I am attempting to pass in a string as input argument to a Python program, from the command line i.e. $python parser_prog.py <pos1> <pos2> --opt1 --opt2 and interpreting these using argparse. Of course if contains any metacharacters these are first interpreted by the shell, so it needs to be quoted.
This seems to work, strings are passed through literally, preserving the \*?! characters:
$ python parser_prog.py 'str\1*?' 'str2!'
However, when I attempt to pass through a '-' (hyphen) character, I cannot seem to mask it. It is interpreted as an invalid option.
$ python parser_prog.py 'str\1*?' '-str2!'
I have tried single and double quotes, is there a way to make sure Python interprets this as a raw string? (I'm not in the interpreter yet, this is on the shell command line, so I can't use pythonic expressions such as r'str1')
Thank you for any hints!
As you said yourself, Python only sees the strings after being processed by the shell. The command-line arguments '-f' and -f look identical to the called program, and there is no way to dsitinguish them. That said, I think that argparse supports a -- argument to denote the end of the options, and everything after this is treated as a positional argument.
Related
Oddly enough, when running this program with the arguments of
program.py "(lp0\nS'cat'\np1\naI5\na."
With program.py being:
import sys,pickle
print sys.argv[1]=="(lp0\nS'cat'\np1\naI5\na."
False is printed... I have narrowed the difference in evaluation to the \n character however I can find no way of ignoring such.
Why is this and how can I fix it?
You need to use raw string literal like this:
sys.argv[1] == r"(lp0\nS'cat'\np1\naI5\na."
Also, you can use a string in the parameters without quotes.
It is because the syntax of strings in Python and in the shell (presumably Bash) is different.
You may want to run the program as
echo $'"(lp0\nS\'cat\'\np1\naI5\na.'
program.py $'"(lp0\nS\'cat\'\np1\naI5\na.'
I have a script that I run from the command line which I would like to be able to pass string arguments into. As in
script.py --string "thing1\nthing2"
such that the program would interpret the '\n' as a new line. If string="thing1\nthing2" I want to get
print string
to return:
thing1
thing2
rather than thing1\nthing2
If I simply hard-code the string "thing1\nthing2" into the script, it does this, but if it's entered as a command line argument via getopt, it doesn't recognize it. I have tried a number of approaches to this: reading in the cl string as r"%s" % arg, various ways of specifying it on the commandline, etc, and nothing seems to work. Ideas? Is this completely impossible?
From https://stackoverflow.com/a/4918413/478656 in Bash, you can use:
script.py --string $'thing1\nthing2'
e.g.
$ python test.py $'1\n2'
1
2
But that's Bash-specific syntax.
This is really a shell question since the shell does all the command parsing. Python doesn't care what's happening with that and only gets what comes through in the exec system call. If you're using bash, it doesn't do certain kinds of escaping between double quotes. If you want things like \n, \t, or \xnn to be escaped, the following syntax is a bash extension:
python test.py $'thing1\nthing2'
Note that the above example uses single quotes and not double quotes. That's important. Using double quotes causes different rules to apply. You can also do:
python test.py "thing1
thing2"
Here's some more info on bash quoting if you're interested. Even if you're not using bash, it's still good reading:
http://mywiki.wooledge.org/Quotes
This one is relatively simple and I am surprised no one has said it.
In your python script just write the following code
print string.replace("\\n", "\n")
and you will get the string printed with the new line and not the \n.
I'm trying to run this script using the code
subprocess.call(["php C:\Python27\a.php"])
and I'm getting this error:
FileNotFoundError: [WinError 2] The system cannot find the file specified
i have tried changing path but nothing seems to work, any ideas?
Either
subprocess.call(["php", "C:\\Python27\\a.php"])
or
subprocess.call(["php", r"C:\Python27\a.php"])
should work.
Try this:
subprocess.call(["php", "C:\\Python27\\a.php"])
From the documentation:
args is required for all calls and should be a string, or a sequence
of program arguments. Providing a sequence of arguments is generally
preferred, as it allows the module to take care of any required
escaping and quoting of arguments (e.g. to permit spaces in file
names). If passing a single string, either shell must be True (see
below) or else the string must simply name the program to be executed
without specifying any arguments.
Also note that in python, like many other languages, the backslash in a normal string has special meaning. You'll have to use double backslashes or a raw string to get the behavior you want.
I am calling a python script through a bash wrapper, but I'm having trouble dealing with arguments that contain quoted spaces.
I assemble the arguments to the python script into a bash variable, such as
opt="-c start.txt"
opt+="--self 'name Na'"
Then call the python script with something like:
python test_args.py $opt
When printing sys.argv in Python, I get
['test-args.py', '-c', 'start.txt', '--self', "'name", "Na'"]
instead of the expected
['test-args.py', '-c', 'start.txt', '--self', 'name Na']
I tried using an array when calling the script, such as
python test_args.py ${opt[#]}
but then I get
['test-args.py', "-c start.txt --self 'name Na'"]
Any other ideas?
Use an array, but store each argument as a separate element in the array:
opt=(-c start.txt)
opt+=(--self 'name Na')
python test_args.py "${opt[#]}"
See BashFAQ #050.
This is what the shlex module is for.
The shlex class makes it easy to write lexical analyzers for simple
syntaxes resembling that of the Unix shell. This will often be useful
for writing minilanguages, (for example, in run control files for
Python applications) or for parsing quoted strings.
Your instinct to embed spaces inside the variable's value was good, but when the value is simply expanded during the command line parsing their special meaning is lost as you saw. You need to expand the variable before the command line to your python script is parsed:
set -f
eval python test_args.py $opt
set +f
That will expand to:
python test_args.py -c start.txt --self 'name Na'
Which will then be parsed correctly with the quotes regaining their special meaning.
Edit: I've added set -f/+f (aka -/+o noglob) around the eval to disable file globbing although that wasn't an issue in the OP's example that's not an unheard of issue with eval. (Another, stronger caveat is to never eval user input unless you take extreme care to make sure it won't blow up into something nasty. If you don't control the value being eval-ed, you can't be sure what will happen.)
Is there any way I can tell argparse to not eat quotation marks?
For example, When I give an argument with quotes, argparse only takes what's inside of the quotes as the argument. I want to capture the quotation marks as well (without having to escape them on the command line.)
pbsnodes -x | xmlparse -t "interactive-00"
produces
interactive-00
I want
"interactive-00"
I think it is the shell that eats them, so python will actually never see them. Escaping them on the command line may be your only option.
If it's the \"backslash\" style escaping you don't like for some reason, then this way should work instead:
pbsnodes -x | xmlparse -t '"interactive-00"'
Command line is parsed into argument vector by python process itself. Depending on how python is built, that would be done by some sort of run-time library. For Windows build, that would be most likely MS Visual C++ runtime library. More details about how it parses command line can be found in Visual C++ documentation: Parsing C++ command-Line arguments.
In particular:
A string surrounded by double quotation marks ("string") is interpreted as a single argument, regardless of white space contained within. A quoted string can be embedded in an argument.
A double quotation mark preceded by a backslash (\") is interpreted as a literal double quotation mark character (").
If you want to see unprocessed command line, on Windows you can do this:
import win32api
print(win32api.GetCommandLine())