Python open+append 没有按预期工作

Question

根据文档，如果我使用 open("file","a") 并将新数据写入文件，则会附加新数据，但在下面的示例中，第二个命令只是覆盖文件。我不太明白为什么。

import subprocess

startupinfo = subprocess.STARTUPINFO()
subprocess.STARTF_USESHOWWINDOW = 1
startupinfo.dwFlags = subprocess.STARTF_USESHOWWINDOW

with open(r"c:\folder\test.txt","a") as log:
    Process = subprocess.Popen(['dir'],
                               stdout = log, stderr = log,
                               startupinfo = startupinfo,
                               shell=True)

with open(r"c:\folder\test.txt","a") as log:
    Process = subprocess.Popen(['dir'],
                               stdout = log, stderr = log,
                               startupinfo = startupinfo,
                               shell=True)

我已经尝试过模式“a+b”，但我得到了相同的最终结果。

Answer 1

在 subprocess 中，文件位置没有增加。 log.tell() 在第二个 with 语句中返回 0。您可以将 log 的位置增加到文件末尾。对于第一个 Process，wait() 似乎很好。以下对我有用:

import subprocess
from os import linesep, stat 

with open(r"test.txt","a") as log:
    Process = subprocess.Popen(['dir'],
                               stdout = log, stderr = log,
                               shell=True)
    Process.wait()

with open(r"test.txt","a") as log:
# this prints 0
    print log.tell()
# get the length of the file log
    pos = stat(r"test.txt").st_size
    print pos
# go to the end of log
    log.seek(pos)
    Process = subprocess.Popen(['dir'],
                               stdout = log, stderr = log,
                               shell=True)