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异常

try & except 块

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:

In [1]:

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)
<ipython-input-1-ceb8cf66641b> in <module>()
 6         break
 7     x = float(text)
----> 8  y = math.log10(x)
 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 ValueErrorexcept 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:

In [2]:

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

捕捉不同的错误类型

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

In [3]:

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)
<ipython-input-3-7607f1ae6af9> in <module>()
 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:

捕捉所有异常

except 的值改成 Exception 类,来捕获所有的异常。

In [4]:

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

指定特定值

这里,我们把 ZeroDivisionError 加入 except

In [5]:

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:

In [6]:

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:

In [7]:

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

得到异常的具体信息

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。

In [8]:

float('a')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-8-99859da4e72c> in <module>()
----> 1  float('a')

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:

In [9]:

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

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:

try:
    pass
except:
    pass

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。

自定义异常

异常是标准库中的类,这意味着我们可以自定义异常类:

In [10]:

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:

In [11]:

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)
<ipython-input-11-0e1f81a1136d> in <module>()
 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。

finally

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。。

In [12]:

try:
    print 1
finally:
    print 'finally was called.'
1
finally was called.

在抛出异常前执行:

In [13]:

try:
    print 1 / 0
finally:
    print 'finally was called.'
finally was called.
---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-13-87ecdf8b9265> in <module>()
 1 try:
----> 2  print 1 / 0
 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:

In [14]:

try:
    print 1 / 0
except ZeroDivisionError:
    print 'divide by 0.'
finally:
    print 'finally was called.'
divide by 0.
finally was called.