如何确定对 sys.stdin.readline() (或更一般地说，任何基于文件描述符的文件对象上的 readline() )的调用是否会阻塞？

当我在 python 中编写基于行的文本过滤程序时会出现这种情况；
也就是说，程序反复从输入中读取一行文本，可能对其进行转换，然后将其写入输出。

我想实现一个合理的输出缓冲策略。
我的标准是:

#!/usr/bin/python
# cat-n.linebuffered.py
import sys
num_lines_read = 0
while True:
  line = sys.stdin.readline()
  if line == '': break
  num_lines_read += 1
  print("%d: %s" % (num_lines_read, line))
  sys.stdout.flush()

#!/usr/bin/python
# cat-n.defaultbuffered.py
import sys
num_lines_read = 0
while True:
  line = sys.stdin.readline()
  if line == '': break
  num_lines_read += 1
  print("%d: %s" % (num_lines_read, line))

#!/usr/bin/python
num_lines_read = 0
while True:
  if sys_stdin_readline_is_about_to_block():  # <--- How do I implement this??
    sys.stdout.flush()
  line = sys.stdin.readline()
  if line == '': break
  num_lines_read += 1
  print("%d: %s" % (num_lines_read, line))

#!/usr/bin/python
# cat-n.threaded.py
import queue
import sys
import threading
def iter_with_abouttoblock_cb(callable, sentinel, abouttoblock_cb, qsize=100):
  # child will send each item through q to parent.
  q = queue.Queue(qsize)
  def child_fun():
    for item in iter(callable, sentinel):
      q.put(item)
    q.put(sentinel)
  child = threading.Thread(target=child_fun)
  # The child thread normally runs until it sees the sentinel,
  # but we mark it daemon so that it won't prevent the parent
  # from exiting prematurely if it wants.
  child.daemon = True
  child.start()
  while True:
    try:
      item = q.get(block=False)
    except queue.Empty:
      # q is empty; call abouttoblock_cb before blocking
      abouttoblock_cb()
      item = q.get(block=True)
    if item == sentinel:
      break  # do *not* yield sentinel
    yield item
  child.join()

num_lines_read = 0
for line in iter_with_abouttoblock_cb(sys.stdin.readline,
                                      sentinel='',
                                      abouttoblock_cb=sys.stdout.flush):
  num_lines_read += 1
  sys.stdout.write("%d: %s" % (num_lines_read, line))

for which in defaultbuffered linebuffered threaded; do
  for python in python2.7 python3.5; do
    echo "$python cat-n.$which.py:"
      (echo z; echo -n a; sleep 1; echo b; sleep 1; echo -n c; sleep 1; echo d; echo x; echo y; echo z; sleep 1; echo -n e; sleep 1; echo f) | $python cat-n.$which.py | cat
  done
done

python2.7 cat-n.defaultbuffered.py:
[... pauses 5 seconds here. Bad! ...]
1: z
2: ab
3: cd
4: x
5: y
6: z
7: ef
python3.5 cat-n.defaultbuffered.py:
[same]
python2.7 cat-n.linebuffered.py:
1: z
[... pauses 1 second here, as expected ...]
2: ab
[... pauses 2 seconds here, as expected ...]
3: cd
4: x
5: y
6: z
[... pauses 2 seconds here, as expected ...]
6: ef
python3.5 cat-n.linebuffered.py:
[same]
python2.7 cat-n.threaded.py:
[same]
python3.5 cat-n.threaded.py:
[same]

for which in defaultbuffered linebuffered threaded; do
  for python in python2.7 python3.5; do
    echo -n "$python cat-n.$which.py:  "
      timings=$(time (yes 01234567890123456789012345678901234567890123456789012345678901234567890123456789 | head -1000000 | $python cat-n.$which.py >| /tmp/REMOVE_ME) 2>&1)
      echo $timings
  done
done
/bin/rm /tmp/REMOVE_ME

python2.7 cat-n.defaultbuffered.py:  real 0m1.490s user 0m1.191s sys 0m0.386s
python3.5 cat-n.defaultbuffered.py:  real 0m1.633s user 0m1.007s sys 0m0.311s
python2.7 cat-n.linebuffered.py:  real 0m5.248s user 0m2.198s sys 0m2.704s
python3.5 cat-n.linebuffered.py:  real 0m6.462s user 0m3.038s sys 0m3.224s
python2.7 cat-n.threaded.py:  real 0m25.097s user 0m18.392s sys 0m16.483s
python3.5 cat-n.threaded.py:  real 0m12.655s user 0m11.722s sys 0m1.540s

最佳答案

你当然可以用 select : 这就是它的用途，它的性能对于少量的文件描述符来说是好的。您必须自己实现行缓冲/中断，以便您可以检测在缓冲(结果是)部分行后是否有更多可用输入。

你可以自己做所有的缓冲(这是合理的，因为 select 在文件描述符级别操作)，或者你可以设置 stdin非阻塞并使用 file.read()或 BufferedReader.read() (取决于您的 Python 版本)使用任何可用的东西。如果您的输入可能是 Internet 套接字，则无论缓冲如何，您都必须使用非阻塞输入，因为 select 的常见实现可以虚假地指示来自套接字的可读数据。 (在这种情况下，Python 2 版本使用 IOError 引发 EAGAIN ；Python 3 版本返回 None 。)

( os.fdopen 在这里没有帮助，因为它不会为 fcntl 使用创建新的文件描述符。在某些系统上，您可以使用 /dev/stdin 打开 O_NONBLOCK 。)

基于默认(缓冲)的 Python 2 实现 file.read() :

import sys,os,select,fcntl,errno

fcntl.fcntl(sys.stdin.fileno(),fcntl.F_SETFL,os.O_NONBLOCK)

rfs=[sys.stdin.fileno()]
xfs=rfs+[sys.stdout.fileno()]

buf=""
lnum=0
timeout=None
rd=True
while rd:
  rl,_,xl=select.select(rfs,(),xfs,timeout)
  if xl: raise IOError          # "exception" occurred (TCP OOB data?)
  if rl:
    try: rd=sys.stdin.read()    # read whatever we have
    except IOError as e:        # spurious readiness?
      if e.errno!=errno.EAGAIN: raise # die on other errors
    else: buf+=rd
    nl0=0                       # previous newline
    while True:
      nl=buf.find('\n',nl0)
      if nl<0:
        buf=buf[nl0:]           # hold partial line for "processing"
        break
      lnum+=1
      print "%d: %s"%(lnum,buf[nl0:nl])
      timeout=0
      nl0=nl+1
  else:                         # no input yet
    sys.stdout.flush()
    timeout=None

if buf: sys.stdout.write("%d: %s"%(lnum+1,buf)) # write any partial last line

关于python - 你如何判断 sys.stdin.readline() 是否会阻塞？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/52893192/