Raw strings and escaping

[Matthew Warren]

Hi,

I would expect this to work,

rawstring=r'som e things\new things\some other things\'

But it fails as the last backslash escapes the single quote.

...although writing this I think I have solved my own problem. Is \' the
only thing escaped in a raw string so you can place ' in a raw string?
Although I thought the correct thing in that case would be;

rawstring=r"raw string'with single-quote"


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[Rob Williscroft]

Re: Raw strings and escaping

Matthew Warren wrote in news:mailman.11 52.1159872720.1 0491.python-
list@python.org in comp.lang.pytho n:

I would expect this to work,
>
rawstring=r'som e things\new things\some other things\'

It in the docs:

<url:http://docs.python.org/ref/strings.html#l2 h-14>

.... Specifically, a raw string cannot end in a single backslash (since the
backslash would escape the following quote character). Note also that a
single backslash followed by a newline is interpreted as those two
characters as part of the string, not as a line continuation.

Rob.

[Duncan Booth]

Re: Raw strings and escaping

"Matthew Warren" <Matthew.Warren @Digica.comwrot e:

I would expect this to work,
>
rawstring=r'som e things\new things\some other things\'
>
But it fails as the last backslash escapes the single quote.
>
..although writing this I think I have solved my own problem. Is \'
the only thing escaped in a raw string so you can place ' in a raw
string? Although I thought the correct thing in that case would be;
>
rawstring=r"raw string'with single-quote"

You cannot end *any* string literal with an odd number of backslash
characters. The "r" prefix changes how the escape sequences are interpreted
after the string literal has been parsed, it doesn't change how the literal
itself is actually parsed. See the Python Reference Manual, 2.4.1:

When an "r" or "R" prefix is present, a character following a
backslash is included in the string without change, and all
backslashes are left in the string. For example, the string literal
r"\n" consists of two characters: a backslash and a lowercase "n".
String quotes can be escaped with a backslash, but the backslash
remains in the string; for example, r"\"" is a valid string literal
consisting of two characters: a backslash and a double quote; r"\" is
not a valid string literal (even a raw string cannot end in an odd
number of backslashes). Specifically, a raw string cannot end in a
single backslash (since the backslash would escape the following quote
character). Note also that a single backslash followed by a newline is
interpreted as those two characters as part of the string, not as a
line continuation.

[Jon Ribbens]

Re: Raw strings and escaping

In article <mailman.1152.1 159872720.10491 .python-list@python.org >, Matthew Warren wrote:

I would expect this to work,
>
rawstring=r'som e things\new things\some other things\'
>
But it fails as the last backslash escapes the single quote.

String constants in Python are weird - raw strings doubly so.
In a raw string, backslashes are not special - unless followed by
a quote character, when they simultaneously escape the quote and
also insert a literal backslash.

I presume there was originally some reason for this bizarre behaviour
- it'd be interesting to know what it is/was, if anyone knows?

[Duncan Booth]

Re: Raw strings and escaping

Jon Ribbens <jon+usenet@une quivocal.co.ukw rote:

I presume there was originally some reason for this bizarre behaviour
- it'd be interesting to know what it is/was, if anyone knows?
>

See the FAQ for the explanation:

http://www.python.org/doc/faq/general/#why-can-t-raw-strings-r-strings-end-with-a-backslash

The idea is that if you use raw strings for regular expressions you can
write things like:

pattern = r'[\'"]'

if the raw string simply ignored the backslash altogether it would be much
harder to write regular expressions that contain both sorts of quotes.

[Jon Ribbens]

Re: Raw strings and escaping

In article <Xns98518349247 01duncanbooth@1 27.0.0.1>, Duncan Booth wrote:

>I presume there was originally some reason for this bizarre behaviour
>- it'd be interesting to know what it is/was, if anyone knows?
>>

See the FAQ for the explanation:
>

http://www.python.org/doc/faq/general/#why-can-t-raw-strings-r-strings-end-with-a-backslash

>
The idea is that if you use raw strings for regular expressions you can
write things like:
>
pattern = r'[\'"]'
>
if the raw string simply ignored the backslash altogether it would be much
harder to write regular expressions that contain both sorts of quotes.

Well, hardly *much* harder:

pattern = r"""foo"""

Personally, I think that raw strings behaving as they do is
unexpected, unintuitive and unnecessary, but it's obviously too late
to change it now anyway ;-)

I think standard strings accepting backslash followed by an unexpected
character is also a mistake, but again it's too late to fix now.

[Duncan Booth]

Re: Raw strings and escaping

Jon Ribbens <jon+usenet@une quivocal.co.ukw rote:

Well, hardly *much* harder:
>
pattern = r"""foo"""

It means you have to always triple quote your raw strings or know in
advance of writing the regular expression which of r'', r"", r'''''',
r"""""" is most appropriate. The way it works at the moment you don't have
to care, you just write the raw string knowing that with r'' you have to
escape single quotes, r"" you have to escape double quotes and everything
works as expected.

Its only when you start trying to use raw strings for things other than
regular expressions that backslash at the end of the string can be a
problem.

[Scott David Daniels]

Re: Raw strings and escaping

Matthew Warren wrote:

Hi,
>
I would expect this to work,
>
rawstring=r'som e things\new things\some other things\'
>
But it fails as the last backslash escapes the single quote.

Note something many people don't when looking over the string rules:

astring = r'some things\new things\some other things' '\\'

gives you exactly what you want, doesn't imply a string concatenation
at run time, and various other things. Two strings in succession are
concatenated at source translation time.

By the way, I renamed the result from being rawstring. It gives people
bad intuitions to refer to some strings as "raw" when what you really
mean is that the notation you are using is a "raw" notation for a
perfectly normal string.

--Scott David Daniels
scott.daniels@a cm.org