python之异常捕获(try...except)

本文简单介绍了 python 中的异常捕获。

Python try…except 异常处理模块
 Python 异常处理 try...except、raise
python中的try/except/else/finally语句

PEP#

doc 网站搜索 except：

PEP 341 -- Unifying try-except and try-finally

Abstract#

try:
    <do something>
except Exception:
    <handle the error>
finally:
    <cleanup>

Rationale/Proposal#

try:
    <suite 1>
except Ex1:
    <suite 2>
<more except: clauses>
else:
    <suite 3>
finally:
    <suite 4>

Reference#

控制台执行 help('except')：

Before an except clause's suite is executed, details about the
exception are stored in the "sys" module and can be accessed via
"sys.exc_info()". "sys.exc_info()" returns a 3-tuple consisting of the
exception class, the exception instance and a traceback object (see
section The standard type hierarchy) identifying the point in the
program where the exception occurred.  "sys.exc_info()" values are
restored to their previous values (before the call) when returning
from a function that handled an exception.

Related help topics: EXCEPTIONS

控制台执行 help('EXCEPTIONS')：

Exceptions are a means of breaking out of the normal flow of control
of a code block in order to handle errors or other exceptional
conditions.  An exception is *raised* at the point where the error is
detected; it may be *handled* by the surrounding code block or by any
code block that directly or indirectly invoked the code block where
the error occurred.

Related help topics: try, except, finally, raise

或在 The Python Language Reference 搜索 Exceptions。

Library#

The Python Standard Library 搜索 Exceptions

Tutorial#

在 The Python Tutorial 搜索 Exceptions

demos#

try:
    mbr_log_file = open('nosuchfile.md')
except Exception as exc:
    print(exc)
except:
    print('A exception flew by!')
    raise
finally:
    # sys.exit()
    print('finally')

当前工作目录下不存在指定文件时，执行结果如下：

[Errno 2] No such file or directory: 'nosuchfile.md'
finally

第一个 except 语句捕获通用的 Exception 并打印出来。
第二个 except 语句没有指定具体的异常对象类型，按默认异常处理。

从 BaseException 的继承体系可知，可以用扩号指定其他三类异常 tuple，并统称别名为 exc：

except (SystemExit, KeyboardInterrupt, GeneratorExit) as exc:

第3个 except 也可以换成 else 声明，打印一句 A exception flew by! 后，调用 raise 抛回给系统执行默认处理。

exc_info#

如果 except 后面不指定具体的异常类型，可以打印 sys.exc_info() 三元组（3-tuple）获取相关异常信息：

import sys

try:
    mbr_log_file = open('nosuchfile.md')
except:
    print(sys.exc_info()[0]) # exception class
    print(sys.exc_info()[1]) # exception instance
    # print(sys.exc_info()[2]) # traceback object
finally:
    # sys.exit()
    print('finally')

执行结果：

<class 'FileNotFoundError'>
[Errno 2] No such file or directory: 'nosuchfile.md'
finally

UnicodeDecodeError #

As for guessing the encoding, then you can just loop through the encoding from most to least specific:

def decode(s):
    for encoding in "utf-8-sig", "utf-16":
        try:
            return s.decode(encoding)
        except UnicodeDecodeError:
            continue
    return s.decode("ISO-8859-1") # fallback, will always work

An UTF-16 encoded file wont decode as UTF-8, so we try with UTF-8 first. If that fails, then we try with UTF-16.
Finally, we use ISO-8859-1 — this will always work since all 256 bytes are legal values in ISO-8859-1.