《head first python》——文件与异常

1. 查看当前路径，转到工作目录

>> import os
>>> os.getcwd()
'C:Python27'
>>> os.chdir("F:/data")
>>> os.getcwd()
'F:data'

>> str = "hello:my name is: Anna"
>>> (say,words,name) = str.split(":")
>>> words
'my name is'

>> (words,name) = str.split(":")
Traceback (most recent call last):
  File "",line 1,in 
    (words,name) = str.split(":")
ValueError: too many values to unpack
>>> (words,name) = str.split(":",1)
>>> name
'my name is: Anna'

3. 异常捕获机制，try-except机制可以让你专注于代码真正需要做的工作，避免不必要的代码逻辑

程序运行过程中出现异常可能导致程序崩溃，通过try/except允许在异常发生时捕获异常，如果发现有问题，会执行你预先设定的恢复代码，然后继续执行程序。也就是在不崩溃的情况下系统地处理错误和异常。

首先是要找出容易出错、要保护的代码。

>> try:
    data = fd
    try:
        data1 = fd1
    except:
        pass
    fd.close
except:
    print "error"
error

<code class="language-python">except ValueError:

<code class="language-python">except IOError:

考虑文件关闭有两种方式：一、使用try/except/finally机制。

试图打开一个不存在的文件，会发生崩溃，可以用except IOError处理异常，fianlly扩展try：里面放着无论任何情况都必须运行的代码，通常是文件关闭。

>> try:
    data = open("missing.txt")
    print(data.readline(),end='')
except IOError:
    print("File Error")
finally:
    data.close()
File Error

Traceback (most recent call last):

File "<pyshell#45>",line 7,in 

data.close()

NameError: name 'data' is not defined

>> try:
    data = open("missing.txt")
    print(data.readline(),end='')
except IOError:
    print("File Error")
finally:
    if 'data' in locals():
        data.close()
File Error

给异常起名字

>> try:
    data = open("missing.txt")
    print(data.readline(),end='')
except IOError as err:
    print("File Error"+err)
finally:
    if 'data' in locals():
        data.close()
Traceback (most recent call last):

File "<pyshell#49>",line 2,in 

data = open("missing.txt")

FileNotFoundError: [Errno 2] No such file or directory: 'missing.txt'
During handling of the above exception,another exception occurred:
Traceback (most recent call last):

File "<pyshell#49>",line 5,in 

print("File Error"+err)

TypeError: Can't convert 'FileNotFoundError' object to str implicitly

<code class="language-python">>>> try:

data = open("missing.txt")

print(data.readline(),end='')

except IOError as err:

print("File Error"+str(err))

finally:

if 'data' in locals():

data.close()

File Error[Errno 2] No such file or directory: 'missing.txt'

<p style="font-size:13.3333339691162px;">二、使用with，python解释器会自动考虑为你关闭文件

>> try:
    with open("missing.txt","r"):
        print(data.readline(),end='')
except IOError as err:
    print("File Error"+str(err))
File Error[Errno 2] No such file or directory: 'missing.txt'

4.pass放过错误

使用split()方法调用导致一个异常，可以报告这是一个错误并使用pass继续执行代码。也就是说python忽略了这个错误。

try/execpt/pass

如果用raise则表示不能放过这个错误，这段代码不能跳过

5.使用pickle腌制python数据，dump()腌制，load()取出

",line 3,in 
    mylist[i] = fb.readline()
IndexError: list assignment index out of range

<code class="language-python">test = 'abc'

if True:

raise Exception(test + 'def')