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POWhy does supplying stdin to subprocess.Popen cause what is written to stdout to change?
 

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  1. POWhy does supplying stdin to subprocess.Popen cause what is written to stdout to change?
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    <p>I'm using Python's subprocess.Popen to perform some FTP using the binary client of the host operating system. I can't use ftplib or any other library for various reasons.</p> <p>The behavior of the binary seems to change if I attach a stdin handler to the Popen instance. For example, using XP's ftp client, which accepts a text file of commands to issue:</p> <pre><code>&gt;&gt;&gt;from subprocess import Popen, PIPE &gt;&gt;&gt;p = Popen(['ftp','-A','-s:commands.txt','example.com'], stdout=PIPE) &gt;&gt;&gt;p.communicate()[0] 'Connected to example.com. 220 ProFTPD 1.3.1 Server (Debian) ... 331 Anonymous login ok, send your complete email address as your password &lt;snip&gt; ftp&gt; binary 200 Type set to I ftp&gt; get /testfiles/100.KiB 200 PORT command successful 150 Opening BINARY mode data connection for /testfiles/100.KiB (102400 bytes) 226 Transfer complete ftp: 102400 bytes received in 0.28Seconds 365.71Kbytes/sec. ftp&gt; quit &gt;&gt;&gt; </code></pre> <p>commands.txt:</p> <pre><code>binary get /testfiles/100.KiB quit </code></pre> <p>When also supplying stdin, all you get in stdout is:</p> <pre><code>&gt;&gt;&gt;from subprocess import Popen, PIPE &gt;&gt;&gt;p = Popen(['ftp','-A','-s:commands.txt','example.com'], stdin=PIPE, stdout=PIPE) &gt;&gt;&gt;p.communicate()[0] 'binary get /testfiles/100.KiB quit' &gt;&gt;&gt; </code></pre> <p>Initially I thought this was a quirk of the XP ftp client, perhaps knowing it wasn't in interactive mode and therefore limiting its output. However, the same behaviour happens with OS X's ftp - all the server responses are missing from stdout if stdin is supplied - which leads me to think that this is normal behaviour.</p> <p>In Windows I can use the -s switch to effectively script ftp without using stdin, but on other platforms one relies on the shell for that kind of interaction.</p> <p>Python version is 2.6.x on both platforms. Why would supplying a handle for stdin change stdout, and where have the server responses gone to?</p>
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    1. COHave you considered using `ftplib`? http://docs.python.org/library/ftplib.html
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      1. USIgnacio Vazquez-Abrams
    2. COwhat are the reasons you can't use ftplib. it comes with your Python distribution right?
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