Pexpect and buffering

[jim.hefferon]

Hello,

I'm trying to use pexpect to grab interactions with Python's REPL. I am having trouble with tracebacks. Possibly it is related to buffering (hence the subject line) but I admit that's a guess.

At the end of this message is a minimal example program. It feeds three commands to a python interpreter. The second command should fail like this.
Traceback (most recent call last):

However, pexpect only returns part of that "Traceback .." message, that is, pexpect is not waiting for the >>> prompt. I have included the output of the program below, after the program.

(In case it helps, spawning Python with -u doesn't make any difference.)

I'd be very glad for any suggestions.
Jim

The program:
--------------------------------------------------
#!/usr/bin/python
import sys, os, re, pprint
import pexpect

cmds = ['1+2', 'a', '3+4']
PROMPT = '>>> '
PROMPT_CONTINUE = '... '

child = pexpect.spawn('python') # start the repl
# child = pexpect.spawn('python', maxread=1) # makes no difference
child.expect([PROMPT])
print " initial child.before=",pprint.pformat(child.before)
print " initial child.after=",pprint.pformat(child.after)

r = []
for cmd in cmds:
print "++++++ cmd=",pprint.pformat(cmd)
child.sendline(cmd)
dex = child.expect([PROMPT, PROMPT_CONTINUE])
print " child.before=",pprint.pformat(child.before)
print " child.after=",pprint.pformat(child.after)
r.append(child.before)
print "r=",pprint.pformat(r)
-----------------------------------------

My screen when I run this (Ubuntu 12.04 with Python 2.7.3).

$ ./minex.py
initial child.before= 'Python 2.7.3 (default, Aug 1 2012, 05:16:07) \r\n[GCC 4.6.3] on linux2\r\nType "help", "copyright", "credits" or "license" for more information.\r\n'
initial child.after= '>>> '
++++++ cmd= '1+2'
child.before= '1+2\r\n3\r\n'
child.after= '>>> '
++++++ cmd= 'a'
child.before= 'a\r\nTraceb'
child.after= 'ack '
++++++ cmd= '3+4'
child.before= '(m'
child.after= 'ost '
r= ['1+2\r\n3\r\n', 'a\r\nTraceb', '(m']

 

[Chris Rebert]

Hello,

I'm trying to use pexpect to grab interactions with Python's REPL. I am

having trouble with tracebacks. Possibly it is related to buffering (hence
the subject line) but I admit that's a guess.

Why are you doing this in the first place? Why invoke an external Python
shell when you're in a Python program to begin with? Seems terribly
unnecessarily roundabout.

 

[jim.hefferon]

Sorry to reply to my own post, but I believe I have my answer and I want to help anyone who might google their way here: I need to change PROMPT and PROMPT_CONTINUE to be regular expressions, for instance by escaping the periods.