Why can't Python's raw string literals end with a single backslash?

Question

Technically, any odd number of backslashes, as described in the documentation.

>>> r\'\\\'
  File \"\", line 1
    r\'\\\'
       ^
Syn         


        

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

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