Python automatically converting some strings to raw strings?

Question

Python seems to be automatically converting strings (not just input) into raw strings. Can somebody explain what is happening here?

Python 3.7.1 (v3.7.1:260ec2c36a, Oct 20 2018, 14:57:15) [MSC v.1915 64 bit 
(AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> s = '\stest'
>>> s
'\\stest'
# looks like a raw string
>>> print(s)
\stest
>>> s = '\ntest'
>>> s
'\ntest'
# this one doesn't
>>> s = '\n test'
>>> s
'\n test'
>>> s = r'\n test'
>>> s
'\\n test'
>>> print(s)
\n test

The question marked as a duplicate for this one seems to be useful, but then I do not understand why

>>> s = '\n test'
>>> s
'\n test'
>>> repr(s)
"'\\n test'"

does not get two backslashes when called, and does when repr() is called on it.

From my understanding, python seems to be detecting whether the character following the backslash has a meaning if combined with it or not, and if it does have it then it does not escape it. If it has it, it leaves it be. — mariogarcc, Dec 21 '18 at 00:05
The proposed duplicate didn't actually explain why the terminal output is different for `s` vs `repr(s)`, so reopened. — jpp, Dec 21 '18 at 00:30

wim · Accepted Answer · 2018-12-21T01:13:28.670

\n is a valid escape sequence and '\n' is a length 1 string (new line character). In contrast, \s is an invalid escape sequence, so Python is assuming that what you wanted there was a two character string: a backlash character plus an s character.

>>> len('\s')
2

What you saw on terminal output was just the usual representation for such a length 2 string. Note that the correct way to create the string which Python gave you back here would have been with r'\s' or with '\\s'.

>>> r'\s' == '\\s' == '\s'
True

This is a deprecated behavior. In a future version of Python, likely the next point release, your code will be a syntax error.

Since you're using v3.7.1, you could enable warnings if you want to be informed about such uses of deprecated features:

$ python -Wall
>>> '\s'
<stdin>:1: DeprecationWarning: invalid escape sequence \s
'\\s'

As for your subsequent question after the edit:

>>> s = '\n test'
>>> s  # this prints the repr(s)
'\n test'
>>> repr(s)  # this prints the repr(repr(s))
"'\\n test'"

That last addition was what made me understand the issue. So the deprecated behaviour is the introduction of a backslash followed by a "non-meaningful" character, like `\s`, and the correct way to try to introduce said strings from now on will be by escaping any backslash that is not followed by a "meaningful" character, like `n`, `t`, [etc.](https://docs.python.org/3/reference/lexical_analysis.html#string-and-bytes-literals) — mariogarcc, Dec 21 '18 at 01:12

Python automatically converting some strings to raw strings?

1 Answers1