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    <p>The answer lies not in any magic related to the subprocess module, but in the typical behaviour of the <code>read()</code> method on Python objects.</p> <p>If you run this:</p> <pre><code>import subprocess p = subprocess.Popen(['ls'], stdout=subprocess.PIPE) help(p.stdout.read) </code></pre> <p>You'll see this:</p> <blockquote> <p>read(...)</p> <pre><code>read([size]) -&gt; read at most size bytes, returned as a string. If the size argument is negative or omitted, read until EOF is reached. Notice that when in non-blocking mode, less data than what was requested may be returned, even if no size parameter was given. </code></pre> <p>(END)</p> </blockquote> <p>The same thing applies to all file-like objects. It's very simple: calling <code>read()</code> with no argument consumes the buffer until it encounters an error (usually EOF).</p> <p><code>EOF</code> is not sent until either:</p> <ul> <li>the subprocess calls sys.stdout.close(), or</li> <li>the subprocess exits and the Python runtime and/or OS kernel clean up its file descriptors</li> </ul> <p>Beware that <code>os.read</code> has different behaviour - much more like typical buffered I/O in C. The built-in Python help function is useless, but if you're on any UNIXy system you should be able to run <code>man 3 read</code>; the Python behaviour more or less matches what's there.</p> <h2>A word of warning</h2> <p>The program above is fine, but patterns like that sometimes lead to a deadlock. The docs for the subprocess module warns about this where Popen.wait() is documented:</p> <blockquote> <p>Warning</p> <p>This will deadlock when using stdout=PIPE and/or stderr=PIPE and the child process generates enough output to a pipe such that it blocks waiting for the OS pipe buffer to accept more data. Use communicate() to avoid that.</p> </blockquote> <p>It's possible to get in a similar situation if you're not careful during two-way communication with a subprocess, depending on what the subprocess is doing.</p> <p><strong>edit</strong>:</p> <p>By the way, <a href="http://man7.org/linux/man-pages/man7/pipe.7.html" rel="nofollow">this page</a> covers the behaviour of pipes with EOF:</p> <blockquote> <p>If all file descriptors referring to the write end of a pipe have been closed, then an attempt to read(2) from the pipe will see end-of-file (read(2) will return 0).</p> </blockquote> <p><strong>edit 2</strong>:</p> <p>As Lennart mentined above, if you want truly two-way communication that goes beyond write-once read-once, you'll also need to beware of buffering. If you read <a href="http://docs.python.org/2/library/stdtypes.html#bltin-file-objects" rel="nofollow">this</a> you'll get some idea of it, but you should be aware that this is how buffered IO almost always works in UNIX-based systems - it's not a Python quirk. Run <code>man stdio.h</code> for more information.</p>
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    1. POAbout piping stdio and subprocess.Popen
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    1. COI'll check out communicate, then, hopefully that will work as I want it. Thanks for that piping info, I'll definitely read it.
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