<p>看一下打开模式(python使用与C<code>fopen</code>)相同的模式<a href="http://www.manpagez.com/man/3/fopen/" rel="nofollow noreferrer">http://www.manpagez.com/man/3/fopen/</a></p>
<pre><code> ``r'' Open text file for reading. The stream is positioned at the
beginning of the file.
``r+'' Open for reading and writing. The stream is positioned at the
beginning of the file.
``w'' Truncate to zero length or create text file for writing. The
stream is positioned at the beginning of the file.
``w+'' Open for reading and writing. The file is created if it does not
exist, otherwise it is truncated. The stream is positioned at
the beginning of the file.
``a'' Open for writing. The file is created if it does not exist. The
stream is positioned at the end of the file. Subsequent writes
to the file will always end up at the then current end of file,
irrespective of any intervening fseek(3) or similar.
``a+'' Open for reading and writing. The file is created if it does not
exist. The stream is positioned at the end of the file. Subse-
quent writes to the file will always end up at the then current
end of file, irrespective of any intervening fseek(3) or similar.
</code></pre>
<p>在<code>'a+'</code>模式的描述中,可以清楚地看到流位于文件的末尾。所以在这一点上,如果你继续从当前位置(文件末尾)读取它,那么你的输出也会随之改变。在</p>
<p>要在这种情况下获得正确的输出,可以使用<code>file.seek()</code>函数,如下所示:</p>
^{pr2}$