readline 功能未按预期工作

Question

以下代码作为线程的一部分运行。当文件打开并且我们继续追加文件时，文件大小会正确增加，但该行是空的。行和文件大小都打印在第 5 行。无法理解 io 模块中的问题是什么。

with io.open(file_path, 'r', encoding='utf-8') as file:
  file.seek(0, 2)  # Seek to the end of the file
  while not self._stop_event.is_set():
    line = file.readline()
    print(f"LINE  = {line}  FILE SIZE = {os.path.getsize(file_path)}") #LINE 5
    if not line:
      time.sleep(1)
    else:
      print(f"LINE : {line.strip()}")

我尝试打开该文件并继续追加内容。当我们不断向文件中添加新行时，它会不断增加文件大小。无法知道具体问题出在哪里。

Answer 1

您的代码不显示您如何附加到文件。

请注意，打开文件进行写入（在文本模式下）将被缓冲。因此，在写入每一行之后，刷新缓冲区非常重要。如果写入的数据包含回车符或换行符，则要么以行缓冲模式打开。

这是对您似乎正在尝试执行的操作的模拟，效果符合预期：

from threading import Thread
from time import sleep
from sys import stdout

RUN = True
FILENAME = "foo.txt"

def tail():
    with open(FILENAME) as fd:
        fd.seek(0, 2)
        while RUN:
            if line := fd.readline():
                # stdout is line-buffered
                # therefore, as our data has a newline character at its end, there's no need to flush
                stdout.write(line)
            sleep(0.25)

if __name__ == "__main__":
    (thread := Thread(target=tail)).start()
    # By opening the file in line-buffered mode, there's no need to flush
    # providing the output contains either a carriage return or newline.
    # If the file is opened in text mode without buffering=1 then it will
    # be necessary to flush after each write
    with open(FILENAME, "a", 1) as fd:
        for i in range(10):
            fd.write(f"Line {i}\n")
            sleep(1)

    RUN = False
    thread.join()

输出：

Line 0
Line 1
Line 2
Line 3
Line 4
Line 5
Line 6
Line 7
Line 8
Line 9

注：

每条输出线之间会有~1s的间隔