python raw string assignment

Question

Given a file contains lines such as:

(?i:\bsys\.user_catalog\b)

While reading those line, I want the value to be a raw string (unescaped), meaning, in memory, line should be

r'(?i:\bsys\.user_catalog\b)'

instead of

(?i:\bsys\.user_catalog\b)

Which is escaped when passed over to libs such as sqlobject.

For instance, with sqlobject, if I state

Table(column=r'(?i:\bsys\.user_catalog\b)')

I get the desired results while if I state

Table(column='(?i:\bsys\.user_catalog\b)')

I do not.

So question is basically, I can I pass a raw string when I am not in declarative/assignment mode (e.g. a = r'string'), rather, the string is already in memory.

Answer 1

The raw string notation is only used in Python source code; all strings declared as raw strings are "converted" to normal strings with the necessary escape sequences added during "compile time" (unlike (in Python 2) the two different string types string/Unicode string):

>>> r"\b"
'\\b'
>>> "Hello"
'Hello' 
>>> u"Hello"
u'Hello'

If you read the string from a file, it will already be correctly escaped.

(Assuming test.txt contains (?i:\bsys\.user_catalog\b)):

f = open("test.txt").read()
print f
print repr(f)

Output:

(?i:\bsys\.user_catalog\b)
'(?i:\\bsys\\.user_catalog\\b)'

Answer 2

You can use raw string anywhere you are using a string. Raw string is just a user friendly way to represent a string when you have lots of escape characters.

The second case is not working because of the '\'. So you need to escape it using another '\'. The second case should work if you give '(?i:\\bsys\\.user_catalog\\b)'. In memory, since ASCII or Unicode is stored, it doesn't make any difference if it is raw string or not.