I have a bunch of code that uses the old deprecated popen from the platform package. Since this is deprecated, I will be moving this to the subprocess package.
What is the equivalent statement to popen("some_command")? Is there a reason that popen was deprecated?
platform.popen has not been deprecated as best as I can tell. However, this is a low-level function that you should not make use of for flexibility and portability reasons.
Lots of other process-launching things were deprecated and some removed in Python 3. Many, many attempts at doing this well were made in the history of Python, and subprocess.Popen and its convenience functions are by far the best. After its existence the others became cruft and most of the retained ones are just there to support legacy code.
If you're going to port your code to use the subprocess module, don't look for an exact equivalent to what you have been doing, or you will miss out on the ways in which it is better. Read and understand the subprocess documentation and understand the ideas it is using to solve the problem of process-launching better than the older alternatives.
How is subprocess.Popen better than the older alternatives?
It is secure. Instead of something('shell command here'), we do Popen(['shell', 'command', 'here']). This doesn't launch an unnecessary shell process, which makes it less errorprone and dangerous.
Consider if I asked the user for their name to be input. I might write something('foo %s" % name) in the old thing. It should work--if the user gives you the name "Mike", then it becomes a command like foo Mike. But what if the user's name is "Mike Graham"? Then I want foo 'Mike Graham'. So now I always put in the apostrophes, but now what if the user's name is "Mike O'Reilley"? Worse yet, what if his name is "Mike; rm -rf /"? The solution here isn't to try to escape these yourself (which is hard to do right, let alone to do cross-platform), but to pass the arguments directly without bothering with the shell--Popen(['foo', name])`.
It is flexible. You can control the input and output fully.
It is nonblocking. Popen can run a process concurrently with yours.
Related
How would I do the following in Node.js? I realize there's probably no builtin feature or written module for this, so how might I implement this?
>>> import shlex
>>> shlex.split("-a arga -b \"argument b\" arg1 arg2")
['-a', 'arga', '-b', 'argument b', 'arg1', 'arg2']
I assume you've already searched http://npmjs.org (either searching, or browsing the shell keyword) instead of just assuming no such thing exists. At a quick glance, for example, various packages like shell-quote seem likely to do what you want, and others like nshell seem likely to either depend on a shlex-like library or to have equivalent code internally, but I haven't actually looked at any of them in detail, so I'm willing to accept that there's nothing out there you can use.
Getting all of the details right is complicated. But fortunately, the source code for Python's shlex.split is written in pure Python, and is reasonably readable. So, you should be able to port it.
If you do this, you should ideally also build a good test suite and publish it as an npm package so that the next time someone else looks, it will exist at http://npmjs.org.
I've done a basic port of shlex to Node.js: https://www.npmjs.com/package/shlex
shell-quote appears to be abandoned, with several open issues and pull requests, but the author has not responded to them in a while.
I have a Python 3 file. I want to use an open-source tool on the internet (nltk), but unfortunately it only supports Python 2. There is no way for me to convert it to Python 3, nor can I convert my Python 3 file to Python 2.
If the user does not give a certain argument (on argparse) then I do something in my file. If the user does give a certain argument, however, I need to use nltk.
Writing a Python 2 script that uses nltk and then executing script that in my Python 3 script
My current idea is to write a script in Python 2 that does what I want with nltk and then run that from my current Python 3 script. However, I don't actually know how to do this.
I found this code: os.system(command) and so I will modify it to be os.system("python py2.py") (where py2.py is my newly written Python 2 file).
I'm not sure if that will work.
I also don't know if that is the most efficient way to solve my problem. I cannot find any information about it on the internet.
The data transferred will probably be quite large. Currently, my test data is about 6600 lines, utf-8. Functionality is more important than how long it takes (to a certain extent) in my case.
Also, how would I pass values from my Python 2 script to my Python 3 script?
Thanks
Is there any other way to do this?
Well, if you're sure you can't convert your script to Python 2, then having one script call the other by running the Python interpreter probably is the best way. (And, this being Python, the best way is, or at least should be, the only way.)
But are you sure? Between the six module, the 3to2 tool, and __future__ statements, it may not be as hard as you think.
Anyway, if you do need to have one script call the other, you should almost never use os.system. As the docs for that function say:
The subprocess module provides more powerful facilities for spawning new processes and retrieving their results; using that module is preferable to using this function. See the Replacing Older Functions with the subprocess Module section in the subprocess documentation for some helpful recipes.
The simplest version is this:
subprocess.check_call(["python", "py2.py"])
This runs your script, waits for it to finish, and raises an exception if the script returns failure—basically, what you wanted to do with os.system, but better. (For example, it doesn't spawn an unnecessary extra shell, it takes care of error handling, etc.)
That assumes whatever other data you need to share is being shared in some implicit, external way (e.g., by accessing files with the same name). You might be better off passing data to py2.py as command-line arguments and/or stdin, passing data back as via stdout, or even opening an explicit pipe or socket to pass things over. Without knowing more about exactly what you need to do, it's hard to suggest anything, but the docs, especially the section Replacing Older Functions with the subprocess Module have lots of discussion on the options.
To give you an idea, here's a simple example: to pass one of your filename arguments to py2.py, and then get data back from py2.py to py3.py, just have py3.py do this:
py2output = subprocess.check_output(["python", "py2.py", my_args[0]])
And then in py2.py, just print whatever you want to send back.
The Anyone hear when NLTK 3.0 will be out? here in SO points out that...
There's a Python 3 branch:
https://github.com/nltk/nltk/tree/nltk-py3k
The answer is from July 2011. It could be improved since then.
I have just looked at https://github.com/nltk/nltk. There is at least the document that talks about Python 3 port related things https://github.com/nltk/nltk/blob/2and3/web/dev/python3porting.rst.
Here is a longer discussion on NLTK and Python 3 that you may be interested in.
And the Grants to Assist Kivy, NLTK in Porting to Python 3 (published 3 days ago) is directly related to the problem.
Spinning off from another thread, when is it appropriate to use os.system() to issue commands like rm -rf, cd, make, xterm, ls ?
Considering there are analog versions of the above commands (except make and xterm), I'm assuming it's safer to use these built-in python commands instead of using os.system()
Any thoughts? I'd love to hear them.
Rule of thumb: if there's a built-in Python function to achieve this functionality use this function. Why? It makes your code portable across different systems, more secure and probably faster as there will be no need to spawn an additional process.
One of the problems with system() is that it implies knowledge of the shell's syntax and language for parsing and executing your command line. This creates potential for a bug where you didn't validate input properly, and the shell might interpet something like variable substitution or determining where an argument begins or ends in a way you don't expect. Also, another OS's shell might have divergent syntax from your own, including very subtle divergence that you won't notice right away. For reasons like these I prefer to use execve() instead of system() -- you can pass argv tokens directly and not have to worry about something in the middle (mis-)parsing your input.
Another problem with system() (this also applies to using execve()) is that when you code that, you are saying, "look for this program, and pass it these args". This makes a couple of assumptions which may lead to bugs. First is that the program exists and can be found in $PATH. Maybe on some system it won't. Second, maybe on some system, or even a future version of your own OS, it will support a different set of options. In this sense, I would avoid doing this unless you are absolutely certain the system you will run on will have the program. (Like maybe you put the callee program on the system to begin with, or the way you invoke it is mandated by something like POSIX.)
Lastly... There's also a performance hit associated with looking for the right program, creating a new process, loading the program, etc. If you are doing something simple like a mv, it's much more efficient to use the system call directly.
These are just a few of the reasons to avoid system(). Surely there are more.
Darin's answer is a good start.
Beyond that, it's a matter of how portable you plan to be. If your program is only ever going to run on a reasonably "standard" and "modern" Linux then there's no reason for you to re-invent the wheel; if you tried to re-write make or xterm they'd be sending the men in the white coats for you. If it works and you don't have platform concerns, knock yourself out and simply use Python as glue!
If compatibility across unknown systems was a big deal you could try looking for libraries to do what you need done in a platform independent way. Or you need to look into a way to call on-board utilities with different names, paths and mechanisms depending on which kind of system you're on.
The only time that os.system might be appropriate is for a quick-and-dirty solution for a non-production script or some kind of testing. Otherwise, it is best to use built-in functions.
Your question seems to have two parts. You mention calling commands like "xterm", "rm -rf", and "cd".
Side Note: you cannot call 'cd' in a sub-shell. I bet that was a trick question ...
As far as other command-level things you might want to do, like "rm -rf SOMETHING", there is already a python equivalent. This answers the first part of your question. But I suspect you are really asking about the second part.
The second part of your question can be rephrased as "should I use system() or something like the subprocess module?".
I have a simple answer for you: just say NO to using "system()", except for prototyping.
It's fine for verifying that something works, or for that "quick and dirty" script, but there are just too many problems with os.system():
It forks a shell for you -- fine if you need one
It expands wild cards for you -- fine unless you don't have any
It handles redirect -- fine if you want that
It dumps output to stderr/stdout and reads from stdin by default
It tries to understand quoting, but it doesn't do very well (try 'Cmd" > "Ofile')
Related to #5, it doesn't always grok argument boundaries (i.e. arguments with spaces in them might get screwed up)
Just say no to "system()"!
I would suggest that you only use use os.system for things that there are not already equivalents for within the os module. Why make your life harder?
The os.system call is starting to be 'frowned upon' in python. The 'new' replacement would be subprocess.call or subprocess.Popen in the subprocess module. Check the docs for subprocess
The other nice thing about subprocess is you can read the stdout and stderr into variables, and process that without having to redirect to other file(s).
Like others have said above, there are modules for most things. Unless you're trying to glue together many other commands, I'd stick with the things included in the library. If you're copying files, use shutil, working with archives you've got modules like tarfile/zipfile and so on.
Good luck.
I'm noticing that even for system modules, code completion doesn't work too well.
For example, if I have a simple file that does:
import re
p = re.compile(pattern)
m = p.search(line)
If I type p., I don't get completion for methods I'd expect to see (I don't see search() for example, but I do see others, such as func_closure(), func_code()).
If I type m., I don't get any completion what so ever (I'd expect .groups(), in this case).
This doesn't seem to affect all modules.. Has any one seen this behaviour and knows how to correct it?
I'm running Vim 7.2 on WinXP, with the latest pythoncomplete.vim from vim.org (0.9), running python 2.6.2.
Completion for this kind of things is tricky, because it would need to execute the actual code to work.
For example p.search() could return None or a MatchObject, depending on the data that is passed to it.
This is why omni-completion does not work here, and probably never will. It works for things that can be statically determined, for example a module's contents.
I never got the builtin omnicomplete to work for any languages. I had the most success with pysmell (which seems to have been updated slightly more recently on github than in the official repo). I still didn't find it to be reliable enough to use consistently but I can't remember exactly why.
I've resorted to building an extensive set of snipMate snippets for my primary libraries and using the default tab completion to supplement.
I need to be able to list the command line arguments (if any) passed to other running processes. I have the PIDs already of the running processes on the system, so basically I need to determine the arguments passed to process with given PID XXX.
I'm working on a core piece of a Python module for managing processes. The code is written as a Python extension in C and will be wrapped by a higher level Python library. The goal of this project is to avoid dependency on third party libs such as the pywin32 extensions, or on ugly hacks like calling 'ps' or taskkill on the command line, so I'm looking for a way to do this in C code.
I've Googled this around and found some brief suggestions of using CreateRemoteThread() to inject myself into the other process, then run GetCommandLine() but I was hoping someone might have some working code samples and/or better suggestions.
UPDATE: I've found full working demo code and a solution using NtQueryProcessInformation on CodeProject: http://www.codeproject.com/KB/threads/GetNtProcessInfo.aspx - It's not ideal since it's "unsupported" to cull the information directly from the NTDLL structures but I'll live with it. Thanks to all for the suggestions.
UPDATE 2: I managed through more Googling to dig up a C version that does not use C++ code, and is a little more direct/concisely pointed toward this problem. See http://wj32.wordpress.com/2009/01/24/howto-get-the-command-line-of-processes/ for details.
Thanks!
To answer my own question, I finally found a CodeProject solution that does exactly what I'm looking for:
http://www.codeproject.com/KB/threads/GetNtProcessInfo.aspx
As #Reuben already pointed out, you can use NtQueryProcessInformation to retrieve this information. Unfortuantely it's not a recommended approach, but given the only other solution seems to be to incur the overhead of a WMI query, I think we'll take this approach for now.
Note that this seems to not work if using code compiled from 32bit Windows on a 64bit Windows OS, but since our modules are compiled from source on the target that should be OK for our purposes. I'd rather use this existing code and should it break in Windows 7 or a later date, we can look again at using WMI. Thanks for the responses!
UPDATE: A more concise and C only (as opposed to C++) version of the same technique is illustrated here:
http://wj32.wordpress.com/2009/01/24/howto-get-the-command-line-of-processes/
The cached solution:
http://74.125.45.132/search?q=cache:-wPkE2PbsGwJ:windowsxp.mvps.org/listproc.htm+running+process+command+line&hl=es&ct=clnk&cd=1&gl=ar&client=firefox-a
in CMD
WMIC /OUTPUT:C:\ProcessList.txt PROCESS get Caption,Commandline,Processid
or
WMIC /OUTPUT:C:\ProcessList.txt path win32_process get Caption,Processid,Commandline
Also:
http://mail.python.org/pipermail/python-win32/2007-December/006498.html
http://tgolden.sc.sabren.com/python/wmi_cookbook.html#running_processes
seems to do the trick:
import wmi
c = wmi.WMI ()
for process in c.Win32_Process ():
print process.CommandLine
By using psutil ( https://github.com/giampaolo/psutil ):
>>> import psutil, os
>>> psutil.Process(os.getpid()).cmdline()
['C:\\Python26\\python.exe', '-O']
>>>
The WMI approach mentioned in another response is probably the most reliable way of doing this. Looking through MSDN, I spotted what looks like another possible approach; it's documented, but its not clear whether it's fully supported. In MSDN's language, it--
may be altered or unavailable in
future versions of Windows...
In any case, provided that your process has the right permissions, you should be able to call NtQueryProcessInformation with a ProcessInformationClass of ProcessBasicInformation. In the returned PROCESS_BASIC_INFORMATION structure, you should get back a pointer to the target process's process execution block (as field PebBaseAddress). The ProcessParameters field of the PEB will give you a pointer to an RTL_USER_PROCESS_PARAMETERS structure. The CommandLine field of that structure will be a UNICODE_STRING structure. (Be careful not too make too many assumptions about the string; there are no guarantees that it will be NULL-terminated, and it's not clear whether or not you'll need to strip off the name of the executed application from the beginning of the command line.)
I haven't tried this approach--and as I mentioned above, it seems a bit... iffy (read: non-portable)--but it might be worth a try. Best of luck...
If you aren't the parent of these processes, then this is not possible using documented functions :( Now, if you're the parent, you can do your CreateRemoteThread trick, but otherwise you will almost certainly get Access Denied unless your app has admin rights.