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

P: n/a
Joe
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.

Jul 18 '05 #1
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8 Replies


P: n/a
Joe wrote:
It appears that Python treats the comand line string as a raw string.

what is the best way to work around the issue?


You probably want to use str.decode with the encoding 'string_escape'[1]

py> s = r'\n\t'
py> s
'\\n\\t'
py> s.decode('string_escape')
'\n\t'

STeVe

[1]http://docs.python.org/lib/standard-encodings.html
Jul 18 '05 #2

P: n/a
Joe
Steve,

THANKS! That is exactly what I was looking for but unable to find.

Joe

"Steven Bethard" <st************@gmail.com> wrote in message
news:oc********************@comcast.com...
Joe wrote:
It appears that Python treats the comand line string as a raw string.

what is the best way to work around the issue?


You probably want to use str.decode with the encoding 'string_escape'[1]

py> s = r'\n\t'
py> s
'\\n\\t'
py> s.decode('string_escape')
'\n\t'

STeVe

[1]http://docs.python.org/lib/standard-encodings.html

Jul 18 '05 #3

P: n/a
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@headrat sholden]$ python -c "import sys; print
repr(sys.argv[1])" "\n"
'\\n'
[sholden@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
--
Meet the Python developers and your c.l.py favorites March 23-25
Come to PyCon DC 2005 http://www.pycon.org/
Steve Holden http://www.holdenweb.com/
Jul 18 '05 #4

P: n/a
Op 2005-03-02, Joe schreef <Jo********@hotmail.com>:
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"
Are you sure this supplies a newline and not the string <backslach> <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.


I'm not sure this confirms anything. IMO it is possible that echo
will translate <backslach> <n> to <newline>, giving you the
impression that you have provideded a newline on the command line
while in fact you have not.

--
Antoon Pardon
Jul 18 '05 #5

P: n/a
Joe
Hi Steve,

I've been using Python for many years, just hadn't had to deal with an
escape sequence in the command line args. :-) and couldn't find the solution
in the docs.

It is easy to prove my assertion:

import sys
c = sys.argv[1]
print len(c)
print 'Line 1', c, 'Line 2'

Output:
2
Line 1 \n Line 2

Versus:

import sys
c = sys.argv[1]
print len(c)
print 'Line 1', c.decode('string_escape'), 'Line 2'

Output:
2
Line 1
Line 2

Agreed, it is much easier on Unix to deal with this, I have not seen a way
to get the XP command interpreter to allow a multiline command line as you
described.

Your solution was exactly what I need. I had an escape sequence entered on
the command line and needed to decode the string so that Python used it as
an escape sequence, in fact the sequence really is part of the output that
the program produces.

Thanks again for your assistance.

Regards,

Joe
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@headrat sholden]$ python -c "import sys; print repr(sys.argv[1])"
"\n"
'\\n'
[sholden@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
--
Meet the Python developers and your c.l.py favorites March 23-25
Come to PyCon DC 2005 http://www.pycon.org/
Steve Holden http://www.holdenweb.com/

Jul 18 '05 #6

P: n/a
Joe
Antoon,

I tested the batch file :-)

The one line batchfile does prove it because it prints out <backslash><n>
and not <newline>.

See other post, decode is exactly what was needed to fix the problem.

Regards,

Joe

"Antoon Pardon" <ap*****@forel.vub.ac.be> wrote in message
news:sl********************@rcpc42.vub.ac.be...
Op 2005-03-02, Joe schreef <Jo********@hotmail.com>:
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"


Are you sure this supplies a newline and not the string <backslach> <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.


I'm not sure this confirms anything. IMO it is possible that echo
will translate <backslach> <n> to <newline>, giving you the
impression that you have provideded a newline on the command line
while in fact you have not.

--
Antoon Pardon

Jul 18 '05 #7

P: n/a
Joe wrote:
Hi Steve,

I've been using Python for many years, just hadn't had to deal with an
escape sequence in the command line args. :-) and couldn't find the solution
in the docs.

It is easy to prove my assertion:

import sys
c = sys.argv[1]
print len(c)
print 'Line 1', c, 'Line 2'

Output:
2
Line 1 \n Line 2
The "2" appears to prove *my* assertion that the argument passed by the
shell to your Python program consists of 2 characters, "\\" followed by "n".
Versus:

import sys
c = sys.argv[1]
print len(c)
print 'Line 1', c.decode('string_escape'), 'Line 2'

Output:
2
Line 1
Line 2
As does this.
Agreed, it is much easier on Unix to deal with this, I have not seen a way
to get the XP command interpreter to allow a multiline command line as you
described.

Your solution was exactly what I need. I had an escape sequence entered on
the command line and needed to decode the string so that Python used it as
an escape sequence, in fact the sequence really is part of the output that
the program produces.
In fairness it was Steven Bethard's solution that gave you the solution
you needed. As long as ytour problem is solved, that's fine, and it
appears that you've solved it in a reasonably cross-platform way.
Thanks again for your assistance.

Always happy to help when I can!

regards
Steve
--
Meet the Python developers and your c.l.py favorites March 23-25
Come to PyCon DC 2005 http://www.pycon.org/
Steve Holden http://www.holdenweb.com/
Jul 18 '05 #8

P: n/a
Joe
Hey no fair changing last names in the middle of a thread :-)

Thanks to BOTH Steve's.

In fairness it was Steven Bethard's solution that gave you the solution
you needed. As long as ytour problem is solved, that's fine, and it
appears that you've solved it in a reasonably cross-platform way.

Steve Holden http://www.holdenweb.com/

Jul 18 '05 #9

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