mirror of
https://github.com/apachecn/ailearning.git
synced 2026-02-08 21:04:33 +08:00
521 lines
10 KiB
Markdown
521 lines
10 KiB
Markdown
# 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：
|
||
|
||
```py
|
||
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]:
|
||
|
||
```py
|
||
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)
|
||
|
||
```
|
||
|
||
```py
|
||
> -1
|
||
|
||
```
|
||
|
||
```py
|
||
---------------------------------------------------------------------------
|
||
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`：
|
||
|
||
```py
|
||
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：
|
||
|
||
In [2]:
|
||
|
||
```py
|
||
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"
|
||
|
||
```
|
||
|
||
```py
|
||
> -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
|
||
|
||
```py
|
||
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]:
|
||
|
||
```py
|
||
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"
|
||
|
||
```
|
||
|
||
```py
|
||
> 1
|
||
|
||
```
|
||
|
||
```py
|
||
---------------------------------------------------------------------------
|
||
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
|
||
|
||
将`except` 的值改成 `Exception` 类，来捕获所有的异常。
|
||
|
||
In [4]:
|
||
|
||
```py
|
||
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"
|
||
|
||
```
|
||
|
||
```py
|
||
> 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]:
|
||
|
||
```py
|
||
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"
|
||
|
||
```
|
||
|
||
```py
|
||
> 1
|
||
invalid value
|
||
> -1
|
||
invalid value
|
||
> 0
|
||
invalid value
|
||
> q
|
||
|
||
```
|
||
|
||
或者另加处理：
|
||
|
||
In [6]:
|
||
|
||
```py
|
||
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"
|
||
|
||
```
|
||
|
||
```py
|
||
> 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]:
|
||
|
||
```py
|
||
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"
|
||
|
||
```
|
||
|
||
```py
|
||
> 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]:
|
||
|
||
```py
|
||
float('a')
|
||
|
||
```
|
||
|
||
```py
|
||
---------------------------------------------------------------------------
|
||
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]:
|
||
|
||
```py
|
||
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
|
||
|
||
```
|
||
|
||
```py
|
||
> 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
|
||
|
||
```py
|
||
ValueError: could not convert string to float: a
|
||
```
|
||
|
||
对应的 `message` 是
|
||
|
||
```py
|
||
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：
|
||
|
||
```py
|
||
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]:
|
||
|
||
```py
|
||
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]:
|
||
|
||
```py
|
||
valid_commands = {'start', 'stop', 'pause'}
|
||
|
||
while True:
|
||
command = raw_input('> ')
|
||
if command.lower() not in valid_commands:
|
||
raise CommandError('Invalid commmand: %s' % command)
|
||
|
||
```
|
||
|
||
```py
|
||
> bad command
|
||
|
||
```
|
||
|
||
```py
|
||
---------------------------------------------------------------------------
|
||
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：
|
||
|
||
```py
|
||
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]:
|
||
|
||
```py
|
||
try:
|
||
print 1
|
||
finally:
|
||
print 'finally was called.'
|
||
|
||
```
|
||
|
||
```py
|
||
1
|
||
finally was called.
|
||
|
||
```
|
||
|
||
在抛出异常前执行：
|
||
|
||
In [13]:
|
||
|
||
```py
|
||
try:
|
||
print 1 / 0
|
||
finally:
|
||
print 'finally was called.'
|
||
|
||
```
|
||
|
||
```py
|
||
finally was called.
|
||
|
||
```
|
||
|
||
```py
|
||
---------------------------------------------------------------------------
|
||
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]:
|
||
|
||
```py
|
||
try:
|
||
print 1 / 0
|
||
except ZeroDivisionError:
|
||
print 'divide by 0.'
|
||
finally:
|
||
print 'finally was called.'
|
||
|
||
```
|
||
|
||
```py
|
||
divide by 0.
|
||
finally was called.
|
||
|
||
``` |