Develop Reference

Send literal string from python-vim script to a tmux pane

I am using Vim (8.0) and tmux (2.3) together in the following way: In a tmux session I have a window split to 2 panes, one pane has some text file open in Vim, the other pane has some program to which I want to send lines of text. A typical use case is sending lines from a Python script to IPython session running in the other pane.
I am doing this by a Vim script which uses python, code snippet example (assuming the target tmux pane is 0):
py import vim
python << endpython
cmd = "print 1+2"
vim_cmd = "silent !tmux send-keys -t 0 -l \"" + cmd + "\"" # -l for literal
vim.command(vim_cmd)
endpython
This works well, except for when cmd has some characters which has to be escaped, like %, #, $, etc. The cmd variable is read from the current line in the text file opened in Vim, so I can do something like cmd = cmd.replace('%', '\%') etc., but this has 2 disadvantages: first, I don't know all the vim characters which have to be escaped, so it has been trial and error up until now, and second, the characters " is not escaped properly - that is in the string Vim gets, the " just disappears, even if I do cmd = cmd.replace('"', '\"').
So, is there a general way to tell Vim to not interpret anything, just get a raw string and send it as is? If not, why is the " not escaped properly?

Vimscript
You're looking for the shellescape() function. If you use the :! Vim command, the {special} argument needs to be 1.
silent execute '!tmux send-keys -t 0 -l ' . shellescape(cmd, 1)
But as you're not interested in (displaying) the shell output, and do not need to interact with the launched script, I would switch from :! to the lower-level system() command.
call system('tmux send-keys -t 0 -l ' . shellescape(cmd))
Python
The use of Python (instead of pure Vimscript) doesn't have any benefits (at least in the small snippet in your question). Instead, if you embed the Python cmd variable in a Vimscript expression, now you also need a Python function to escape the value as a Vimscript string (something like '%s'" % str(cmd).replace("'", "''"))). Alternatively, you could maintain the value in a Vim variable, and access it from Python via vim.vars.

python -c vs python -<< heredoc

I am trying to run some piece of Python code in a Bash script, so i wanted to understand what is the difference between:
#!/bin/bash
#your bash code
python -c "
#your py code
"
vs
python - <<DOC
#your py code
DOC
I checked the web but couldn't compile the bits around the topic. Do you think one is better over the other?
If you wanted to return a value from Python code block to your Bash script then is a heredoc the only way?

The main flaw of using a here document is that the script's standard input will be the here document. So if you have a script which wants to process its standard input, python -c is pretty much your only option.
On the other hand, using python -c '...' ties up the single-quote for the shell's needs, so you can only use double-quoted strings in your Python script; using double-quotes instead to protect the script from the shell introduces additional problems (strings in double-quotes undergo various substitutions, whereas single-quoted strings are literal in the shell).
As an aside, notice that you probably want to single-quote the here-doc delimiter, too, otherwise the Python script is subject to similar substitutions.
python - <<'____HERE'
print("""Look, we can have double quotes!""")
print('And single quotes! And `back ticks`!')
print("$(and what looks to the shell like process substitutions and $variables!)")
____HERE
As an alternative, escaping the delimiter works identically, if you prefer that (python - <<\____HERE)

If you are using bash, you can avoid heredoc problems if you apply a little bit more of boilerplate:
python <(cat <<EoF
name = input()
print(f'hello, {name}!')
EoF
)
This will let you run your embedded Python script without you giving up the standard input. The overhead is mostly the same of using cmda | cmdb. This technique is known as Process Substitution.
If want to be able to somehow validate the script, I suggest that you dump it to a temporary file:
#!/bin/bash
temp_file=$(mktemp my_generated_python_script.XXXXXX.py)
cat > $temp_file <<EoF
# embedded python script
EoF
python3 $temp_file && rm $temp_file
This will keep the script if it fails to run.

If you prefer to use python -c '...' without having to escape with the double-quotes you can first load the code in a bash variable using here-documents:
read -r -d '' CMD << '--END'
print ("'quoted'")
--END
python -c "$CMD"
The python code is loaded verbatim into the CMD variable and there's no need to escape double quotes.

How to use here-docs with input
tripleee's answer has all the details, but there's Unix tricks to work around this limitation:
So if you have a script which wants to process its standard input, python -c is pretty much your only option.
This trick applies to all programs that want to read from a redirected stdin (e.g., ./script.py < myinputs) and also take user input:
python - <<'____HERE'
import os
os.dup2(1, 0)
print(input("--> "))
____HERE
Running this works:
$ bash heredocpy.sh
--> Hello World!
Hello World!
If you want to get the original stdin, run os.dup(0) first. Here is a real-world example.
This works because as long as either stdout or stderr are a tty, one can read from them as well as write to them. (Otherwise, you could just open /dev/tty. This is what less does.)
In case you want to process inputs from a file instead, that's possible too -- you just have to use a new fd:
Example with a file
cat <<'____HERE' > file.txt
With software there are only two possibilites:
either the users control the programme
or the programme controls the users.
____HERE
python - <<'____HERE' 4< file.txt
import os
for line in os.fdopen(4):
print(line.rstrip().upper())
____HERE
Example with a command
Unfortunately, pipelines don't work here -- but process substitution does:
python - <<'____HERE' 4< <(fortune)
import os
for line in os.fdopen(4):
print(line.rstrip().upper())
____HERE

Using cat command in Python for printing

In the Linux kernel, I can send a file to the printer using the following command
cat file.txt > /dev/usb/lp0
From what I understand, this redirects the contents in file.txt into the printing location. I tried using the following command
>>os.system('cat file.txt > /dev/usb/lp0')
I thought this command would achieve the same thing, but it gave me a "Permission Denied" error. In the command line, I would run the following command prior to concatenating.
sudo chown root:lpadmin /dev/usb/lp0
Is there a better way to do this?

While there's no reason your code shouldn't work, this probably isn't the way you want to do this. If you just want to run shell commands, bash is much better than python. On the other hand, if you want to use Python, there are better ways to copy files than shell redirection.
The simplest way to copy one file to another is to use shutil:
shutil.copyfile('file.txt', '/dev/usb/lp0')
(Of course if you have permissions problems that prevent redirect from working, you'll have the same permissions problems with copying.)
You want a program that reads input from the keyboard, and when it gets a certain input, it prints a certain file. That's easy:
import shutil
while True:
line = raw_input() # or just input() if you're on Python 3.x
if line == 'certain input':
shutil.copyfile('file.txt', '/dev/usb/lp0')
Obviously a real program will be a bit more complex—it'll do different things with different commands, and maybe take arguments that tell it which file to print, and so on. If you want to go that way, the cmd module is a great help.

Remember, in UNIX - everything is a file. Even devices.
So, you can just use basic (or anything else, e.g. shutil.copyfile) files methods (http://docs.python.org/2/tutorial/inputoutput.html#reading-and-writing-files).
In your case code may (just a way) be like that:
# Read file.txt
with open('file.txt', 'r') as content_file:
content = content_file.read()
with open('/dev/usb/lp0', 'w') as target_device:
target_device.write(content)
P. S. Please, don't use system() call (or similar) to solve your issue.

under windows OS there is no cat command you should usetype instead of cat under windows
(**if you want to run cat command under windows please look at: https://stackoverflow.com/a/71998867/2723298 )
import os
os.system('type a.txt > copy.txt')
..or if your OS is linux and cat command didn't work anyway here are other methods to copy file..
with grep:
import os
os.system('grep "" a.txt > b.txt')
*' ' are important!
copy file with sed:
os.system('sed "" a.txt > sed.txt')
copy file with awk:
os.system('awk "{print $0}" a.txt > awk.txt')

Open a new window in Vim-embedded python script

I've just started wrapping my head around vim+python scripts (having no experience with native vim scripts).
How can I open a new window to contain the stdout from a background process?
Currently, after reading some :help python, the only option I see is something like:
cmd = ":bel new"
vim.command(cmd)

Since vim.command can execute most (if not all?) ex commands, you can simply call :new +read!ls from within it.
:new splits the current window and puts a new (empty, no name) buffer into the upper window. It takes an argument +[cmd] which we use to execute read!cmd which reads the stdout of cmd after the bang into the buffer. Be aware that you need to escape spaces in your command with \
All in all you get vim.command("new +read!cmd")
:python vim.command("new +read!ls")
to read in the contents of the current directory into a new buffer in a n cichew, horizontally split window.
If you want to handle escaping of special characters, consider using python's re.escape():
:py import re;vim.command("new +read!"+re.escape("ls Dire*"))
which should be sufficient for most cases. If in doubt, check its documentation and compare it to that of your shell.

Passing a multi-line string as an argument to a script in Windows

I have a simple python script like so:
import sys
lines = sys.argv[1]
for line in lines.splitlines():
print line
I want to call it from the command line (or a .bat file) but the first argument may (and probably will) be a string with multiple lines in it. How does one do this?
Of course, this works:
import sys
lines = """This is a string
It has multiple lines
there are three total"""
for line in lines.splitlines():
print line
But I need to be able to process an argument line-by-line.
EDIT: This is probably more of a Windows command-line problem than a Python problem.
EDIT 2: Thanks for all of the good suggestions. It doesn't look like it's possible. I can't use another shell because I'm actually trying to invoke the script from another program which seems to use the Windows command-line behind the scenes.

I know this thread is pretty old, but I came across it while trying to solve a similar problem, and others might as well, so let me show you how I solved it.
This works at least on Windows XP Pro, with Zack's code in a file called
"C:\Scratch\test.py":
C:\Scratch>test.py "This is a string"^
More?
More? "It has multiple lines"^
More?
More? "There are three total"
This is a string
It has multiple lines
There are three total
C:\Scratch>
This is a little more readable than Romulo's solution above.

Just enclose the argument in quotes:
$ python args.py "This is a string
> It has multiple lines
> there are three total"
This is a string
It has multiple lines
there are three total

The following might work:
C:\> python something.py "This is a string^
More?
More? It has multiple lines^
More?
More? There are three total"

This is the only thing which worked for me:
C:\> python a.py This" "is" "a" "string^
More?
More? It" "has" "multiple" "lines^
More?
More? There" "are" "three" "total
For me Johannes' solution invokes the python interpreter at the end of the first line, so I don't have the chance to pass additional lines.
But you said you are calling the python script from another process, not from the command line. Then why don't you use dbr' solution? This worked for me as a Ruby script:
puts `python a.py "This is a string\nIt has multiple lines\nThere are three total"`
And in what language are you writing the program which calls the python script? The issue you have is with argument passing, not with the windows shell, not with Python...
Finally, as mattkemp said, I also suggest you use the standard input to read your multi-line argument, avoiding command line magic.

Not sure about the Windows command-line, but would the following work?
> python myscript.py "This is a string\nIt has multiple lines\there are three total"
..or..
> python myscript.py "This is a string\
It has [...]\
there are [...]"
If not, I would suggest installing Cygwin and using a sane shell!

Have you tried setting you multiline text as a variable and then passing the expansion of that into your script. For example:
set Text="This is a string
It has multiple lines
there are three total"
python args.py %Text%
Alternatively, instead of reading an argument you could read from standard in.
import sys
for line in iter(sys.stdin.readline, ''):
print line
On Linux you would pipe the multiline text to the standard input of args.py.
$ <command-that-produces-text> | python args.py