Develop Reference

Calling a subprocess with mixed data type arguments in Python

I am a bit confused as to how to get this done.
What I need to do is call an external command, from within a Python script, that takes as input several arguments, and a file name.
Let's call the executable that I am calling "prog", the input file "file", so the command line (in Bash terminal) looks like this:
$ prog --{arg1} {arg2} < {file}
In the above {arg1} is a string, and {arg2} is an integer.
If I use the following:
#!/usr/bin/python
import subprocess as sbp
sbp.call(["prog","--{arg1}","{arg2}","<","{file}"])
The result is an error output from "prog", where it claims that the input is missing {arg2}
The following produces an interesting error:
#!/usr/bin/python
import subprocess as sbp
sbp.call(["prog","--{arg1} {arg2} < {file}"])
all the spaces seem to have been removed from the second string, and equal sign appended at the very end:
command not found --{arg1}{arg2}<{file}=
None of this behavior seems to make any sense to me, and there isn't much that one can go by from the Python man pages found online. Please note that replacing sbp.call with sbp.Popen does not fix the problem.

The issue is that < {file} isn’t actually an argument to the program, but is syntax for the shell to set up redirection. You can tell Python to use the shell, or you can setup the redirection yourself.
from subprocess import *
# have shell interpret redirection
check_call('wc -l < /etc/hosts', shell=True)
# set up redirection in Python
with open('/etc/hosts', 'r') as f:
check_call(['wc', '-l'], stdin=f.fileno())
The advantage of the first method is that it’s faster and easier to type. There are a lot of disadvantages, though: it’s potentially slower since you’re launching a shell; it’s potentially non-portable because it depends on the operating system shell’s syntax; and it can easily break when there are spaces or other special characters in filenames.
So the second method is preferred.

Running Another program from python

I want to call a program multiple times from a python code, and save the output of that program in a text file. My first problem right now is just calling the other code. I have to redirect to a different directory and call ./rank on output.txt. This is how Im trying to do it:
TheCommand = "~/src/rank-8-9-2011/rank output.txt"
os.system(TheCommand)
but im getting a parsing error.
[Parsing error on line ]Unknown error: 0
Im running python2.7 on Mac OS 10.5.8. Im not sure what the problem is. I also tried using subprocess:
subprocess.call(["~/src/rank-8-9-2011/rank", "output.txt"])
This does not find the directory (I have a feeling Im using the subprocess incorrectly), but I dont know what is wrong with the os.system.

the name of the program in the first argument to subprocess.Popen must not contain ~ as it doesn't pass the string to the shell for processing (which like always using parameterized queries in sql, protects one from string injection attacks, e.g. if instead of output.text one had ;rm -rf /, the system version would run rank and then run rm -rf . but the subprocess.Popen would only have rank open a file named ;rm -rf .), so one should expand it by calling os.path.expanduser:
subprocess.Popen([os.path.expanduser('~/src/rank-8-9-2011/rank'), "output.txt"])
although it is possible to turn shell processing on by passing shell=True, it is not recommended for the aforementioned reason.

you should try http://docs.python.org/library/os.path.html#os.path.expanduser
import os.path
subprocess.call([os.path.expanduser("~/src/rank-8-9-2011/rank"), "output.txt"])

I'm fairly certain your parsing error is coming from rank, not from your os.system command, as nothing there looks weird. What happens if you run rank manually?
subprocess seems to have a problem with '~', although I'm not immediately sure why. Put the full path and it should work (although you'll likely get that parsing error if it is indeed a problem with rank).

How can I add a command to the Python interactive shell?

I'm trying to save myself just a few keystrokes for a command I type fairly regularly in Python.
In my python startup script, I define a function called load which is similar to import, but adds some functionality. It takes a single string:
def load(s):
# Do some stuff
return something
In order to call this function I have to type
>>> load('something')
I would rather be able to simply type:
>>> load something
I am running Python with readline support, so I know there exists some programmability there, but I don't know if this sort of thing is possible using it.
I attempted to get around this by using the InteractivConsole and creating an instance of it in my startup file, like so:
import code, re, traceback
class LoadingInteractiveConsole(code.InteractiveConsole):
def raw_input(self, prompt = ""):
s = raw_input(prompt)
match = re.match('^load\s+(.+)', s)
if match:
module = match.group(1)
try:
load(module)
print "Loaded " + module
except ImportError:
traceback.print_exc()
return ''
else:
return s
console = LoadingInteractiveConsole()
console.interact("")
This works with the caveat that I have to hit Ctrl-D twice to exit the python interpreter: once to get out of my custom console, once to get out of the real one.
Is there a way to do this without writing a custom C program and embedding the interpreter into it?
Edit
Out of channel, I had the suggestion of appending this to the end of my startup file:
import sys
sys.exit()
It works well enough, but I'm still interested in alternative solutions.

You could try ipython - which gives a python shell which does allow many things including automatic parentheses which gives you the function call as you requested.

I think you want the cmd module.
See a tutorial here:
http://wiki.python.org/moin/CmdModule

Hate to answer my own question, but there hasn't been an answer that works for all the versions of Python I use. Aside from the solution I posted in my question edit (which is what I'm now using), here's another:
Edit .bashrc to contain the following lines:
alias python3='python3 ~/py/shellreplace.py'
alias python='python ~/py/shellreplace.py'
alias python27='python27 ~/py/shellreplace.py'
Then simply move all of the LoadingInteractiveConsole code into the file ~/py/shellreplace.py Once the script finishes executing, python will cease executing, and the improved interactive session will be seamless.

Dropping a file onto a script to run as argument causes exception in Vista

edit:OK, I could swear that the way I'd tested it showed that the getcwd was also causing the exception, but now it appears it's just the file creation. When I move the try-except blocks to their it actually does catch it like you'd think it would. So chalk that up to user error.
Original Question:
I have a script I'm working on that I want to be able to drop a file on it to have it run with that file as an argument. I checked in this question, and I already have the mentioned registry keys (apparently the Python 2.6 installer takes care of it.) However, it's throwing an exception that I can't catch. Running it from the console works correctly, but when I drop a file on it, it throws an exception then closes the console window. I tried to have it redirect standard error to a file, but it threw the exception before the redirection occurred in the script. With a little testing, and some quick eyesight I saw that it was throwing an IOError when I tried to create the file to write the error to.
import sys
import os
#os.chdir("C:/Python26/Projects/arguments")
try:
print sys.argv
raw_input()
os.getcwd()
except Exception,e:
print sys.argv + '\n'
print e
f = open("./myfile.txt", "w")
If I run this from the console with any or no arguments, it behaves as one would expect. If I run it by dropping a file on it, for instance test.txt, it runs, prints the arguments correctly, then when os.getcwd() is called, it throws the exception, and does not perform any of the stuff from the except: block, making it difficult for me to find any way to actually get the exception text to stay on screen. If I uncomment the os.chdir(), the script doesn't fail. If I move that line to within the except block, it's never executed.
I'm guessing running by dropping the file on it, which according to the other linked question, uses the WSH, is somehow messing up its permissions or the cwd, but I don't know how to work around it.

Seeing as this is probably not Python related, but a Windows problem (I for one could not reproduce the error given your code), I'd suggest attaching a debugger to the Python interpreter when it is started. Since you start the interpreter implicitly by a drag&drop action, you need to configure Windows to auto-attach a debugger each time Python starts. If I remember correctly, this article has the needed information to do that (you can substitute another debugger if you are not using Visual Studio).
Apart from that, I would take a snapshot with ProcMon while dragging a file onto your script, to get an idea of what is going on.

As pointed out in my edit above, the errors were caused by the working directory changing to C:\windows\system32, where the script isn't allowed to create files. I don't know how to get it to not change the working directory when started that way, but was able to work around it like this.
if len(sys.argv) == 1:
files = [filename for filename in os.listdir(os.getcwd())
if filename.endswith(".txt")]
else:
files = [filename for filename in sys.argv[1:]]
Fixing the working directory can be managed this way I guess.
exepath = sys.argv[0]
os.chdir(exepath[:exepath.rfind('\\')])

"Unknown object" error when trying to capture artwork from a pict file and embed it into a track

I'm trying to capture artwork from a pict file and embed it into a track on iTunes using python appscript.
I did something like this:
imFile = open('/Users/kartikaiyer/temp.pict','r')
data = imFile.read()
it = app('iTunes')
sel = it.current_track.get()
sel.artworks[0].data_.set(data[513:])
I get an error OSERROR: -1731
MESSAGE: Unknown object
Similar applescript code looks like this:
tell application "iTunes"
set the_artwork to read (POSIX file "/Users/kartikaiyer/temp.pict") from 513 as picture
set data of artwork 1 of current track to the_artwork
end tell
I tried using ASTranslate but it never instantiates the_artwork and then throws an error when there is a reference to the_artwork.

This is an older question, but since I was having trouble doing this same thing now, I thought I'd post my solution in case someone else might benefit.
selected = appscript.app('iTunes').selection.get()
for t in selected:
myArt = open(/path/to/image.jpg,'r')
data = myArt.read()
t.artworks[1].data_.set(data) # no need to remove header but one-indexed as has said earlier
myArt.close()
Hope this helps.

At a quick guess, Appscript references, like AppleScript references, use 1-indexing, not zero-indexing like Python lists. So you probably need to write:
it.current_track.artworks[1].data_.set(...)
(Incidentally, the extra get command in your original script is unnecessary, though harmless in this case.)
As for ASTranslate, you need to enable the 'Send events to app' checkbox if you want it to actually send commands to applications and scripting additions and receive their results. As a rule, it's best to disable this option so that you don't have any unfortunate accidents when translating potentially destructive commands such as set or delete, so only to enable it if you really need it, and be careful what code you run when you do.

The read command is part of Scripting Additions, which ASTranslate doesn't translate to. Use ASDictionary to create a glue for Scripting Additions, by clicking "Choose Installed Scripting Additions" Under the Dictionary menu, and then selecting "Scripting Additions" from the list.