API Overview

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:

# This connects to the openbsd ftp site and
# downloads the recursive directory listing.
import pexpect
child = pexpect.spawn('ftp ftp.openbsd.org')
child.expect('Name .*: ')
child.sendline('anonymous')
child.expect('Password:')
child.sendline('noah@example.com')
child.expect('ftp> ')
child.sendline('lcd /tmp')
child.expect('ftp> ')
child.sendline('cd pub/OpenBSD')
child.expect('ftp> ')
child.sendline('get README')
child.expect('ftp> ')
child.sendline('bye')

Obviously you could write an ftp client using Python’s own ftplib 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.

There are two important methods in Pexpect – expect() and send() (or sendline() which is like send() with a linefeed). The expect() 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 send() 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 expect() the before and after properties will be set to the text printed by child application. The before property will contain all text up to the expected string pattern. The after string will contain the text that was matched by the expected pattern. The match property is set to the re match object.

An example of Pexpect in action may make things more clear. This example uses ftp to login to the OpenBSD site; list files in a directory; and then pass interactive control of the ftp session to the human user:

import pexpect
child = pexpect.spawn ('ftp ftp.openbsd.org')
child.expect ('Name .*: ')
child.sendline ('anonymous')
child.expect ('Password:')
child.sendline ('noah@example.com')
child.expect ('ftp> ')
child.sendline ('ls /pub/OpenBSD/')
child.expect ('ftp> ')
print child.before   # Print the result of the ls command.
child.interact()     # Give control of the child to the user.

Special EOF and TIMEOUT patterns

There are two special patterns to match the End Of File (EOF) or a Timeout condition (TIMEOUT). You can pass these patterns to expect(). These patterns are not regular expressions. Use them like predefined constants.

If the child has died and you have read all the child’s output then ordinarily expect() will raise an EOF exception. You can read everything up to the EOF without generating an exception by using the EOF pattern expect. In this case everything the child has output will be available in the before property.

The pattern given to expect() may be a regular expression or it may also be a list of regular expressions. This allows you to match multiple optional responses. The expect() 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:

child.expect('password:')
child.sendline(my_secret_password)
# We expect any of these three patterns...
i = child.expect (['Permission denied', 'Terminal type', '[#\$] '])
if i==0:
    print('Permission denied on host. Can\'t login')
    child.kill(0)
elif i==1:
    print('Login OK... need to send terminal type.')
    child.sendline('vt100')
    child.expect('[#\$] ')
elif i==2:
    print('Login OK.')
    print('Shell command prompt', child.after)

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():

# Wait no more than 2 minutes (120 seconds) for password prompt.
child.expect('password:', timeout=120)

Find the end of line – CR/LF conventions

Pexpect matches regular expressions a little differently than what you might be used to.

The $ pattern for end of line match is useless. 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.

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.

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".

UNIX uses just linefeeds to end lines of text, but not when it comes to TTY devices! TTY devices are more like the Windows world. Each line of text ends 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 0D 0A). Since Pexpect emulates a terminal, to match ends of lines you have to expect the CR/LF combination:

child.expect('\r\n')

If you just need to skip past a new line then expect('\n') 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:

child.expect('\w+\r\n')

But the following would both fail:

child.expect('\w+\n')

And as explained before, trying to use $ to match the end of line would not work either:

child.expect ('\w+$')

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.

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. Pexpect must implicitly always do a NON greedy match (minimal) at the end of a input.

Pexpect compiles all regular expressions with the re.DOTALL flag. With the DOTALL flag, a "." will match a newline.

Beware of + and * at the end of patterns

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:

child.expect ('.+')

This example will match successfully, but will always return no characters:

child.expect ('.*')

Generally any star * expression will match as little as possible.

One thing you can do is to try to force a non-ambiguous character at the end of your \d+ pattern. Expect that character to delimit the string. For example, you might try making the end of your pattern be \D+ instead of \D*. Number digits alone would not satisfy the (\d+)\D+ pattern. You would need some numbers and at least one non-number at the end.

Debugging

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:

try:
    i = child.expect ([pattern1, pattern2, pattern3, etc])
except:
    print("Exception was thrown")
    print("debug information:")
    print(str(child))

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):

child = pexpect.spawn(foo)
child.logfile = sys.stdout

Exceptions

EOF

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:

  • “End Of File (EOF) in read(). Exception style platform.”
  • “End Of File (EOF) in read(). Empty string style platform.”

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.

If you wish to read up to the end of the child’s output without generating an EOF exception then use the expect(pexpect.EOF) method.

TIMEOUT

The expect() and read() 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 TIMEOUT exception. You can have these methods ignore timeout and block indefinitely by passing None for the timeout parameter:

child.expect(pexpect.EOF, timeout=None)

Pexpect on Windows

New in version 4.0: Windows support

Pexpect can be used on Windows to wait for a pattern to be produced by a child process, using pexpect.popen_spawn.PopenSpawn, or a file descriptor, using pexpect.fdpexpect.fdspawn.

pexpect.spawn and pexpect.run() are not available on Windows, as they rely on Unix pseudoterminals (ptys). Cross platform code must not use these.

PopenSpawn is not a direct replacement for spawn. Many programs only offer interactive behaviour if they detect that they are running in a terminal. When run by PopenSpawn, they may behave differently.

See also

winpexpect and wexpect

Two unmaintained pexpect-like modules for Windows, which work with a hidden console.