Why can't I end a raw string with a backslash? [duplicate]

问题

This question already has an answer here:

Why can't Python's raw string literals end with a single backslash? 12 answers

I am confused here, even though raw strings convert every \\ to \\\\ but when this \\ appears in the end it raises error.

>>> r\'so\\m\\e \\te\\xt\'
\'so\\\\m\\\\e \\\\te\\\\xt\'

>>> r\'so\\m\\e \\te\\xt\\\'
SyntaxError: EOL while scanning string literal

Update:

This is now covered in Python FAQs as well: Why can’t raw strings (r-strings) end with a backslash?

回答1:

You still need \ to escape ' or " in raw strings, since otherwise the python interpreter doesn't know where the string stops. In your example, you're escaping the closing '.

Otherwise:

r'it wouldn\'t be possible to store this string'
r'since it'd produce a syntax error without the escape'

Look at the syntax highlighting to see what I mean.

回答2:

Raw strings can't end in single backslashes because of how the parser works (there is no actual escaping going on, though). The workaround is to add the backslash as a non-raw string literal afterwards:

>>> print(r'foo\')
  File "<stdin>", line 1
    print(r'foo\')
                 ^
SyntaxError: EOL while scanning string literal
>>> print(r'foo''\\')
foo\

Not pretty, but it works. You can add plus to make it clearer what is happening, but it's not necessary:

>>> print(r'foo' + '\\')
foo\

回答3:

Python strings are processed in two steps:

First the tokenizer looks for the closing quote. It recognizes backslashes when it does this, but doesn't interpret them - it just looks for a sequence of string elements followed by the closing quote mark, where "string elements" are either (a character that's not a backslash, closing quote or a newline - except newlines are allowed in triple-quotes), or (a backslash, followed by any single character).
Then the contents of the string are interpreted (backslash escapes are processed) depending on what kind of string it is. The r flag before a string literal only affects this step.

回答4:

Quote from https://docs.python.org/3.4/reference/lexical_analysis.html#literals:

Even in a raw literal, quotes can be escaped with a backslash, but the backslash remains in the result; 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 literal 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 literal, not as a line continuation.

So in raw string, backslash are not treated specially, except when preceding " or '. Therefore, r'\' or r"\" is not a valid string cause right quote is escaped thus making the string literal invalid. In such case, there's no difference whether r exists, i.e. r'\' is equivalent to '\' and r"\" is equivalent to "\".