Modifying escape sequences in strings

[Thomas Philips]

I have been playing around with reading strings with embedded escape
sequences from files both using readline() and codecs.open() and have
a question.I create a file "test.txt" with exactly one line:
1\na\n\n2\n\n3

I then open test.txt and then read it using readline():[color=blue][color=green][color=darkred]
>>> input_file=file ("test.txt")
>>> x=input_file.re adline()[/color][/color][/color]
[color=blue][color=green][color=darkred]
>>> x
>>> 1\\na\\n\\n2\\n \\n3
>>> print x
>>> 1\na\n\n2\n\n3[/color][/color][/color]

The readline has escaped the backslashes, so that they print
correctly. I tried to replace the double backslashes with single
backslashes to escape the "n"[color=blue][color=green][color=darkred]
>>> x.replace("\\", "\")[/color][/color][/color]

SyntaxError: EOL while scanning single-quoted string[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

How can I replace the escaped backslash with a backslash? I realize
that I can solve the problem by reading the file using
codecs.open("te st.txt","r","st ring_escape") as suggested by Peter
Otten. I'm trying to do the same thing in different ways to better
understand Python

Sincerely
Thomas Philips

[Larry Bates]

Re: Modifying escape sequences in strings

The problem is that the backslash (\) has special
meaning to Python. It means that the next character
is escaped (has special meaning).

Try following:

x=x.replace("\\ n","\n")

This works for me.

-Larry


"Thomas Philips" <tkpmep@hotmail .com> wrote in message
news:b4a8ffb6.0 403020814.283e6 4d6@posting.goo gle.com...[color=blue]
> I have been playing around with reading strings with embedded escape
> sequences from files both using readline() and codecs.open() and have
> a question.I create a file "test.txt" with exactly one line:
> 1\na\n\n2\n\n3
>
> I then open test.txt and then read it using readline():[color=green][color=darkred]
> >>> input_file=file ("test.txt")
> >>> x=input_file.re adline()[/color][/color]
>[color=green][color=darkred]
> >>> x
> >>> 1\\na\\n\\n2\\n \\n3
> >>> print x
> >>> 1\na\n\n2\n\n3[/color][/color]
>
> The readline has escaped the backslashes, so that they print
> correctly. I tried to replace the double backslashes with single
> backslashes to escape the "n"[color=green][color=darkred]
> >>> x.replace("\\", "\")[/color][/color]
>
> SyntaxError: EOL while scanning single-quoted string[color=green][color=darkred]
> >>>[/color][/color]
>
> How can I replace the escaped backslash with a backslash? I realize
> that I can solve the problem by reading the file using
> codecs.open("te st.txt","r","st ring_escape") as suggested by Peter
> Otten. I'm trying to do the same thing in different ways to better
> understand Python
>
> Sincerely
> Thomas Philips[/color]

[Jeff Epler]

Re: Modifying escape sequences in strings

If you want to translate two backslashes into a single backslash,
have to write
x.replace("\\\\ ", "\\")
the first is a string of length 2 and the second is a string of length
1. I don't know why the tutorial doesn't cover this point explicitly
(http://python.org/doc/current/tut/no...00000000000000)
but the language reference does (http://python.org/doc/current/ref/strings.html)
.... but there are no sequences of two backslashes in the strings you
were working with.

However, I think that referring to "escaped backslashes" in the string
you read shows that there's some other misunderstandin g of what is going
on. Is your final goal to turn the backslash-n sequences into actual
newlines, or what? If this is your goal, then you should use the
"string_esc ape" codec in Python 2.3:[color=blue][color=green][color=darkred]
>>> '\\n'.decode("s tring_escape")[/color][/color][/color]
'\n'
This took a string containing backslash-n and returned a string
containing a newline character.

Jeff