Python read file as raw string

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.

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Karl Pflästerer said unto the world upon 08/04/2004 12:32:

> On  8 Apr 2004, Brian van den Broek <- bvande at po-box.mcgill.ca wrote:
> 
> [...]
> 
>>Why I care:
>>I am wanting to store a series of directory names in a plain text
>>config file. I can read the file just fine and have can do all I've
>>tried with the output of the readline() method so far. But, since I am
>>on a Windows machine, I would prefer to turn the readline() output
>>into a raw string. Otherwise, the directory separator '\' gets read as
>>an escape character. I know I can solve it by using '/' in place of
> 
> 
> Did you actually try that?  Here's a small example:
> 
> $ cat > raw_string
> c:\foo\bar\test\new\end
> 
> $ python
> Python 2.3.3 (#1, Dec 30 2003, 08:29:25) 
> [GCC 3.3.1 (cygming special)] on cygwin
> Type "help", "copyright", "credits" or "license" for more information.
> 
>>>>f = file('raw_string')
>>>>f.readline()
> 
> 'c:\\foo\\bar\\test\\new\\end\r\n'
> 
>>>>f = file('raw_string')
>>>>print f.readline()
> 
> c:\foo\bar\test\new\end
> 
> No interpolation happens (\t is tab and \n newline).
> 
> 
> 
>    Karl

Karl,

right you are. Thanks.

I think I confused myself by assuming that stuff that worked one 
way in the interactive prompt on strings typed in as literals and 
assigned to variables would work the same on a string taken from 
file.readline(). Clearly I need to know more about Python before 
running around making assumptions!

Sorry. On the plus side, I am getting the hang of it, if slowly.

Thanks again and best,

Brian vdB

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Python raw string is created by prefixing a string literal with ‘r’ or ‘R’. Python raw string treats backslash (\) as a literal character. This is useful when we want to have a string that contains backslash and don’t want it to be treated as an escape character.

Python Raw String

Let’s say we want to create a string Hi\nHello in python. If we try to assign it to a normal string, the \n will be treated as a new line.

s = 'Hi\nHello'
print(s)

Output:

Hi
Hello

Let’s see how raw string helps us in treating backslash as a normal character.

raw_s = r'Hi\nHello'
print(raw_s)

Output: Hi\nHello Let’s see another example where the character followed by backslash doesn’t have any special meaning.

>>> s = 'Hi\xHello'
  File "", line 1
SyntaxError: (unicode error) 'unicodeescape' codec can't decode bytes in position 2-3: truncated \xXX escape

We got the error because python doesn’t know how to decode ‘\x’ as it doesn’t have any special meaning. Let’s see how we can create the same string using raw strings.

>>> s = r'Hi\xHello'
>>> print(s)
Hi\xHello

If you are on Python console and create a raw-string like below.

>>> r'Hi\xHello'
'Hi\\xHello'

Don’t get confused with the output having two backslashes. It’s just to show it as a normal python string where backslash is being escaped.

Python Raw String and Quotes

When a backslash is followed by a quote in a raw string, it’s escaped. However, the backslash also remains in the result. Because of this feature, we can’t create a raw string of single backslash. Also, a raw string can’t have an odd number of backslashes at the end. Some of the invalid raw strings are:

r'\'  # missing end quote because the end quote is being escaped
r'ab\\\'  # first two backslashes will escape each other, the third one will try to escape the end quote.

Let’s look at some of the valid raw string examples with quotes.

raw_s = r'\''
print(raw_s)

raw_s = r'ab\\'
print(raw_s)

raw_s = R'\\\"'  # prefix can be 'R' or 'r'
print(raw_s)

Output:

\'
ab\\
\\\"

That’s all for a quick introduction of python raw string.

You can checkout complete python script and more Python examples from our GitHub Repository.

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