Python用户输入问题

Question

问题

在执行这两个函数时与STDIN发生冲突。什么会导致第二个函数不能正确读取STDIN？如果没有执行第一个函数，则第二个函数在读取输入时没有问题。

要清除STDIN缓冲区，我尝试过：

'sys.stdin.flush'

'tcflush(sys.stdin, TCIOFLUSH)'

Python代码

import sys, select

def stdin_read():

    print "[stdin read] You must answer Y/N and press ENTER"
    sys.stdout.flush()
    response = sys.stdin.read(1)
    print "You said '{}'".format(response)


def stdin_timeout():

    print "[stdin timeout] You must answer YES / NO in 10 seconds and press ENTER"
    sys.stdout.flush()

    sys.stdin.flush()

    i, o, e = select.select( [sys.stdin], [], [], 10 )

    if (i):
        print "You said '{}'".format(sys.stdin.readline().strip())
        exit(0)
    else:
        print "You said nothing!"
        exit(1)


stdin_read()
stdin_timeout()

Python输出两个函数：

:~# python input.py
[stdin read] You must answer Y/N and press ENTER
n
You said 'n'
[stdin timeout] You must answer YES / NO in 10 seconds and press ENTER
no
You said ''
:~# no
-bash: no: command not found

Python输出第二个函数

~# python input.py
[stdin timeout] You must answer YES / NO in 10 seconds and press ENTER
no
You said 'no'

Answer 1

这个问题来自于the behaviour of stdin flushing in unix

我将通过批注您的代码来回答：

import sys, select

def stdin_read():

    print "[stdin read] You must answer Y/N and press ENTER"
    # sys.stdout.flush() # not needed
    # you are reading one byte here, but the user enters at least two bytes
    # (e.g 'y' + LF(enter key)) so the LF byte (and possibly additional bytes)
    # remain in the stdin buffer.
    response = sys.stdin.read(1) # (->LF REMAINS)
    print "You said '{}'".format(response)


def stdin_timeout():

    print "[stdin timeout] You must answer YES / NO in 10 seconds and press ENTER"
    #sys.stdout.flush() #  not needed
    # the behaviour of stdin flushing in unix is not defined (see 
    # link above)
    sys.stdin.flush()
    # thus the LF still remains in the stdin buffer

    # the select call blocks until the file discriptor is 'ready' for
    # the corresponding i/o operation. DISCLAIMER: assumptions are
    # following.... By default python does a open call which uses
    # buffered reads, so even if you did a read(1) it will likely read
    # more data from stdin and the select call blocks until the next LF
    # arrives.
    i, o, e = select.select( [sys.stdin], [], [], 10 )

    if (i):
        # you are just reading the left over bytes from above (->LF REMAINS)
        print "You said '{}'".format(sys.stdin.readline().strip())
        # Proof: readline() again:
        print "You missed '{}'".format(sys.stdin.readline().strip())
        exit(0)
    else:
        print "You said nothing!"
        exit(1)


stdin_read()
stdin_timeout()

但是也许使用另一种方法Keyboard input with timeout in Python会更好