Why can’t I end a raw string with a backslash?

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

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

>>> r'some text'
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?

Answers:

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Method 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.

Method 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

Method 3

Python strings are processed in two steps:

1. 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).
2. 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.

Method 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 "".

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0