[Python编程(第4版)].(Programming.Python.4th.Edition).Mark.Lutz.文字版

In fact, we can use obtain both the input and output streams of a spawned program with
this module. Let’s reuse the simple writer and reader scripts we wrote earlier to
demonstrate:

C:\...\PP4E\System\Streams> type writer.py

print("Help! Help! I'm being repressed!")

print(42)

C:\...\PP4E\System\Streams> type reader.py

print('Got this: "%s"' % input())

import sys

data = sys.stdin.readline()[:-1]

print('The meaning of life is', data, int(data) * 2)

Code like the following can both read from and write to the reader script—the pipe
object has two file-like objects available as attached attributes, one connecting to the
input stream, and one to the output (Python 2.X users might recognize these as equiv-
alent to the tuple returned by the now-defunct os.popen2):

>>> pipe = Popen('python reader.py', stdin=PIPE, stdout=PIPE)

>>> pipe.stdin.write(b'Lumberjack\n')

11

>>> pipe.stdin.write(b'12\n')

3

>>> pipe.stdin.close()

>>> output = pipe.stdout.read()

>>> pipe.wait()

0

>>> output

b'Got this: "Lumberjack"\r\nThe meaning of life is 12 24\r\n'

As we’ll learn in Chapter 5, we have to be cautious when talking back and forth to a
program like this; buffered output streams can lead to deadlock if writes and reads are
interleaved, and we may eventually need to consider tools like the Pexpect utility as a
workaround (more on this later).

Finally, even more exotic stream control is possible—the following connects two pro-
grams, by piping the output of one Python script into another, first with shell syntax,
and then with the subprocess module:

C:\...\PP4E\System\Streams> python writer.py | python reader.py

Got this: "Help! Help! I'm being repressed!"

The meaning of life is 42 84

C:\...\PP4E\System\Streams> python

>>> from subprocess import Popen, PIPE

>>> p1 = Popen('python writer.py', stdout=PIPE)

>>> p2 = Popen('python reader.py', stdin=p1.stdout, stdout=PIPE)

>>> output = p2.communicate()[0]

>>> output

b'Got this: "Help! Help! I\'m being repressed!"\r\nThe meaning of life is 42 84\r\n'

>>> p2.returncode

0

Standard Streams | 131