Redirect output from stdin using code module in python?

Question

I am trying to figure out a way to redirect output from a script that I'm writing that runs the interactive console.

I need to be able to:

capture the output in a string, and then check it, or print it out.

Is this doable? Should I be using a different python module?

I tried this with the subprocess module, but ran into a problem where I could only do 1 read at the end of the process. That won't work, I need to read multiple times from stdout. This is what I have so far:

import code
console = code.InteractiveConsole()
console.push('from sim import newsteam\n')
console.push('assembler = newsteam()\n')
console.push('assembler.__init__()\n')

line = console.push('assembler.reportr()\n')
#I need to be able to interact with this output
line1 = console.push('assembler.reportpc()\n')
line2 = console.push('assembler.reportm()\n')

print("this is the result: " + line + line1 + line2)

score 1 · Accepted Answer · answered Mar 16 '14 at 08:55

1

You could redirect the stdout temporarily:

import code
import sys
from contextlib import contextmanager

try:
    from StringIO import StringIO
except ImportError: # Python 3
    from io import StringIO

try:
    from contextlib import redirect_stdout 
except ImportError: # Python < 3.4
    @contextmanager
    def redirect_stdout(new_target):
        old_target, sys.stdout = sys.stdout, new_target # replace sys.stdout
        try:
            yield new_target # run some code with the replaced stdout
        finally:
            sys.stdout = old_target # restore to the previous value

f = StringIO()
with redirect_stdout(f):
    console = code.InteractiveConsole()
    console.push('print("hello world")')
    console.push('i = int(input("give me a number"))')
    console.push('sq = i*i')
    console.push('print("%d squared is %d" % (i, sq))')
print("Got: %r" % f.getvalue())

Example:

$ echo 2 | python code-interactive.py 
Got: 'hello world\ngive me a number2 squared is 4\n'

answered Mar 16 '14 at 08:55

jfs

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1,670

Will this get me multiple values? Like say afterwards, I wanted to take j as input, do the square of j in the console, and depending on that value, say, `if (j < 10):` I want to run a new console.push with k, otherwise, I want to end the program. – Derek Halden Mar 16 '14 at 16:18
Nevermind, I realized why this works, and what you are doing. Thank you very much for your help. – Derek Halden Mar 16 '14 at 16:31
@DerekHalden: you could check the output: `f.getvalue()` after each `.push()`, just reset the `StringIO()` object: `f.seek(0); f.truncate()` to avoid reading the same output multiple times. – jfs Mar 16 '14 at 16:57
Yes, I realized that after I asked the question. Thank you very much for your help! – Derek Halden Mar 16 '14 at 21:36

score 0 · Answer 2 · edited May 23 '17 at 12:31

0

Perhaps you could try using subprocess Popen, spawn IDLE as a subprocess, and manually handle stdin and stdout using pipes. However, if you go that route read up on solutions for non-blocking read/writes. By default, reads (ie. readlines()) will keep reading until the subprocess ends. It appears you need it to be interactive (handling both stdin and stdout). Check out this thread.

edited May 23 '17 at 12:31

Community

1
1

answered Mar 15 '14 at 15:24

dotslashb

54
4

I had seen that thread before, and tried to use it, but I kept running into an broken pipe error. – Derek Halden Mar 15 '14 at 15:28
@DerekHalden: Regardless of whether you choose to use `subprocess` to interact with python or not: if you don't know why your code is broken; [ask](http://stackoverflow.com/questions/ask). – jfs Mar 16 '14 at 09:02
[there are multiple issues with using `subprocess` for interaction with `python`](http://pexpect.readthedocs.org/en/latest/FAQ.html#whynotpipe) – jfs Mar 16 '14 at 09:03
@J.F.Sebastian I would have, but I was trying to use this console method first instead, because you had recommended it to me in a previous question's comment thread. – Derek Halden Mar 16 '14 at 16:35

esnadr · Answer 3 · 2014-03-16T12:19:06.840

0

pop = subprocess.Popen([ "command",'argument'] , stdin=subprocess.PIPE , stdout =subprocess.PIPE  )  
while True:
    l = pop.stdout.readline()
    if not l :
        break 
    if l == '':
        time.sleep(1)
        continue 
    # do some function with l 
    # ....

N.B:. Not tested, but it should work.

thanks for @ J.F. Sebastian_comment

edited Mar 16 '14 at 12:19

answered Mar 15 '14 at 22:33

esnadr

427
3
18

`.communicate()` passes the whole input exactly once. And returns the whole output at once. There is no more interactivity. `pop.stdout.readline()`-based solution has [buffering issues that may easily result in a deadlock](http://pexpect.readthedocs.org/en/latest/FAQ.html#whynotpipe) – jfs Mar 16 '14 at 05:40

score 0 · Answer 4 · answered Mar 16 '14 at 09:27

You can use a local variable to store the results generated inside the console.

For instance:

import code
output = []
console = code.InteractiveConsole({'output':output})
console.push('import sys')
line = console.push('output.append(sys.version)')
print('Result value:', output)

You'll get a list with the output lines you want to check in your main module.

In your case you could do something like:

import code
output = []
console = code.InteractiveConsole({'output': output})
console.push('from sim import newsteam\n')
console.push('assembler = newsteam()\n')
console.push('assembler.__init__()\n')

console.push('output.append(assembler.reportr())')
console.push('output.append(assembler.reportpc())')
console.push('output.append(assembler.reportm())')

print("this is the result: ", output)

I suspect that functions named `report*` return nothing (`None`) and print to stdout. You can [redirect stdout](http://stackoverflow.com/a/22434858/4279) — jfs, Mar 16 '14 at 10:39
In such case, you are right. I considered that the methods return a str. — Roberto, Mar 16 '14 at 11:04

Redirect output from stdin using code module in python?

4 Answers4