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| <h1>Pexpect version VERSION<br> |
| a Pure Python Expect-like module |
| </h1> |
| </div> |
| <div id="Content"> |
| <p>Pexpect makes Python a better tool for controlling other |
| applications.</p> |
| <p>Pexpect is a pure Python module for spawning child applications; |
| controlling them; and responding to expected patterns in their output. |
| Pexpect works like Don Libes' Expect. Pexpect allows your script to |
| spawn a child application and control it as if a human were typing |
| commands.</p> |
| <p>Pexpect can be used for automating interactive applications such as |
| ssh, ftp, passwd, telnet, etc. It can be used to a automate setup |
| scripts for duplicating software package installations on different |
| servers. It can be used for automated software testing. Pexpect is in |
| the spirit of Don Libes' Expect, but Pexpect is pure Python. Unlike |
| other Expect-like modules for Python, Pexpect does not require TCL or |
| Expect nor does it require C extensions to be compiled. It should work |
| on any platform that supports the standard Python pty module. The |
| Pexpect interface was designed to be easy to use.</p> |
| <table border="0"> |
| <tbody> |
| <tr> |
| <td align="right" valign="top">Send questions to:</td> |
| <td align="left"><a href="http://www.noah.org/email/"><img |
| src="email.png" alt="Click to send email." border="0" height="16" |
| width="100"></a></td> |
| </tr> |
| </tbody> |
| </table> |
| <hr noshade="noshade" size="1"> |
| <h1><a name="license"></a>License: MIT style</h1> |
| <p> |
| Free, open source, and all that good stuff.<br> |
| <br> |
| Permission is hereby granted, free of charge, to any person obtaining a copy |
| of this software and associated documentation files (the "Software"), to deal |
| in the Software without restriction, including without limitation the rights |
| to use, copy, modify, merge, publish, distribute, sublicense, and/or sell |
| copies of the Software, and to permit persons to whom the Software is |
| furnished to do so, subject to the following conditions:<br> |
| <br> |
| The above copyright notice and this permission notice shall be included in all |
| copies or substantial portions of the Software.<br> |
| <br> |
| THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, |
| EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF |
| MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. |
| IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, |
| DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR |
| OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE |
| USE OR OTHER DEALINGS IN THE SOFTWARE.<br> |
| <br> |
| Pexpect Copyright (c) 2008 Noah Spurrier<br> |
| http://pexpect.sourceforge.net/ |
| </p> |
| |
| <hr noshade="noshade" size="1"> |
| <h1><a name="download"></a><a |
| href="http://sourceforge.net/project/showfiles.php?group_id=59762">Download</a></h1> |
| <p>Download the <a |
| href="http://sourceforge.net/project/showfiles.php?group_id=59762"> |
| current version here</a> from the SourceForge site. Grab the current Pexpect tarball. |
| </p> |
| <h2>Installing Pexpect</h2> |
| <p>The Pexpect tarball is a standard Python Distutil distribution.</p> |
| <ol> |
| <li>download <span class="code">pexpect-VERSION.tar.gz</span></li> |
| <li><span class="code">tar zxf pexpect-VERSION.tar.gz</span></li> |
| <li><span class="code">cd pexpect-VERSION</span></li> |
| <li><span class="code">python setup.py install</span> <i>do this as root</i></li> |
| </ol> |
| <h2>Examples</h2> |
| <p> |
| Under the <span class="code">pexpect-VERSION</span> directory you should find |
| the <span class="code">examples</span> directory. |
| This is the best way to learn to use Pexpect. |
| See the descriptions of <a href="examples.html">Pexpect Examples</a>. |
| </p> |
| <h2><a name="doc"></a>API Documentation</h2> |
| <p> |
| <blockquote> |
| <a href="pexpect.html">pexpect</a> This is the main module that you want.<br> |
| <a href="pxssh.html">pxssh</a> Pexpect SSH is an extension of 'pexpect.spawn' that specializes in SSH.<br> |
| </blockquote> |
| the following are experimental extensions to Pexpect<br> |
| <blockquote> |
| <a href="fdpexpect.html">fdpexpect</a> fdpexpect extension of 'pexpect.spawn' that uses an open file descriptor.<br> |
| <a href="screen.html">SCREEN</a> This represents a virtual 'screen'.<br> |
| <a href="ANSI.html">ANSI</a> This parses ANSI/VT-100 terminal escape codes.<br> |
| <a href="FSM.html">FSM</a> This is a finite state machine used by ANSI.<br> |
| </blockquote> |
| </p> |
| <hr noshade="noshade" size="1"> |
| <h1><a name="status"></a>Project Status</h1> |
| <p>Automated pyunit tests reach over 80% |
| code coverage on pexpect.py. I regularly test on Linux and BSD |
| platforms. I try to test on Solaris and Irix. |
| </p> |
| <hr noshade="noshade" size="1"> |
| <h1><a name="requirements"></a>Requirements for use of Pexpect</h1> |
| <h2>Python</h2> |
| <blockquote> |
| <p>Pexpect was written and tested with Python 2.4. It should work on |
| earlier versions that have the <span class="code">pty</span> module. I |
| sometimes even manually test it with Python 1.5.2, but I can't easily |
| run the PyUnit test framework against Python 1.5.2, so I have less |
| confidence in Pexpect on Python 1.5.2.</p> |
| </blockquote> |
| <h2>pty module</h2> |
| <blockquote> |
| <p>Any POSIX system (UNIX) with a working <span class="code">pty</span> |
| module should be able to run Pexpect. The <span class="code">pty</span> |
| module is part of the Standard Python Library, so if you are running on |
| a POSIX system you should have it. The <span class="code">pty</span> |
| module does not run the same on all platforms. It should be solid on Linux |
| and BSD systems. I have taken effort to try to smooth the wrinkles out of the different platforms. To learn more |
| about the wrinkles see <a href="#bugs">Bugs</a> and <a href="#testing">Testing</a>.</p> |
| </blockquote> |
| <p>Pexpect does not currently work on the standard Windows Python (see |
| the pty requirement); however, it seems to work fine using <a |
| href="http://www.cygwin.com/">Cygwin</a>. It is possible to build |
| something like a pty for Windows, but it would have to use a different |
| technique that I am still investigating. I know it's possible because |
| Libes' Expect was ported to Windows. <i>If you have any ideas or |
| skills to contribute in this area then I would really appreciate some |
| tips on how to approach this problem.</i> </p> |
| <hr noshade="noshade" size="1"> |
| <h1><a name="overview"></a>Overview</h1> |
| <p>Pexpect can be used for automating interactive applications such as |
| ssh, ftp, mencoder, passwd, etc. The Pexpect interface was designed to be |
| easy to use. Here is an example of Pexpect in action:</p> |
| <blockquote> |
| <pre class="code"># This connects to the openbsd ftp site and<br># downloads the recursive directory listing.<br>import pexpect<br>child = pexpect.spawn ('ftp ftp.openbsd.org')<br>child.expect ('Name .*: ')<br>child.sendline ('anonymous')<br>child.expect ('Password:')<br>child.sendline ('noah@example.com')<br>child.expect ('ftp> ')<br>child.sendline ('cd pub')<br>child.expect('ftp> ')<br>child.sendline ('get ls-lR.gz')<br>child.expect('ftp> ')<br>child.sendline ('bye')<br></pre> |
| </blockquote> |
| <p> Obviously you could write an ftp client using Python's own <span |
| class="code">ftplib</span> module, but this is just a demonstration. |
| You can use this technique with any application. This is especially |
| handy if you are writing automated test tools.</p> |
| |
| <p>There are two important methods in Pexpect -- <span class="code"><b>expect()</b></span> |
| and <span class="code"><b>send()</b></span> (or <span class="code">sendline()</span> |
| which is like <span class="code">send()</span> with a linefeed). |
| The <span class="code">expect()</span> method waits for the child application |
| to return a given string. The string you specify is a regular expression, so |
| you can match complicated patterns. The <span class="code"><b>send()</b></span> method |
| writes a string to the child application. From the child's point of |
| view it looks just like someone typed the text from a terminal. After |
| each call to <span class="code"><b>expect()</b></span> the <span |
| class="code"><b>before</b></span> and <span class="code"><b>after</b></span> |
| properties will be set to the text printed by child application. The <span |
| class="code"><b>before</b></span> property will contain all text up to |
| the expected string pattern. The <span class="code"><b>after</b></span> string |
| will contain the text that was matched by the expected pattern. |
| The <span class="code">match</span> property is set to the <span class="code">re MatchObject</span>. |
| </p> |
| |
| <p>An example of Pexpect in action may make things more clear. This example uses |
| <span class="code">ftp</span> to login to the OpenBSD site; list files |
| in a directory; and then pass interactive control of the ftp session to |
| the human user.</p> |
| <blockquote> |
| <pre class="code">import pexpect<br>child = pexpect.spawn ('ftp ftp.openbsd.org')<br>child.expect ('Name .*: ')<br>child.sendline ('anonymous')<br>child.expect ('Password:')<br>child.sendline ('noah@example.com')<br>child.expect ('ftp> ')<br>child.sendline ('ls /pub/OpenBSD/')<br>child.expect ('ftp> ')<br>print child.before # Print the result of the ls command.<br>child.interact() # Give control of the child to the user.<br></pre> |
| </blockquote> |
| <h2>Special EOF and TIMEOUT patterns</h2> |
| <p> |
| There are two special patterns to match the End Of File or a Timeout condition. |
| You you can pass these patterns to <span class="code">expect()</span>. |
| These patterns are not regular expressions. Use them like predefined constants. |
| </p> |
| <p>If the child has died and you have read all the child's output then ordinarily |
| <span class="code">expect()</span> will raise an <span class="code">EOF</span> |
| exception. You can read everything up to the EOF without generating an |
| exception by using the EOF pattern <span class="code">expect(pexpect.EOF)</span>. |
| In this case everything the child has output will be available in the <span |
| class="code">before</span> property.</p> |
| <p>The pattern given to <span class="code">expect()</span> may be a |
| regular expression or it may also be a <b>list</b> of regular expressions. |
| This allows you to match multiple optional responses. The <span class="code">expect()</span> |
| method returns the index of the pattern that was matched. For example, |
| say you wanted to login to a server. After entering a password you |
| could get various responses from the server -- your password could be |
| rejected; or you could be allowed in and asked for your terminal type; |
| or you could be let right in and given a command prompt. The following |
| code fragment gives an example of this:</p> |
| <blockquote> |
| <pre class="code">child.expect('password:')<br>child.sendline (my_secret_password)<br># We expect any of these three patterns...<br>i = child.expect (['Permission denied', 'Terminal type', '[#\$] '])<br>if i==0:<br> print 'Permission denied on host. Can't login'<br> child.kill(0)<br>elif i==2:<br> print 'Login OK... need to send terminal type.'<br> child.sendline('vt100')<br> child.expect ('[#\$] ')<br>elif i==3:<br> print 'Login OK.'<br> print 'Shell command prompt', child.after</pre> |
| </blockquote> |
| <p>If nothing matches an expected pattern then expect will eventually |
| raise a TIMEOUT exception. The default time is 30 seconds, but you can |
| change this by passing a timeout argument to expect():</p> |
| <blockquote> |
| <pre class="code"># Wait no more than 2 minutes (120 seconds) for password prompt.<br>child.expect('password:', timeout=120)</pre> |
| </blockquote> |
| <h2>Find the end of line -- CR/LF conventions<br> |
| Matching at the end of a line can be tricky<br> |
| $ regex pattern is useless.<br> |
| </h2> |
| <p>Pexpect matches regular expressions a little differently than what |
| you might be used to. |
| </p> |
| <p><i><b>The $ pattern for end of line match is useless</b></i>. |
| The $ matches the end of string, but Pexpect reads from the child |
| one character at a time, so each character looks like the end of a line. |
| Pexpect can't do a look-ahead into the child's output stream. |
| In general you would have this situation when using regular expressions |
| with any stream.<br> |
| <i>Note, pexpect does have an internal buffer, so reads are faster |
| than one character at a time, but from the user's perspective the regex |
| patterns test happens one character at a time.</i></p> |
| <p>The best way to match the end of a line is to look for the |
| newline: "\r\n" (CR/LF). Yes, that does appear to be DOS-style. |
| It may surprise some UNIX people to learn that terminal TTY device drivers |
| (dumb, vt100, ANSI, xterm, etc.) all use the CR/LF combination to signify |
| the end of line. Pexpect uses a Pseudo-TTY device to talk to the child application, so |
| when the child app prints "\n" you actually see "\r\n". |
| </p> |
| <p><b>UNIX uses just linefeeds to end lines of text, but not when it |
| comes to TTY devices!</b> TTY devices are more like the Windows world. |
| Each line of text end with a CR/LF combination. When you intercept data |
| from a UNIX command from a TTY device you will find that the TTY device |
| outputs a CR/LF combination. A UNIX command may only write a linefeed |
| (\n), but the TTY device driver converts it to CR/LF. This means that |
| your terminal will see lines end with CR/LF (hex <span class="code">0D 0A</span>). |
| Since Pexpect emulates a terminal, to match ends of lines you have to |
| expect the CR/LF combination.</p> |
| <blockquote> |
| <p class="code">child.expect ('\r\n')</p> |
| </blockquote> |
| <p>If you just need to skip past a new line then <span class="code">expect |
| ('\n')</span> by itself will work, but if you are expecting a specific |
| pattern before the end of line then you need to explicitly look for the |
| \r. For example the following expects a word at the end of a line:</p> |
| <blockquote> |
| <p class="code">child.expect ('\w+\r\n')</p> |
| </blockquote> |
| <p>But the following would both fail:</p> |
| <blockquote> |
| <p class="code">child.expect ('\w+\n')</p> |
| </blockquote> |
| <p>And as explained before, trying to use '$' to match the end of line |
| would not work either:</p> |
| <blockquote> |
| <p class="code">child.expect ('\w+$')</p> |
| </blockquote> |
| <p>So if you need to explicitly look for the END OF LINE, you want to |
| look for the CR/LF combination -- not just the LF and not the $ pattern.</p> |
| <p>This problem is not limited to Pexpect. This problem happens any |
| time you try to perform a regular expression match on a stream. Regular |
| expressions need to look ahead. With a stream it is hard to look ahead |
| because the process generating the stream may not be finished. There is no |
| way to know if the process has paused momentarily or is finished and |
| waiting for you. <font color="#cc0000">Pexpect must implicitly always |
| do a NON greedy match (minimal) at the end of a input {### already said |
| this}.</font> </p> |
| <p>Pexpect compiles all regular expressions with the DOTALL flag. With |
| the DOTALL flag a "." will match a newline. See the Python <a |
| href="http://www.python.org/doc/current/lib/node115.html#l2h-733">documentation</a></p> |
| <h2>Beware of + and * at the end of input.</h2> |
| <p>Remember that any time you try to match a pattern that needs |
| look-ahead that you will always get a minimal match (non greedy). For |
| example, the following will always return just one character:</p> |
| <blockquote> |
| <p class="code">child.expect ('.+')</p> |
| </blockquote> |
| <p>This example will match successfully, but will always return no |
| characters:</p> |
| <blockquote> |
| <p class="code">child.expect ('.*')</p> |
| </blockquote> |
| <p>Generally any star * expression will match as little as possible</p> |
| <p>One thing you can do is to try to force a non-ambiguous character at |
| the end of your <span class="code">\d+</span> pattern. Expect that |
| character to delimit the string. For example, you might try making the |
| end of your pattrn be <span class="code">\D+</span> instead of <span |
| class="code">\D*</span>. That means number digits alone would not |
| satisfy the (<span class="code">\d+</span>) pattern. You would need |
| some number(s) and at least one <span class="code">\D</span> at the |
| end. </p> |
| <h2>Matching groups</h2> |
| <p>You can group regular expression using parenthesis. After a match, |
| the <span class="code">match</span> parameter of the spawn object will |
| contain the Python Match object. </p> |
| <h2>Examples</h2> |
| <p>Using "match" and groups...</p> |
| <h2>Debugging</h2> |
| <p>If you get the string value of a pexpect.spawn object you will get |
| lots of useful debugging information. For debugging it's very useful to |
| use the following pattern:</p> |
| <p>try:<br> |
| i = child.expect ([pattern1, pattern2, pattern3, |
| etc])<br> |
| except:<br> |
| print "Exception was thrown"<br> |
| print "debug information:"<br> |
| print str(child)<br> |
| </p> |
| <p>It is also useful to log the child's input and out to a file or the |
| screen. The following will turn on logging and send output to stdout |
| (the screen).<br> |
| </p> |
| <p> child = pexpect.spawn (foo)<br> |
| child.logfile = sys.stdout<br> |
| <br> |
| </p> |
| <hr noshade="noshade" size="1"> |
| <h1>Exceptions</h1> |
| <p><b>EOF</b></p> |
| <p>Note that two flavors of EOF Exception may be thrown. They are |
| virtually identical except for the message string. For practical |
| purposes you should have no need to distinguish between them, but they |
| do give a little extra information about what type of platform you are |
| running. The two messages are:</p> |
| <blockquote> |
| <p class="code">End Of File (EOF) in read(). Exception style platform.</p> |
| <p class="code">End Of File (EOF) in read(). Empty string style |
| platform.</p> |
| </blockquote> |
| <p>Some UNIX platforms will throw an exception when you try to read |
| from a file descriptor in the EOF state. Other UNIX platforms instead |
| quietly return an empty string to indicate that the EOF state has been |
| reached.</p> |
| <p><b>Expecting EOF</b></p> |
| <p>If you wish to read up to the end of the child's output without |
| generating an <span class="code">EOF</span> exception then use the <span |
| class="code">expect(pexpect.EOF)</span> method.</p> |
| <p><b>TIMEOUT</b></p> |
| <p>The <span class="code">expect()</span> and <span class="code">read()</span> |
| methods will also timeout if the child does not generate any output for |
| a given amount of time. If this happens they will raise a <span |
| class="code">TIMEOUT</span> exception. You can have these method |
| ignore a timeout and block indefinitely by passing None for the timeout |
| parameter.</p> |
| <blockquote> |
| <p class="code">child.expect(pexpect.EOF, timeout=None)</p> |
| </blockquote> |
| <hr noshade="noshade" size="1"> |
| <h1><a name="faq"></a>FAQ</h1> |
| <p><b>Q: Why don't shell pipe and redirect (| and >) work when I |
| spawn a command?</b></p> |
| <p> |
| |
| A: Remember that Pexpect does NOT interpret shell meta characters such as |
| redirect, pipe, or wild cards (>, |, or *). That's done by a shell not the |
| command you are spawning. This is a common mistake. If you want to run a |
| command and pipe it through another command then you must also start a shell. |
| For example: |
| |
| <pre> |
| child = pexpect.spawn('/bin/sh -c "ls -l | grep LOG > log_list.txt"') |
| child.expect(pexpect.EOF) |
| </pre> |
| |
| The second form of spawn (where you pass a list of arguments) is useful in |
| situations where you wish to spawn a command and pass it its own argument list. |
| This can make syntax more clear. For example, the following is equivalent to |
| the previous example: |
| |
| <pre> |
| shell_cmd = 'ls -l | grep LOG > log_list.txt' |
| child = pexpect.spawn ('/bin/sh', ['-c', shell_cmd]) |
| child.expect (pexpect.EOF) |
| </pre> |
| |
| </p> |
| <p><b>Q: Isn't there already a Python Expect?</b></p> |
| <p>A: Yes, there are several of them. They usually require you to |
| compile C. I wanted something that was pure Python and preferably a |
| single module that was simple to install. I also wanted something that |
| was easy to use. This pure Python expect only recently became possible |
| with the introduction of the pty module in the standard Python library. |
| Previously C extensions were required.</p> |
| |
| <p><strong>Q: The before and after properties sound weird.</strong></p> |
| <p>Originally I was going to model Pexpect more after Expect, but then |
| I found that I could never remember how to get the context of the stuff |
| I was trying to parse. I hate having to read my own documentation. I |
| decided that it was easier for me to remember what before and after |
| was. It just so happens that this is how the -B and -A options in grep |
| works, so that made it even easier for me to remember. Whatever makes |
| my life easier is what's best.</p> |
| |
| <p><b>Q: Why not just use Expect?</b></p> |
| <p>A: I love it. It's great. I has bailed me out of some real jams, but |
| I wanted something that would do 90% of what I need from Expect; be 10% |
| of the size; and allow me to write my code in Python instead of TCL. |
| Pexpect is not nearly as big as Expect, but Pexpect does everything I |
| have ever used Expect for. |
| <!-- :-P If I liked TCL then you wouldn't be reading this. My appologies to Don Libes -- Expect is cool, TK is cool, but TCL is only slightly better than Perl in my book. Hopefully after Expyct is done I will not need to use Expect anymore -- except for that lovely autoexpect tool. Damn, I wish I had that! --> </p> |
| |
| <p><b>Q: Why not just use a pipe (popen())?</b></p> |
| <p>A: A pipe works fine for getting the output to non-interactive |
| programs. If you just want to get the output from <span class="code">ls</span>, |
| <span class="code">uname</span>, or <span class="code">ping</span> |
| then this works. Pipes do not work very well for interactive programs |
| and pipes will almost certainly fail for most applications that ask for |
| passwords such as telnet, ftp, or ssh.</p> |
| <p>There are two reasons for this. </p> |
| <p>First an application may bypass stdout and print directly to its |
| controlling TTY. Something like SSH will do this when it asks you for a |
| password. This is why you cannot redirect the password prompt because |
| it does not go through stdout or stderr.</p> |
| <p>The second reason is because most applications are built using the C |
| Standard IO Library (anything that uses <span class="code">#include |
| <stdio.h></span>). One of the features of the stdio library is |
| that it buffers all input and output. Normally output is <b><i>line |
| buffered</i></b> when a program is printing to a TTY (your terminal |
| screen). Every time the program prints a line-feed the currently |
| buffered data will get printed to your screen. The problem comes when |
| you connect a pipe. The stdio library is smart and can tell that it is |
| printing to a pipe instead of a TTY. In that case it switches from line |
| buffer mode to <i><b>block buffered</b></i>. In this mode the |
| currently buffered data is flushed when the buffer is full. This causes |
| most interactive programs to deadlock. Block buffering is more |
| efficient when writing to disks and pipes. Take the situation where a |
| program prints a message "Enter your user name:\n" and then waits for |
| you type type something. In block buffered mode, the stdio library will |
| not put the message into the pipe even though a linefeed is printed. |
| The result is that you never receive the message, yet the child |
| application will sit and wait for you to type a response. Don't confuse |
| the stdio lib's buffer with the pipe's buffer. The pipe buffer is |
| another area that can cause problems. You could flush the input side of |
| a pipe, whereas you have no control over the stdio library buffer. </p> |
| <p>More information: the Standard IO library has three states for a |
| FILE *. These are: _IOFBF for block buffered; _IOLBF for line buffered; |
| and _IONBF for unbuffered. The STDIO lib will use block buffering when |
| talking to a block file descriptor such as a pipe. This is usually not |
| helpful for interactive programs. Short of recompiling your program to |
| include fflush() everywhere or recompiling a custom stdio library there |
| is not much a controlling application can do about this if talking over |
| a pipe.</p> |
| <p> The program may have put data in its output that remains unflushed |
| because the output buffer is not full; then the program will go and |
| deadlock while waiting for input -- because you never send it any |
| because you are still waiting for its output (still stuck in the |
| STDIO's output buffer).</p> |
| <p>The answer is to use a pseudo-tty. A TTY device will force <i><b>line</b></i> |
| buffering (as opposed to block buffering). Line buffering means that |
| you will get each line when the child program sends a line feed. This |
| corresponds to the way most interactive programs operate -- send a line |
| of output then wait for a line of input.</p> |
| <p>I put "answer" in quotes because it's ugly solution and because |
| there is no POSIX standard for pseudo-TTY devices (even though they |
| have a TTY standard...). What would make more sense to me would be to |
| have some way to set a mode on a file descriptor so that it will tell |
| the STDIO to be line-buffered. I have investigated, and I don't think |
| there is a way to set the buffered state of a child process. The STDIO |
| Library does not maintain any external state in the kernel or whatnot, |
| so I don't think there is any way for you to alter it. I'm not quite |
| sure how this line-buffered/block-buffered state change happens |
| internally in the STDIO library. I think the STDIO lib looks at the |
| file descriptor and decides to change behavior based on whether it's a |
| TTY or a block file (see isatty()).</p> |
| <p>I hope that this qualifies as helpful.</p> |
| |
| <h1>Don't use a pipe to control another application...</h1> |
| <p>Pexpect may seem similar to <span class="code">os.popen()</span> or |
| <span class="code">commands</span> module. The main difference is that |
| Pexpect (like Expect) uses a pseudo-TTY to talk to the child |
| application. Most applications do no work well through the system() |
| call or through pipes. And probably all applications that ask a user to |
| type in a password will fail. These applications bypass the stdin and |
| read directly from the TTY device. Many applications do not explicitly |
| flush their output buffers. This causes deadlocks if you try to control |
| an interactive application using a pipe. What happens is that most UNIX |
| applications use the stdio (#include <stdio.h>) for input and |
| output. The stdio library behaves differently depending on where the |
| output is going. There is no way to control this behavior from the |
| client end.<br> |
| </p> |
| |
| <p><b>Q: Can I do screen scraping with this thing?</b></p> |
| <p>A: That depends. If your application just does line-oriented output |
| then this is easy. If it does screen-oriented output then it may work, |
| but it could be hard. For example, trying to scrape data from the 'top' |
| command would be hard. The top command repaints the text window. </p> |
| <p>I am working on an ANSI / VT100 terminal emulator that will have |
| methods to get characters from an arbitrary X,Y coordinate of the |
| virtual screen. It works and you can play with it, but I have no |
| working examples at this time.</p> |
| <hr noshade="noshade" size="1"> |
| <h1><a name="bugs"></a>Bugs</h1> |
| <h2>Threads</h2> |
| <p>On Linux (RH 8) you cannot spawn a child from a different thread and |
| pass the handle back to a worker thread. The child is successfully |
| spawned but you can't interact with it. The only way to make it work is |
| to spawn and interact with the child all in the same thread. [Adam |
| Kerrison] </p> |
| <h2><a name="echo_bug"></a>Timing issue with send() and sendline()</h2> |
| <p>This problem has been addressed and should not effect most users.</p> |
| <p>It is sometimes possible to read an echo of the string sent with <span |
| class="code">send()</span> and <span class="code">sendline()</span>. |
| If you call <span class="code">sendline()</span> and then immediately |
| call <span class="code">readline()</span> you may get part of your |
| output echoed back. You may read back what you just wrote even if the |
| child application does not explicitly echo it. Timing is critical. This |
| could be a security issue when talking to an application that asks for |
| a password; otherwise, this does not seem like a big deal. <i>But why |
| do TTYs do this</i>?</p> |
| <p>People usually report this when they are trying to control SSH or |
| some other login. For example, if your code looks something like this: </p> |
| <pre class="code">child.expect ('[pP]assword:')<br>child.sendline (my_password)</pre> |
| <p><br> |
| <blockquote> |
| 1. SSH prints "password:" prompt to the user.<br> |
| 2. SSH turns off echo on the TTY device.<br> |
| 3. SSH waits for user to enter a password.<br> |
| </blockquote> |
| When scripting with Pexpect what can happen is that Pexpect will response to the "password:" prompt |
| before SSH has had time to turn off TTY echo. In other words, Pexpect sends the password between |
| steps 1. and 2., so the password gets echoed back to the TTY. I would call this an SSH bug. |
| </p> |
| <p> |
| Pexpect now automatically adds a short delay before sending data to a child process. |
| This more closely mimics what happens in the usual human-to-app interaction. |
| The delay can be tuned with the 'delaybeforesend' attribute of the spawn class. |
| In general, this fixes the problem for everyone and so this should not be an issue |
| for most users. For some applications you might with to turn it off. |
| child = pexpect.spawn ("ssh user@example.com") |
| child.delaybeforesend = 0 |
| </p> |
| <p><br> |
| </p> |
| <p>Try changing it to look like the following. I know that this fix |
| does not look correct, but it works. I have not figured out exactly |
| what is happening. You would think that the sleep should be after the |
| sendline(). The fact that the sleep helps when it's between the |
| expect() and the sendline() must be a clue.</p> |
| <pre class="code">child.expect ('[pP]assword:')<br>child.sendline (my_password)</pre> |
| <h2>Timing issue with isalive()</h2> |
| <p>Reading the state of isalive() immediately after a child exits may |
| sometimes return 1. This is a race condition. The child has closed its |
| file descriptor, but has not yet fully exited before Pexpect's |
| isalive() executes. Addings a slight delay before the isalive() will |
| help. In the following example <span class="code">isalive()</span> |
| sometimes returns 1:</p> |
| <blockquote> |
| <pre class="code">child = pexpect.spawn('ls')<br>child.expect(pexpect.EOF)<br>print child.isalive()</pre> |
| </blockquote> |
| <p>But if there is any delay before the call to <span class="code">isalive()</span> |
| then it will always return 0 as expected.</p> |
| <blockquote> |
| <pre class="code">child = pexpect.spawn('ls')<br>child.expect(pexpect.EOF)<br>time.sleep(0.1)<br>print child.isalive()</pre> |
| </blockquote> |
| |
| <h2>Truncated output just before child exits</h2> |
| <p><i>So far I have seen this only on older versions of <b>Apple's MacOS X</b>.</i> |
| If the child application quits it may not flush its output buffer. This |
| means that your Pexpect application will receive an EOF even though it |
| should have received a little more data before the child died. This is |
| not generally a problem when talking to interactive child applications. |
| One example where it is a problem is when trying to read output from a |
| program like '<span class="code">ls</span>'. You may receive most of |
| the directory listing, but the last few lines will get lost before you |
| receive an EOF. The reason for this is that '<span class="code">ls</span>' |
| runs; completes its task; and then exits. The buffer is not flushed |
| before exit so the last few lines are lost. The following example |
| demonstrates the problem:</p> |
| <p> </p> |
| <blockquote> |
| <pre class="code">child = pexpect.spawn ('ls -l')<br>child.expect (pexpect.EOF)<br>print child.before <br> </pre> |
| </blockquote> |
| <p></p> |
| |
| <h2>Controlling SSH on Solaris</h2> |
| <p>Pexpect does not yet work perfectly on Solaris. |
| One common problem is that SSH sometimes will not allow TTY password |
| authentication. For example, you may expect SSH to ask you for a |
| password using code like this: |
| </p> |
| <pre class="code">child = pexpect.spawn ('ssh user@example.com')<br>child.expect ('assword')<br>child.sendline ('mypassword')<br></pre> |
| You may see the following error come back from a spawned |
| child SSH: |
| <p></p> |
| <blockquote>Permission denied (publickey,keyboard-interactive). </blockquote> |
| <p> |
| This means that SSH thinks it can't access the TTY to ask you for your |
| password. |
| The only solution I have found is to use public key authentication with |
| SSH. |
| This bypasses the need for a password. I'm not happy with this |
| solution. |
| The problem is due to poor support for Solaris Pseudo TTYs in the |
| Python |
| Standard Library. </p> |
| <hr noshade="noshade" size="1"> |
| <h1><a name="changes"></a>CHANGES</h1> |
| <h2>Current Release</h2> |
| <p>Fixed OSError exception when a pexpect object is cleaned up. |
| Previously you might have seen this exception:</p> |
| <blockquote> |
| <pre class="code">Exception exceptions.OSError: (10, 'No child processes') <br>in <bound method spawn.__del__ of<br><pexpect.spawn instance at 0xd248c>> ignored</pre> |
| </blockquote> |
| <p>You should not see that anymore. Thanks to Michael Surette.</p> |
| <p>Added support for buffering reads. This greatly improves speed when |
| trying to match long output from a child process. When you create an |
| instance of the spawn object you can then set a buffer size. For now |
| you MUST do the following to turn on buffering -- it may be on by |
| default in future version.</p> |
| <blockquote> |
| <pre class="code">child = pexpect.spawn ('my_command')<br>child.maxread=1000 # Sets buffer to 1000 characters.</pre> |
| </blockquote> |
| <div> |
| <p>I made a subtle change to the way TIMEOUT and EOF exceptions behave. |
| Previously you could either expect these states in which case pexpect |
| will not raise an exception, or you could just let pexpect raise an |
| exception when these states were encountered. If you expected the |
| states then the 'before' property was set to everything before the |
| state was encountered, but if you let pexpect raise the exception then |
| 'before' was not set. Now the 'before' property will get set either way |
| you choose to handle these states.</p> |
| <h2><i>Older changes...</i></h2> |
| <p>The spawn object now provides iterators for a <i>file-like interface</i>. |
| This makes Pexpect a more complete file-like object. You can now write |
| code like this:</p> |
| <blockquote> |
| <pre class="code">child = pexpect.spawn ('ls -l')<br>for line in child:<br> print line<br></pre> |
| </blockquote> |
| <p>I added the attribute <span class="code">exitstatus</span>. This |
| will give the exit code returned by the child process. This will be set |
| to <span class="code">None</span> while the child is still alive. When |
| <span class="code">isalive()</span> returns 0 then <span class="code">exitstatus</span> |
| will be set.</p> |
| <p>I made a few more tweaks to <span class="code">isalive()</span> so |
| that it will operate more consistently on different platforms. Solaris |
| is the most difficult to support.</p> |
| <p> </p> |
| <p>You can now put <span class="code">TIMEOUT</span> in a list of |
| expected patterns. This is just like putting <span class="code">EOF</span> |
| in the pattern list. Expecting for a <span class="code">TIMEOUT</span> |
| may not be used as often as <span class="code">EOF</span>, but this |
| makes Pexpect more consitent.</p> |
| <p>Thanks to a suggestion and sample code from Chad J. Schroeder I |
| added the ability for Pexpect to operate on a file descriptor that is |
| already open. This means that Pexpect can be used to control streams |
| such as those from serial port devices. Now you just pass the integer |
| file descriptor as the "command" when contsructing a spawn open. For |
| example on a Linux box with a modem on ttyS1:</p> |
| <blockquote> |
| <pre class="code">fd = os.open("/dev/ttyS1", os.O_RDWR|os.O_NONBLOCK|os.O_NOCTTY)<br>m = pexpect.spawn(fd) # Note integer fd is used instead of usual string.<br>m.send("+++") # Escape sequence<br>m.send("ATZ0\r") # Reset modem to profile 0<br>rval = m.expect(["OK", "ERROR"])</pre> |
| </blockquote> |
| <h3>Pexpect now tests itself on Compile Farm!</h3> |
| <p>I wrote a nice script that uses ssh to connect to each machine on |
| Source Forge's Compile Farm and then run the testall.py script for each |
| platform. The result of the test is then recorded for each platform. |
| Now it's easy to run regression tests across multiple platforms.</p> |
| <h3>Pexpect is a file-like object</h3> |
| <p>The spawn object now provides a <i>file-like interface</i>. It |
| supports most of the methods and attributes defined for Python File |
| Objects. </p> |
| <p>I changed write and writelines() so that they no longer return a |
| value. Use send() if you need that functionality. I did this to make |
| the Spawn object more closely match a file-like object.</p> |
| <p>read() was renamed to read_nonblocking(). I added a new read() |
| method that matches file-like object interface. In general, you should |
| not notice the difference except that read() no longer allows you to |
| directly set the timeout value. I hope this will not effect any |
| existing code. Switching to read_nonblocking() should fix existing code.</p> |
| <p>I changed the name of <span class="code">set_echo()</span> to <span |
| class="code">setecho()</span>.</p> |
| <p>I changed the name of <span class="code">send_eof()</span> to <span |
| class="code">sendeof()</span>.</p> |
| <p>I modified <span class="code">kill()</span> so that it checks to |
| make sure the pid isalive().</p> |
| <p>I modified <span class="code">spawn()</span> (really called from <span |
| class="code">__spawn()</span>)so that it does not raise an expection |
| if <span class="code">setwinsize()</span> fails. Some platforms such |
| as Cygwin do not like setwinsize. This was a constant problem and since |
| it is not a critical feature I decided to just silence the error. |
| Normally I don't like to do that, but in this case I'm making an |
| exception.</p> |
| <p>Added a method <span class="code">close()</span> that does what you |
| think. It closes the file descriptor of the child application. It makes |
| no attempt to actually kill the child or wait for its status. </p> |
| <p>Add variables <span class="code">__version__</span> and <span |
| class="code">__revision__</span> (from cvs) to the pexpect modules. |
| This is mainly helpful to me so that I can make sure that I'm testing |
| with the right version instead of one already installed.</p> |
| <h3>Logging changes</h3> |
| <blockquote> |
| <p><span class="code">log_open()</span> and <span class="code">log_close()</span> |
| have been removed. Now use <span class="code">setlog()</span>. The <span |
| class="code">setlog()</span> method takes a file object. This is far |
| more flexible than the previous log method. Each time data is written |
| to the file object it will be flushed. To turn logging off simply call <span |
| class="code">setlog()</span> with None.</p> |
| </blockquote> |
| <h2>isalive changes</h2> |
| <blockquote> |
| <p>I renamed the <span class="code">isAlive()</span> method to <span |
| class="code">isalive()</span> to match the more typical naming style |
| in Python. Also the technique used to detect child process status has |
| been drastically modified. Previously I did some funky stuff with |
| signals which caused indigestion in other Python modules on some |
| platforms. It's was a big headache. It still is, but I think it works |
| better now.</p> |
| </blockquote> |
| <h3>attribute name changes</h3> |
| <blockquote> |
| <p>The names of some attributes have been changed. This effects the |
| names of the attributes that are set after called the <span |
| class="code">expect()</span> method.</p> |
| <table class="pymenu" border="0" cellpadding="5"> |
| <tbody> |
| <tr> |
| <th class="pymenu">NEW NAME</th> |
| <th class="pymenu">OLD NAME</th> |
| </tr> |
| <tr> |
| <td><span class="code">before</span><br> |
| <i>Everything before the match.</i></td> |
| <td><span class="code">before</span></td> |
| </tr> |
| <tr> |
| <td><span class="code">after</span><br> |
| <i>Everything after and including the first character of the |
| match</i></td> |
| <td><span class="code">matched</span></td> |
| </tr> |
| <tr> |
| <td><span class="code">match</span><br> |
| <i>This is the re MatchObject from the match.<br> |
| You can get groups() from this.<br> |
| See '<span class="code">uptime.py</span>' in the examples tar ball.</i></td> |
| <td><i>New -- Did not exist</i></td> |
| </tr> |
| </tbody> |
| </table> |
| </blockquote> |
| <h3>EOF changes</h3> |
| <blockquote> |
| <p>The <span class="code">expect_eof()</span> method is gone. You |
| can now simply use the <span class="code">expect()</span> method to |
| look for EOF.</p> |
| <p>Was:</p> |
| <blockquote> |
| <p><span class="code">p.expect_eof ()</span></p> |
| </blockquote> |
| <p>Now:</p> |
| <blockquote> |
| <p><span class="code">p.expect (pexpect.EOF)</span></p> |
| </blockquote> |
| </blockquote> |
| <hr noshade="noshade" size="1"> |
| <h1><a name="testing"></a>TESTING</h1> |
| <p>The following platforms have been tested:</p> |
| <!-- |
| <table class="pymenu" border="0" cellpadding="5"> |
| <tbody> |
| <tr> |
| <th class="pymenu">PLATFORM</th> |
| <th class="pymenu">RESULTS</th> |
| </tr> |
| <tr> |
| <td>Linux 2.4.9-ac10-rmk2-np1-cerf2<br> |
| armv4l</td> |
| <td><b><i>all tests passed</i></b></td> |
| </tr> |
| <tr> |
| <td>Linux 2.4.18 #2<br> |
| sparc64</td> |
| <td><b><i>all tests passed</i></b></td> |
| </tr> |
| <tr> |
| <td>MacOS X Darwin Kernel Version 5.5<br> |
| powerpc</td> |
| <td> |
| <p>failed more than one test.</p> |
| <p>Generally Pexpect works on OS X, but the nature of the quirks |
| cause a many of the tests to fail. See <a href="#bugs">bugs</a> |
| (Incomplete Child Output). The problem is more than minor, but Pexpect |
| is still more than useful for most tasks. The problem is an edge case.</p> |
| </td> |
| </tr> |
| <tr> |
| <td>Linux 2.2.20<br> |
| alpha<br> |
| </td> |
| <td><b><i>all tests passed</i></b></td> |
| </tr> |
| <tr> |
| <td>Linux 2.4.18-5smp<br> |
| i686</td> |
| <td><b><i>all tests passed</i></b></td> |
| </tr> |
| <tr> |
| <td>OpenBSD 2.9 GENERIC#653<br> |
| i386</td> |
| <td><b><i>all tests passed</i></b></td> |
| </tr> |
| <tr> |
| <td>Solaris</td> |
| <td> |
| <p>failed <span class="code">test_destructor</span></p> |
| <p>Otherwise, this is working pretty well. The destructor problem |
| is minor. For some reason, the <i>second</i> time a pty file |
| descriptor is created and deleted it never gets returned for use. It |
| does not effect the first time or the third time or any time after |
| that. It's only the second time. This is weird... This could be a file |
| descriptor leak, or it could be some peculiarity of how Solaris |
| recycles them. I thought it was a UNIX requirement for the OS to give |
| you the lowest available filedescriptor number. In any case, this |
| should not be a problem unless you create hundreds of pexpect |
| instances... It may also be a pty module bug. </p> |
| </td> |
| </tr> |
| <tr> |
| <td>Windows XP Cygwin</td> |
| <td>failed <span class="code">test_destructor</span>. That it |
| works at all is amazing to me. Cygwin rules!</td> |
| </tr> |
| </tbody> |
| </table> |
| --> |
| <h1> </h1> |
| <h1><a name="todo">TO DO</a></h1> |
| <p>Add an option to add a delay after each expect() or before each |
| read()/readline() call to automatically avoid the <a href="#echo_bug">echo |
| bug</a>.</p> |
| <p> </p> |
| </div> |
| <hr noshade="noshade" size="1"> |
| <table border="0"> |
| <tbody> |
| <tr> |
| <td> <a href="http://www.noah.org/email/"><img src="email.png" |
| alt="Click to send email." border="0" height="16" width="100"></a> </td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <div id="Menu"><b>INDEX</b><br> |
| <hr noshade="noshade" size="1"> <a href="#license" |
| title="Python Software Foundation License">License</a><br> |
| <a href="#download" title="Download and setup instructions">Download</a><br> |
| <a href="#doc" title="Documentation and overview">Documentation</a><br> |
| <a href="#status" title="Project Status">Project Status</a><br> |
| <a href="#requirements" title="System requirements to use Pexpect">Requirements</a><br> |
| <a href="#overview" title="Overview of what Pexpect does">Overview</a><br> |
| <a href="#faq" title="FAQ">FAQ</a><br> |
| <a href="#bugs" title="Bugs and work-arounds">Known Bugs</a><br> |
| <a href="#changes" title="What's new with Pexpect">Recent Changes</a><br> |
| <a href="#testing" title="Test results on various platforms">Testing</a><br> |
| <a href="#todo" title="What to do next">To do</a><br> |
| <a href="http://pexpect.svn.sourceforge.net/viewvc/pexpect/trunk/pexpect/" title="browse SVN">Browse SVN</a><br> |
| <br> |
| <a href="http://sourceforge.net/projects/pexpect/" |
| title="The Pexpect project page on SourceForge.net"> <img |
| src="http://sourceforge.net/sflogo.php?group_id=59762&type=5" |
| alt="The Pexpect project page on SourceForge.net" border="0" |
| height="31" width="105"> </a> </div> |
| </body> |
| </html> |