python - 文件读/写意外行为

Question

当一些奇怪的事情发生时，我正在用 python 读写文本文件。当我跑的时候

f = open('test.txt', 'w+')
f.write('hello')
print f.read()

输出不是预期的hello，而是:

hellodف('test.txt', 'w+')
f.write('hello')
print f.read()
)

 i   LOGNAMEt   USERt   LNAMEt   USERNAMEi    (   R9   R:   R;   R<   (   R   t   environt   gett   pwdt   getpwuidt   getuid(   R   t   namet   userR?   (    (    s   E:\Python\lib\getpass.pyR   ̀  s    
(   t   AskPassword(   t   __doc__R   R   R3   t   __all__t   UserWarningR   R   R(   R1   R   R   R   R   R   R   t   ImportErrorR   R+   t   EasyDialogsRD   R    (    (    (    s   E:\Python\lib\getpass.pyt   <module>   s,   $?       




sued when getpass() cannot prevent
                 echoing of the password contents while reading.

On Windows, the msvcrt module will be used.
On the Mac EasyDialogs.AskPassword is used, if available.

i   getpasst   getusert   GetPassWarningc           B   s   e  Z RS(    (   t   __name__t
   __module__(    (    (    s   E:\Python\lib\getpass.pyR      s    s
   Password: c   
      C   ś  d } d } yJ t j d t j t j B } t j | d d à } | } | sU | } n  Wnj t k
 r} y t j j  À } Wn& t
 t f k
 rÀt |  | Â } n Xt j } | sÀt j
 } qÀn X| d k    rہd } yÀt j | Á } | } | d c t j M<t j }     t t d  r*|     t j O}  n  z, t j | |   | à t |  | d | Â} Wd t j | |    | à | j À XWqہt j k
 rׁ} | d k   r߁ n  ~ ~ t |  | Â } qہXn  | j d Á | S(       s  Prompt for a password, with echo turned off.

    Args:
      prompt: Written on stream to ask for the input.  Default: 'Password: '
      stream: A writable file object to display the prompt.  Defaults to
              the tty.  If no tty is available defaults to sys.stderr.
    Returns:
      The seKr3t input.
    Raises:
      EOFError: If our input tty or stdin was closed.
      GetPassWarning: When we were unable to turn echo off on the input.

    Always restores terminal settings before returning.
    s   /dev/ttys   w+i   i   t   TCSASOFTt   inputNs   
(   t   Nonet   ost   opent   O_RDWRt   O_NOCTTYt   fdopent   EnvironmentErrort   syst   stdint   filenot   AttributeErrort
   ValueErrort   fallback_getpasst   stderrt   termiost    tcgetattrt   ECHOt      TCSAFLUSHt   hasattrR   t       tcsetattrt
   _raw_inputt   flusht   errort   write(
   t   promptt   streamt   fdt   ttyR   t   et   passwdt   oldt   newt   tcsetattr_flags(    (    s   E:\Python\lib\getpass.pyt   unix_getpass   sJ    

c         C   sӀ  t  j t  j k    r t |  | Â Sd d l } x |  D] } | j | Á q2 Wd } xc | j À } | d k sv | d k rz Pn  | d k rπt  n  | d k rȀ| d  } qR | | } qR | j d Á | j d Á | S(   s9   Prompt for password with echo off, using Windows getch().i    s   
s   
s   s   (   R   R   t      __stdin__R   t   msvcrtt   putcht   getcht   KeyboardInterrupt(   R   R    R+   t   ct   pw(    (    s   E:\Python\lib\getpass.pyt   win_getpassY   s$    





c         C   s?   t  j d t d d Â| s( t j } n  | d IJt |  | Â S(   Ns%   Can not control echo on the terminal.t
   stackleveli   s&   Warning: Password input may be echoed.(   t   warningst   warnR   R   R   R   (   R   R    (    (    s   E:\Python\lib\getpass.pyR   p   s    
R)   c         C   s̀  | s t  j } n  | s$ t  j } n  t |  Á }  |  rP | j |  Á | j À n  | j À } | sk t  n  | d d k rȀ| d  } n  | S(   Niﳁ   
(   R   R   R   t   strR   R   t   readlinet   EOFError(   R   R    R   t   line(    (    s   E:\Python\lib\getpass.pyR   y   s    


c          C   s\   d d l  }  x* d D]" } |  j j | Á } | r | Sq Wd d l } | j |  j À Á d S(      s  Get the username from the environment or password database.

    First try various environment variables, then the password
    dat

我知道我应该做

with open('test.txt', 'w') as f:
    f.write('hello')

with open('test.txt', 'r') as f:
    print f.read()

这确实给出了预期的 hello，但为什么在第一种情况下没有发生这种情况，getpass.py 模块在那里做什么？

我正在使用 python 2.7.3 运行 Windows 7

Answer 1

“hello”还没有被刷新到磁盘(它仍然在你的计算机内存中)所以 .read() 可能正在读取刚刚分配的磁盘 block 上的内容到文件，或为文件分配的内存缓冲区。这种行为很奇怪，但可以通过刷新文件轻松避免。

f.flush()

您还需要告诉文件回到开头，以便您可以从中读取“hello”。 (否则，当您开始阅读时，您已经到了文件的末尾。)

f.seek(0)