So it seems that python would convert \sto \s.

Don't confuse string representations with the actual content of the string. String representation is the way you write a string in source code, which may not exactly be the same as the string actually in memory. Backslashes are parsed specially to allow you to write non-printable characters using the backslash syntax. In this case, \s is not a valid escape sequence so the python parser interprets it literally as backslash-s. In memory, the string is still a character sequence containing the letters: `\, s

str class have a __repr__()/repr() method that returns a string that contains the source-code representation of the string, this is the string that gets printed when you don't use print statement in the REPL. This allows you to copy paste those string and reuse it in another part of the shell, but it isn't really what is stored in memory and how python interprets the string. When printing repr, python always escapes a literal backslash, this is to remove ambiguity on whether the backslash is interpreted as escape sequence or as a literal character.

Why would Python do this and what is this for? Is it the same in other languages like Java?

Most languages' string literal do interpret backslash escape sequence, although different languages treats invalid escape sequence differently. In Python, invalid backslash escape sequence is silently treated as literal backslash instead of producing an error. You'd probably encounter this kind of issue more often in Python because it has an ubiquitous repr() protocol and the default use of repr in the REPL shell.

A backslash at the end of a line tells Python to extend the current logical line over across to the next physical line. See the Line Structure section of the Python reference documentation:

2.1.5. Explicit line joining

Two or more physical lines may be joined into logical lines using backslash characters (\), as follows: when a physical line ends in a backslash that is not part of a string literal or comment, it is joined with the following forming a single logical line, deleting the backslash and the following end-of-line character. For example:

if 1900 < year < 2100 and 1 <= month <= 12 \
   and 1 <= day <= 31 and 0 <= hour < 24 \
   and 0 <= minute < 60 and 0 <= second < 60:   # Looks like a valid date
        return 1

There is also the option to use implicit line joining, by using parentheses or brackets or curly braces; Python will not end the logical line until it finds the matching closing bracket or brace for each opening bracket or brace. This is the recommended code style, the sample you found should really be written as:

if ((i < len(words_and_emoticons) - 1 and item.lower() == "kind" and
        words_and_emoticons[i+1].lower() == "of") or
        item.lower() in BOOSTER_DICT):
    sentiments.append(valence)
    continue

See the Python Style Guide (PEP 8) (but note the exception; some Python statements don't support (...) parenthesising so backslashes are acceptable there).

Note that Python is not the only programming language using backslashes for line continuation; bash, C and C++ preprocessor syntax, Falcon, Mathematica and Ruby also use this syntax to extend lines; see Wikipedia.

You're being mislead by output -- the second approach you're taking actually does what you want, you just aren't believing it. :)

>>> foo = 'baz "\\"'
>>> foo
'baz "\\"'
>>> print(foo)
baz "\"

Incidentally, there's another string form which might be a bit clearer:

>>> print(r'baz "\"')
baz "\"