Technically, any odd number of backslashes, as described in the documentation.
>>> r'\'
File "<stdin>", line 1
r'\'
^
SyntaxError: EOL while scanning string literal
>>> r'\\'
'\\\\'
>>> r'\\\'
File "<stdin>", line 1
r'\\\'
^
SyntaxError: EOL while scanning string literal
It seems like the parser could just treat backslashes in raw strings as regular characters (isn't that what raw strings are all about?), but I'm probably missing something obvious.
The reason is explained in the part of that section which I highlighted in bold:
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.
So raw strings are not 100% raw, there is still some rudimentary backslash-processing.
r"foo\bar\baz" "\\" (wrap in parens if ambiguous) will create a single literal at compile time, the first part of which is raw, and only the last tiny bit is non-raw, to allow the trailing backslash. –
Agneta @"\" to represent single backslash. so I think this is not technical problem. –
Porterhouse The whole misconception about python's raw strings is that most of people think that backslash (within a raw string) is just a regular character as all others. It is NOT. The key to understand is this python's tutorial sequence:
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
So any character following a backslash is part of raw string. Once parser enters a raw string (non Unicode one) and encounters a backslash it knows there are 2 characters (a backslash and a char following it).
This way:
r'abc\d' comprises a, b, c, \, d
r'abc\'d' comprises a, b, c, \, ', d
r'abc\'' comprises a, b, c, \, '
and:
r'abc\' comprises a, b, c, \, ' but there is no terminating quote now.
Last case shows that according to documentation now a parser cannot find closing quote as the last quote you see above is part of the string i.e. backslash cannot be last here as it will 'devour' string closing char.
x = r"I have ""an apple""" stands for I have "an apple". One problem is python allows something like a="a""b" or a="a" "b" resulting in a="ab". So to use doubling quotation marks, python needs to ban use case of a="a""b". –
Preeminence r'c:\\', but, of course, it's a bug if Python then creates this string: "c:\\" –
Lefthand The reason is explained in the part of that section which I highlighted in bold:
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.
So raw strings are not 100% raw, there is still some rudimentary backslash-processing.
r"foo\bar\baz" "\\" (wrap in parens if ambiguous) will create a single literal at compile time, the first part of which is raw, and only the last tiny bit is non-raw, to allow the trailing backslash. –
Agneta @"\" to represent single backslash. so I think this is not technical problem. –
Porterhouse That's the way it is! I see it as one of those small defects in python!
I don't think there's a good reason for it, but it's definitely not parsing; it's really easy to parse raw strings with \ as a last character.
The catch is, if you allow \ to be the last character in a raw string then you won't be able to put " inside a raw string. It seems python went with allowing " instead of allowing \ as the last character.
However, this shouldn't cause any trouble.
If you're worried about not being able to easily write windows folder pathes such as c:\mypath\ then worry not, for, you can represent them as r"C:\mypath", and, if you need to append a subdirectory name, don't do it with string concatenation, for it's not the right way to do it anyway! use os.path.join
>>> import os
>>> os.path.join(r"C:\mypath", "subfolder")
'C:\\mypath\\subfolder'
r"raw" "\"" or r'raw"quote' –
Trickster In order for you to end a raw string with a slash I suggest you can use this trick:
>>> print r"c:\test"'\\'
test\
It uses the implicit concatenation of string literals in Python and concatenates one string delimited with double quotes with another that is delimited by single quotes. Ugly, but works.
Another trick is to use chr(92) as it evaluates to "\".
I recently had to clean a string of backslashes and the following did the trick:
CleanString = DirtyString.replace(chr(92),'')
I realize that this does not take care of the "why" but the thread attracts many people looking for a solution to an immediate problem.
"\\" (non-raw string with backslash) –
Trickster Since \" is allowed inside the raw string. Then it can't be used to identify the end of the string literal.
Why not stop parsing the string literal when you encounter the first "?
If that was the case, then \" wouldn't be allowed inside the string literal. But it is.
\" anywhere within a double-quoted raw string, OR \ at end of double-quoted raw string. The usage statistics must favor the two character sequence anywhere vs. the one-character sequence at the end. –
Luann The reason for why r'\' is syntactical incorrect is that although the string expression is raw the used quotes (single or double) always have to be escape since they would mark the end of the quote otherwise. So if you want to express a single quote inside single quoted string, there is no other way than using \'. Same applies for double quotes.
But you could use:
'\\'
Another user who has since deleted their answer (not sure if they'd like to be credited) suggested that the Python language designers may be able to simplify the parser design by using the same parsing rules and expanding escaped characters to raw form as an afterthought (if the literal was marked as raw).
I thought it was an interesting idea and am including it as community wiki for posterity.
Given the confusion around the arbitrary-seeming restriction against an odd number of backslashes at the end of a Python raw-string, it's fair to say that this is a design mistake or legacy issue originating in a desire to have a simpler parser.
While workarounds (such as r'C:\some\path' '\\' yielding (in Python notation:) 'C:\\some\\path\\' or (verbatim:) C:\some\path\) are simple, it's counterintuitive to be needing them. For comparison, let's have a look at C++ and Perl.
In C++, we can straightforwardly use raw string literal syntax
#include <iostream>
int main() {
std::cout << R"(Hello World!)" << std::endl;
std::cout << R"(Hello World!\)" << std::endl;
std::cout << R"(Hello World!\\)" << std::endl;
std::cout << R"(Hello World!\\\)" << std::endl;
}
to get the following output:
Hello World!
Hello World!\
Hello World!\\
Hello World!\\\
If we want to use the closing delimiter (above: )) within the string literal, we can even extend the syntax in an ad-hoc way to R"delimiterString(quotedMaterial)delimiterString". For example, R"asdf(some random delimiters: ( } [ ] { ) < > just for fun)asdf" produces the string some random delimiters: ( } [ ] { ) < > just for fun in the output. (Ain't that a good use of "asdf"!)
In Perl, this code
my $str = q{This is a test.\\};
print ($str);
print ("This is another test.\n");
will output the following: This is a test.\This is another test.
Replacing the first line by
my $str = q{This is a test.\};
would lead to an error message: Can't find string terminator "}" anywhere before EOF at main.pl line 1.
However, Perl treating a pre-delimiter \ as an escape character doesn't prevent the user from having an odd number of backslashes at the end of the resulting string; eg to place 3 backslashes \\\ into the end of $str, simply end the code with 6 backslashes: my $str = q{This is a test.\\\\\\};. Importantly, while we need to double the backslashes in the input, there is no Python-like inconsistent-seeming syntactic restriction.
Another way of looking at things is that these 3 languages use different ways to address the parsing issue of interaction between escape characters and closing delimiters:
r'stringWithoutFinalBackslash' '\\'¹ The custom delimiterString itself cannot be more than 16 characters long, but that's hardly a limitation.
² If you need the delimiter itself, just escape it with \.
However, to be fair in a comparison to Python, we need to acknowledge that (1) C++ didn't have such string literals until C++11 and is famously hard to parse and (2) Perl is even harder to parse.
The naive idea of a raw string is
If I put an r in front of a pair of quotes, I can put whatever I want between the quotes and it will mean itself.
Unfortunately, this does not work, because if the whatever happens to contain a quote, the raw string would end at that point.
It is simply impossible that I can put "whatever I want" between fixed delimiters, because some of it could look like the terminating delimiter -- no matter what that delimiter is.
One possible approach to this problem would be to say
If I put an r in front of a pair of quotes, I can put whatever I want between the quotes as long as it does not contain a quote and it will mean itself.
This restriction sounds harsh, until one recognizes that Python's large offering of quotes can accommodate most situations with this rule. The following are all valid Python quotes:
'
"
'''
"""
With this many possibilities for the delimiter, almost anything can be made to work. About the only exception would be if the string literal is supposed to contain a complete list of all allowed Python quotes.
Python, however, takes a different route using an extended version of the above rule. It effectively states
If I put an r in front of a pair of quotes, I can put whatever I want between the quotes as long as it does not contain a quote and it will mean itself. If I insist on including a quote, even that is allowed, but I have to put a backslash before it.
So the Python approach is, in a sense, even more liberal than variant 1 above -- but it has the side effect of "mis"interpreting the closing quote as part of the string if the last intended character of the string is a backslash.
Variant 2 is not helpful:
After writing this, I go with several of the other posters that variant 1 would have been easier to understand and to accept and therefore more pythonic. That's life!
Comming from C it pretty clear to me that a single \ works as escape character allowing you to put special characters such as newlines, tabs and quotes into strings.
That does indeed disallow \ as last character since it will escape the " and make the parser choke. But as pointed out earlier \ is legal.
some tips :
1) if you need to manipulate backslash for path then standard python module os.path is your friend. for example :
os.path.normpath('c:/folder1/')
2) if you want to build strings with backslash in it BUT without backslash at the END of your string then raw string is your friend (use 'r' prefix before your literal string). for example :
r'\one \two \three'
3) if you need to prefix a string in a variable X with a backslash then you can do this :
X='dummy'
bs=r'\ ' # don't forget the space after backslash or you will get EOL error
X2=bs[0]+X # X2 now contains \dummy
4) if you need to create a string with a backslash at the end then combine tip 2 and 3 :
voice_name='upper'
lilypond_display=r'\DisplayLilyMusic \ ' # don't forget the space at the end
lilypond_statement=lilypond_display[:-1]+voice_name
now lilypond_statement contains "\DisplayLilyMusic \upper"
long live python ! :)
n3on
os.path.normpath will remove the tailing backslash... Then how should I concat the filename into the path... –
Saturation Despite its role, even a raw string cannot end in a single backslash, because the backslash escapes the following quote character—you still must escape the surrounding quote character to embed it in the string. That is, r"...\" is not a valid string literal—a raw string cannot end in an odd number of backslashes.
If you need to end a raw string with a single backslash, you can use two and slice off the second.
I encountered this problem and found a partial solution which is good for some cases. Despite python not being able to end a string with a single backslash, it can be serialized and saved in a text file with a single backslash at the end. Therefore if what you need is saving a text with a single backslash on you computer, it is possible:
x = 'a string\\'
x
'a string\\'
# Now save it in a text file and it will appear with a single backslash:
with open("my_file.txt", 'w') as h:
h.write(x)
BTW it is not working with json if you dump it using python's json library.
Finally, I work with Spyder, and I noticed that if I open the variable in spider's text editor by double clicking on its name in the variable explorer, it is presented with a single backslash and can be copied to the clipboard that way (it's not very helpful for most needs but maybe for some..).
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