Raw Strings Suppress Escapes

As we’ve seen, escape sequences are handy for embedding special byte codes within strings. Sometimes, though, the special treatment of backslashes for introducing escapes can lead to trouble. It’s surprisingly common, for instance, to see Python newcomers in classes trying to open a file with a filename argument that looks something like this:

myfile = open('C:\new\text.dat', 'w')

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thinking that they will open a file called text.dat in the directory C:\new. The problem here is that \n is taken to stand for a newline character, and \t is replaced with a tab. In effect, the call tries to open a file named C:(newline)ew(tab)ext.dat, with usually less than stellar results.

This is just the sort of thing that raw strings are useful for. If the letter r (uppercase or lowercase) appears just before the opening quote of a string, it turns off the escape mechanism. The result is that Python retains your backslashes literally, exactly as you type them. Therefore, to fix the filename problem, just remember to add the letter r on Windows:

myfile = open(r'C:\new\text.dat', 'w')

Alternatively, because two backslashes are really an escape sequence for one backslash, you can keep your backslashes by simply doubling them up:

myfile = open('C:\\new\\text.dat', 'w')

In fact, Python itself sometimes uses this doubling scheme when it prints strings with embedded backslashes:

>>> path = r'C:\new\text.dat'
>>> path                          # Show as Python code
'C:\\new\\text.dat'
>>> print(path)                   # User-friendly format
C:\new\text.dat
>>> len(path)                     # String length
15

As with numeric representation, the default format at the interactive prompt prints results as if they were code, and therefore escapes backslashes in the output. The print statement provides a more user-friendly format that shows that there is actually only one backslash in each spot. To verify this is the case, you can check the result of the built-in len function, which returns the number of bytes in the string, independent of display formats. If you count the characters in the print(path) output, you’ll see that there really is just 1 character per backslash, for a total of 15.

Besides directory paths on Windows, raw strings are also commonly used for regular expressions (text pattern matching, supported with the re module introduced in Chapter 4). Also note that Python scripts can usually use forward slashes in directory paths on Windows and Unix because Python tries to interpret paths portably (i.e., 'C:/new/text.dat' works when opening files, too). Raw strings are useful if you code paths using native Windows backslashes, though.

Note

Despite its role, even a raw string cannot end in a single backslash, because the backslash escapes the following quote character—you still must escape the surrounding quote character to embed it in the string. That is, r"...\" is not a valid string literal—a raw string cannot end in an odd number of backslashes. If you need to end a raw string with a single backslash, you can use two and slice off the second (r'1\nb\tc\\'[:-1]), tack one on manually (r'1\nb\tc' + '\\'), or skip the raw string syntax and just double up the backslashes in a normal string ('1\\nb\\tc\\'). All three of these forms create the same eight-character string containing three backslashes.