(text) |
| |
| 9 print "log10({0}) = {1}".format(x, y) |
| |
| ValueError: math domain error |
log10
函数会报错,因为不能接受非正值。
一旦报错,程序就会停止执行,如果不希望程序停止执行,那么我们可以添加一对 try & except
:
| import math |
| |
| while True: |
| try: |
| text = raw_input('> ') |
| if text[0] == 'q': |
| break |
| x = float(text) |
| y = math.log10(x) |
| print "log10({0}) = {1}".format(x, y) |
| except ValueError: |
| print "the value must be greater than 0" |
一旦 try
块中的内容出现了异常,那么 try
块后面的内容会被忽略,Python 会寻找 except
里面有没有对应的内容,如果找到,就执行对应的块,没有则抛出这个异常。
在上面的例子中,try
抛出的是 ValueError
,except
中有对应的内容,所以这个异常被 except
捕捉到,程序可以继续执行:
| import math |
| |
| while True: |
| try: |
| text = raw_input('> ') |
| if text[0] == 'q': |
| break |
| x = float(text) |
| y = math.log10(x) |
| print "log10({0}) = {1}".format(x, y) |
| except ValueError: |
| print "the value must be greater than 0" |
| > -1 |
| the value must be greater than 0 |
| > 0 |
| the value must be greater than 0 |
| > 1 |
| log10(1.0) = 0.0 |
| > q |
| import math |
| |
| while True: |
| try: |
| text = raw_input('> ') |
| if text[0] == 'q': |
| break |
| x = float(text) |
| y = 1 / math.log10(x) |
| print "log10({0}) = {1}".format(x, y) |
| except ValueError: |
| print "the value must be greater than 0" |
假设我们将这里的 y
更改为 1 / math.log10(x)
,此时输入 1
:
| import math |
| |
| while True: |
| try: |
| text = raw_input('> ') |
| if text[0] == 'q': |
| break |
| x = float(text) |
| y = 1 / math.log10(x) |
| print "log10({0}) = {1}".format(x, y) |
| except ValueError: |
| print "the value must be greater than 0" |
| > 1 |
| |
| --------------------------------------------------------------------------- |
| |
| ZeroDivisionError Traceback (most recent call last) |
| |
| in () |
| 7 break |
| 8 x = float(text) |
| ----> 9 y = 1 / math.log10(x) |
| 10 print "log10({0}) = {1}".format(x, y) |
| 11 except ValueError: |
| |
| ZeroDivisionError: float division by zero |
因为我们的 except
里面并没有 ZeroDivisionError
,所以会抛出这个异常,我们可以通过两种方式解决这个问题:
将 except
的值改成 Exception
类,来捕获所有的异常。
| import math |
| |
| while True: |
| try: |
| text = raw_input('> ') |
| if text[0] == 'q': |
| break |
| x = float(text) |
| y = 1 / math.log10(x) |
| print "1 / log10({0}) = {1}".format(x, y) |
| except Exception: |
| print "invalid value" |
| > 1 |
| invalid value |
| > 0 |
| invalid value |
| > -1 |
| invalid value |
| > 2 |
| 1 / log10(2.0) = 3.32192809489 |
| > q |
这里,我们把 ZeroDivisionError
加入 except
。
| import math |
| |
| while True: |
| try: |
| text = raw_input('> ') |
| if text[0] == 'q': |
| break |
| x = float(text) |
| y = 1 / math.log10(x) |
| print "1 / log10({0}) = {1}".format(x, y) |
| except (ValueError, ZeroDivisionError): |
| print "invalid value" |
| > 1 |
| invalid value |
| > -1 |
| invalid value |
| > 0 |
| invalid value |
| > q |
或者另加处理:
| import math |
| |
| while True: |
| try: |
| text = raw_input('> ') |
| if text[0] == 'q': |
| break |
| x = float(text) |
| y = 1 / math.log10(x) |
| print "1 / log10({0}) = {1}".format(x, y) |
| except ValueError: |
| print "the value must be greater than 0" |
| except ZeroDivisionError: |
| print "the value must not be 1" |
| > 1 |
| the value must not be 1 |
| > -1 |
| the value must be greater than 0 |
| > 0 |
| the value must be greater than 0 |
| > 2 |
| 1 / log10(2.0) = 3.32192809489 |
| > q |
事实上,我们还可以将这两种方式结合起来,用 Exception
来捕捉其他的错误:
| import math |
| |
| while True: |
| try: |
| text = raw_input('> ') |
| if text[0] == 'q': |
| break |
| x = float(text) |
| y = 1 / math.log10(x) |
| print "1 / log10({0}) = {1}".format(x, y) |
| except ValueError: |
| print "the value must be greater than 0" |
| except ZeroDivisionError: |
| print "the value must not be 1" |
| except Exception: |
| print "unexpected error" |
| > 1 |
| the value must not be 1 |
| > -1 |
| the value must be greater than 0 |
| > 0 |
| the value must be greater than 0 |
| > q |
在上面的例子中,当我们输入不能转换为浮点数的字符串时,它输出的是 the value must be greater than 0
,这并没有反映出实际情况。
| |
| |
| ValueError Traceback (most recent call last) |
| |
| in () |
| |
| |
| ValueError: could not convert string to float: a |
为了得到异常的具体信息,我们将这个 ValueError
具现化:
| import math |
| |
| while True: |
| try: |
| text = raw_input('> ') |
| if text[0] == 'q': |
| break |
| x = float(text) |
| y = 1 / math.log10(x) |
| print "1 / log10({0}) = {1}".format(x, y) |
| except ValueError as exc: |
| if exc.message == "math domain error": |
| print "the value must be greater than 0" |
| else: |
| print "could not convert '%s' to float" % text |
| except ZeroDivisionError: |
| print "the value must not be 1" |
| except Exception as exc: |
| print "unexpected error:", exc.message |
| > 1 |
| the value must not be 1 |
| > -1 |
| the value must be greater than 0 |
| > aa |
| could not convert 'aa' to float |
| > q |
同时,我们也将捕获的其他异常的信息显示出来。
这里,exc.message
显示的内容是异常对应的说明,例如
| ValueError: could not convert string to float: a |
对应的 message
是
| could not convert string to float: a |
当我们使用 except Exception
时,会捕获所有的 Exception
和它派生出来的子类,但不是所有的异常都是从 Exception
类派生出来的,可能会出现一些不能捕获的情况,因此,更加一般的做法是使用这样的形式:
这样不指定异常的类型会捕获所有的异常,但是这样的形式并不推荐。
异常是标准库中的类,这意味着我们可以自定义异常类:
| class CommandError(ValueError): |
| pass |
这里我们定义了一个继承自 ValueError
的异常类,异常类一般接收一个字符串作为输入,并把这个字符串当作异常信息,例如:
| valid_commands = {'start', 'stop', 'pause'} |
| |
| while True: |
| command = raw_input('> ') |
| if command.lower() not in valid_commands: |
| raise CommandError('Invalid commmand: %s' % command) |
| > bad command |
| |
| --------------------------------------------------------------------------- |
| |
| CommandError Traceback (most recent call last) |
| |
| in () |
| 4 command = raw_input('> ') |
| 5 if command.lower() not in valid_commands: |
| ----> 6 raise CommandError('Invalid commmand: %s' % command) |
| |
| CommandError: Invalid commmand: bad command |
我们使用 raise
关键词来抛出异常。
我们可以使用 try/except
块来捕捉这个异常:
| valid_commands = {'start', 'stop', 'pause'} |
| |
| while True: |
| command = raw_input('> ') |
| try: |
| if command.lower() not in valid_commands: |
| raise CommandError('Invalid commmand: %s' % command) |
| except CommandError: |
| print 'Bad command string: "%s"' % command |
由于 CommandError
继承自 ValueError
,我们也可以使用 except ValueError
来捕获这个异常。
try/catch 块还有一个可选的关键词 finally。
不管 try 块有没有异常, finally 块的内容总是会被执行,而且会在抛出异常前执行,因此可以用来作为安全保证,比如确保打开的文件被关闭。。
| try: |
| print 1 |
| finally: |
| print 'finally was called.' |
在抛出异常前执行:
| try: |
| print 1 / 0 |
| finally: |
| print 'finally was called.' |
| finally was called. |
| |
| |
| |
| ZeroDivisionError Traceback (most recent call last) |
| |
| in () |
| 1 try: |
| |
| 3 finally: |
| 4 print 'finally was called.' |
| |
| ZeroDivisionError: integer division or modulo by zero |
如果异常被捕获了,在最后执行:
| try: |
| print 1 / 0 |
| except ZeroDivisionError: |
| print 'divide by 0.' |
| finally: |
| print 'finally was called.' |
| divide by 0. |
| finally was called. |