What is the best way to handle a command line argument that includes an escape sequence like \n?

[Steve Holden]

Joe wrote:
> I'm using Python 2.4 on Windows XP SP2.
> 
> I'm trying to receive a command line argument that is a newline (\n)
> 
> Here is the command line to use
> 
> sample.py "\n"
> 
> Here is a sample.py script
> 
> import sys
> 
> c = sys.argv[1]
> 
> # when run c is set to \\n instead of \n.
> 
> I created a test batch file
> 
> echo %1
> 
> to confirm that it was not the cmd.exe command processor causing the issue.
> 
> It appears that Python treats the comand line string as a raw string.
> 
> Is this a bug?  If not what is the best way to work around the issue?
> 
> Obviously I could use a hack
> 
> if c == '\\n':
>     c = '\n'
> 
> But surely there is a better way.
> 
> NOTE that I used \n in my sample but I also want to support other escape 
> sequences too.
> 
> 
> 
I don't want you getting more confused rather than less - newcomers are 
sometimes confused by Python's encoding of backslashes and such when 
printing out strings. What is your evidence for the assertion that c is 
set to \\n rather than \n?

In Unix, it's easier to avoid this, since the shell lets you enter 
multi-line strings (note that these lines may wrap in the mail):

[sholden at headrat sholden]$ python -c "import sys; print 
repr(sys.argv[1])" "\n"
'\\n'
[sholden at headrat sholden]$ python -c "import sys; print repr(sys.argv[1])" "
 > "
'\n'

The first case simply demonstrates that the shell doesn't interpret 
backslash escape sequences. I used repr() because that's what the 
interpreter will print if you enter an expression at the interactive 
prompt. The first case shows that the argument is two characters - you 
can verify this using the len() function.

The second case shows how (with a sensible command shell) you can 
provide a newline as an argument.

In Windows we can emulate the first case exactly:
C:\Steve>C:\python24\python -c "import sys; print repr(sys.argv[1])" "\n"
'\\n'

Unfortunately Windows XP's command interpreter doesn't bother to wait 
until you close a double-quote left open at the end of a line:

C:\Steve>\python24\python -c "import sys; print repr(sys.argv[1])" "
''

So someone else will have to tell you how to do that, but you should be 
clear about what's happening before you try and correct the problem.

regards
  Steve
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