Python 2 异常处理 (try 和 except)

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Python 2 异常处理 [try 和 except]

1. Example 1
2. Example 2
3. Example 3
References

异常处理使程序能检测错误，处理它们，然后继续运行。当试图用一个数除以零时，就会发生 ZeroDivisionError。

1. Example 1

#!/usr/bin/env python
# -*- coding: utf-8 -*-from __future__ import absolute_import
from __future__ import print_function
from __future__ import divisionimport os
import sys
import numpy as np
import tensorflow as tfsys.path.append(os.path.dirname(os.path.abspath(__file__)))
current_directory = os.path.dirname(os.path.abspath(__file__))print(16 * "++--")
print("current_directory:", current_directory)
print(16 * "++--")def forever(divisor):return 42 / divisorprint(forever(2))
print(forever(12))
print(forever(0))
print(forever(1))

/usr/bin/python2.7 /home/strong/tensorflow_work/R2CNN_Faster-RCNN_Tensorflow/yongqiang.py
++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--
current_directory: /home/strong/tensorflow_work/R2CNN_Faster-RCNN_Tensorflow
++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--
21.0
3.5
Traceback (most recent call last):File "/home/strong/tensorflow_work/R2CNN_Faster-RCNN_Tensorflow/yongqiang.py", line 27, in <module>print(forever(0))File "/home/strong/tensorflow_work/R2CNN_Faster-RCNN_Tensorflow/yongqiang.py", line 22, in foreverreturn 42 / divisor
ZeroDivisionError: division by zeroProcess finished with exit code 1

根据错误信息中给出的行号，我们知道 forever() 中的 return 语句导致了一个错误。错误可以由 try 和 except 语句来处理。那些可能出错的语句被放在 try 子句中。如果错误发生，程序执行就转到接下来的 except 子句开始处。可以将前面除数为零的代码放在一个 try 子句中，让 except 子句包含代码，来处理该错误发生时应该做的事。

2. Example 2

#!/usr/bin/env python
# -*- coding: utf-8 -*-from __future__ import absolute_import
from __future__ import print_function
from __future__ import divisionimport os
import sys
import numpy as np
import tensorflow as tfsys.path.append(os.path.dirname(os.path.abspath(__file__)))
current_directory = os.path.dirname(os.path.abspath(__file__))print(16 * "++--")
print("current_directory:", current_directory)
print(16 * "++--")def forever(divisor):try:return 42 / divisorexcept ZeroDivisionError:print('Error: Invalid argument.')print(forever(2))
print(forever(12))
print(forever(0))
print(forever(1))

如果在 try 子句中的代码导致一个错误，程序执行就立即转到 except 子句的代码。在运行那些代码之后，执行照常继续。

/usr/bin/python2.7 /home/strong/tensorflow_work/R2CNN_Faster-RCNN_Tensorflow/yongqiang.py
++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--
current_directory: /home/strong/tensorflow_work/R2CNN_Faster-RCNN_Tensorflow
++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--
21.0
3.5
Error: Invalid argument.
None
42.0Process finished with exit code 0

在函数调用中的 try 语句块中，发生的所有错误都会被捕捉。请考虑以下程序，它的做法不一样，将 forever() 调用放在语句块中。

3. Example 3

#!/usr/bin/env python
# -*- coding: utf-8 -*-from __future__ import absolute_import
from __future__ import print_function
from __future__ import divisionimport os
import sys
import numpy as np
import tensorflow as tfsys.path.append(os.path.dirname(os.path.abspath(__file__)))
current_directory = os.path.dirname(os.path.abspath(__file__))print(16 * "++--")
print("current_directory:", current_directory)
print(16 * "++--")def forever(divisor):return 42 / divisortry:print(forever(2))print(forever(12))print(forever(0))print(forever(1))except ZeroDivisionError:print('Error: Invalid argument.')

/usr/bin/python2.7 /home/strong/tensorflow_work/R2CNN_Faster-RCNN_Tensorflow/yongqiang.py
++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--
current_directory: /home/strong/tensorflow_work/R2CNN_Faster-RCNN_Tensorflow
++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--
21.0
3.5
Error: Invalid argument.Process finished with exit code 0

print(forever(1)) 从未被执行是因为，一旦执行跳到 except 子句的代码，就不会回到 try 子句。它会继续照常向下执行。