Is there a difference between os.execl() and os.execv() in python? I was using
os.execl(python, python, *sys.argv)
to restart my script (from here). But it seems to start from where the previous script left.
I want the script to start from the beginning when it restarts. Will this
os.execv(__file__,sys.argv)
do the job? command and idea from here. I couldn't find difference between them from the python help/documentation. Is there a way do clean restart?
For a little more background on what I am trying to do please see my other question
At the low level they do the same thing: they replace the running process image with a new process.
The only difference between execv and execl is the way they take arguments. execv expects a single list of arguments (the first of which should be the name of the executable), while execl expects a variable list of arguments.
Thus, in essence, execv(file, args) is exactly equivalent to execl(file, *args).
Note that sys.argv[0] is already the script name. However, this is the script name as passed into Python, and may not be the actual script name that the program is running under. To be correct and safe, your argument list passed to exec* should be
['python', __file__] + sys.argv[1:]
I have just tested a restart script with the following:
os.execl(sys.executable, 'python', __file__, *sys.argv[1:])
and this works fine. Be sure you're not ignoring or silently catching any errors from execl - if it fails to execute, you'll end up "continuing where you left off".
According to the Python documentation there's no real functional difference between execv and execl:
The “l” and “v” variants of the exec* functions differ in how command-line arguments are passed. The “l” variants are perhaps the easiest to work with if the number of parameters is fixed when the code is written; the individual parameters simply become additional parameters to the execl*() functions. The “v” variants are good when the number of parameters is variable, with the arguments being passed in a list or tuple as the args parameter. In either case, the arguments to the child process should start with the name of the command being run, but this is not enforced.
No idea why one seems to restart the script where it left off but I'd guess that that is unrelated.
Related
I know a very similar question has already been asked but since none of the solutions posted there worked for my problem I try to make it replicable:
So I'm calling this script to merge some shapefiles (all files in one folder) like this:
shpfiles = 'shapefile_a.shp shapefile_b.shp'
subprocess.call(['python', 'shapemerger.py', '%s' % shpfiles])
I only get the Usage Instructions from the script so I cant determine what goes wrong. If i call the script directly in the terminal it works.
Any help is appreciated.
Every time a program is started, it receives a list of arguments it was invoked with. This is often called argv (v stands for vector, i.e. one-dimensional array). The program parses this list, extracts options, parameters, filenames, etc. depending on its own invocation syntax.
When working at the command line, the shell takes care of parsing the input line, starting new program or programs and passing them their argument list.
When a program is called from another program, the caller is responsible to provide the correct arguments. It could delegate this work to shell. The price for it is very high. There is substantial overhead and possibly a security risk! Avoid this approach whenever possible.
Finally to the question itself:
shpfiles = 'shapefile_a.shp shapefile_b.shp'
subprocess.call(['python', 'shapemerger.py', '%s' % shpfiles])
This will invoke python to run the script shapemerger.py with one argument shapefile_a.shp shapefile_b.shp. The script expects filenames and receives this one name. The file "shapefile_a.shp shapefile_b.shp" does not exist, but the script probably stops before attempting to access that file, because it expect 2 or more files to process.
The correct way is to pass every filename as one argument. Assuming shpfiles is a whitespace separated list:
subprocess.call(['python', 'shapemerger.py'] + shpfiles.split())
will generate a list with 4 items. It is important to understand that this approach will fail if there is a space in a filename.
I am trying to use Python to run an executable (Windows 7) with parameters. I have been able to make the program run, but the amount of parameters I can use that will prove the Python script worked with parameters is limited. The best one is formatted like so:
-debugoutput debug.txt
I have tested this using a windows shortcut with an edited target and it works, it creates a debug output in the program directory.
Here is the code I am using:
import subprocess
args = [r"C:\Users\MyName\LevelEditor\LevelEditor.exe", "-debugoutput debug.txt"]
subprocess.call(args)
This does run the program, but the debug output is not created. I have tried putting an "r" in front of the parameter but this made no difference. I assume it is a simple formatting error but I can't find any examples to learn from that are doing the same thing.
UPDATE:
Thanks for the answers everyone, all the same, simple formatting error indeed.
In-code definition results in invocation of shell command line:
C:\Users\MyName\LevelEditor\LevelEditor.exe "-debugoutput debug.txt"
As you can see, by merging -debugoutput debug.txt to single list element, you explicitly stated that space between them shouldn't be parsed as command line argument separator.
To achieve expected behavior put file name string as separate element to argument list.
[r"C:\Users\MyName\LevelEditor\LevelEditor.exe", "-debugoutput", "debug.txt"]
As far as I know you need to split the arguments by the space, so your args would look like:
args = [r"C:\Users\MyName\LevelEditor\LevelEditor.exe", "-debugoutput", "debug.txt"]
Does that work?
I do not know if it works, but
import subprocess
args = [r"C:\Users\MyName\LevelEditor\LevelEditor.exe", "-debugoutput", "debug.txt"]
subprocess.run(args)
Following the docs
I try to invoke an external perl script in my python script. I used subprocess.Popen(). If run it like
subprocess.Popen([mybinary, '-arg1 argv1', '-arg2 argv2'])
the arguments are not sent to mybinary. But if I separate arguments from values, then it runs properly:
subprocess.Popen([mybinary, '-arg1', 'argv1', '-arg2', 'argv2'])
Why is it so? args needs to be a string or list. If I concatenate mybinary and the arguments into a single string for Popen(), Popen() does not work, either. I suspect it is relevant to the key-worded arguments (**kwargs). But the script invoked is external. I don't see the reason.
I try to invoke an external perl script in my python script. I used
subprocess.Popen(). If run it like
subprocess.Popen([mybinary, '-arg1 argv1', '-arg2 argv2'])
the arguments are not sent to mybinary.
I doubt that. I rather think they come along there in a non-proper way.
The script expects options and their values as two separate arguments. If they are combined, it doesn't work.
The arguments are passed in shape and count exactly as you give them in the list.
Is anyone able to tell me how to write a conditional for an argument on a python script? I want it to print "Argument2 Entered" if it is run with a second command line arguments such as:
python script.py argument1 argument2
And print "No second argument" if it is run without command line arguments, like this:
python script.py argument1
Is this possible?
import sys
if len(sys.argv)==2: # first entry in sys.argv is script itself...
print "No second argument"
elif len(sys.argv)==3:
print "Second argument"
There are many answers to this, depending on what exactly you want to do and how much flexibility you are likely to need.
The simplest solution is to examine the variable sys.argv, which is a list containing all of the command-line arguments. (It also contains the name of the script as the first element.) To do this, simply look at len(sys.argv) and change behaviour based on its value.
However, this is often not flexible enough for what people expect command-line programs to do. For example, if you want a flag (-i, --no-defaults, ...) then it's not obvious how to write one with just sys.argv. Likewise for arguments (--dest-dir="downloads"). There are therefore many modules people have written to simplify this sort of argument parsing.
The built-in solution is argparse, which is powerful and pretty easy-to-use but not particularly concise.
A clever solution is plac, which inspects the signature of the main function to try to deduce what the command-line arguments should be.
There are many ways to do this simple thing in Python. If you are interested to know more than I recommend to read this article. BTW I am giving you one solution below:
import click
'''
Prerequisite: # python -m pip install click
run: python main.py ttt yyy
'''
#click.command(context_settings=dict(ignore_unknown_options=True))
#click.argument("argument1")
#click.argument("argument2")
def main(argument1, argument2):
print(f"argument1={argument1} and argument2={argument2}")
if __name__ == '__main__':
main()
Following block should be self explanatory
$ ./first.py second third 4th 5th
5
$ cat first.py
#!/usr/bin/env python
import sys
print (len(sys.argv))
This is related to many other posts depending upon where you are going with this, so I'll put four here:
What's the best way to grab/parse command line arguments passed to a Python script?
Implementing a "[command] [action] [parameter]" style command-line interfaces?
How can I process command line arguments in Python?
How do I format positional argument help using Python's optparse?
But the direct answer to your question from the Python docs:
sys.argv -
The list of command line arguments passed to a Python script. argv[0] is the script name (it is operating system dependent whether this is a full pathname or not). If the command was executed using the -c command line option to the interpreter, argv[0] is set to the string '-c'. If no script name was passed to the Python interpreter, argv[0] is the empty string.
To loop over the standard input, or the list of files given on the command line, see the fileinput module.
recently I started using Emacs as a Python IDE, and it not quite intuitive... The problem I am struggling with right now is how to pass command line arguments to the inferior python shell when the buffer is evaluated with C-c C-c. Thanks for help.
This doesn't appear to be easily possible; the inferior process managed by the python.el module is designed to persist across many invocations of python-send-buffer (and friends). One solution I've found is to write your own function that sets sys.argv programmatically from within the inferior process:
(defun python-send-buffer-with-my-args (args)
(interactive "sPython arguments: ")
(let ((source-buffer (current-buffer)))
(with-temp-buffer
(insert "import sys; sys.argv = '''" args "'''.split()\n")
(insert-buffer-substring source-buffer)
(python-send-buffer))))
Execute this function in your *scratch* buffer and/or save it in your .emacs file, then, if you want, bind it to a convenient key sequence. C-c C-a doesn't seem to be used by python-mode, so perhaps:
(global-set-key "\C-c\C-a" 'python-send-buffer-with-my-args)
The command will prompt you for arguments to use, then copy your source buffer into a temporary buffer, prepending it with a code snippet that sets sys.argv to the list of arguments you supplied, and finally will call python-send-buffer.
The above code will just naively split the string you type on whitespace, so if you need to supply arguments that have whitespace in them, you'll need a more sophisticated algorithm.