CODEWITHSUNDEEP
pythonstringfileliterals
user3202419
user3202419

Reputation: 13

String literals for file names

I am new to Python - but not to programming, and on a bit of a steep learning curve.

I have a programme that reads several input files - the first input file contains (amongst other things) the path and name the other files.

I can open the file and read the name OK. If I print the string it looks like this

Z:\ \python\ \rb_data.dat\n'

all my "\" become "\ \" I think I can fix this by using the "r" prefix to convert it to a literal.

My question is how do I attach the prefix to a string variable ??

This is what I want to do :

modat = open('z:\\python\mot1 input.txt') # first input file containing names of other file

rbfile = modat.readline()  # get new file name

rbdat = open(rbfile)   # open new file

Upvotes: 0

Views: 2779

Answers (3)

Burhan Khalid
Burhan Khalid

Reputation: 174708

Why not use os.path.normcase, like this:

with open(r'z:\python\mot1 input.txt') as f:
   for line in f:
      if line.strip():
         if os.path.isfile(os.path.normcase(line.strip())):
              with open(line.strip()) as f2:
                  # do something with
                  # f2

From the documentation of os.path.normcase:

Normalize the case of a pathname. On Unix and Mac OS X, this returns the path unchanged; on case-insensitive filesystems, it converts the path to lowercase. On Windows, it also converts forward slashes to backward slashes.

Upvotes: 0

poke
poke

Reputation: 388313

The \\ is an escape sequence for the backslash character \. When you specify a string literal, they are enquoted by either ' or ". Because there are some characters you might need to specify to be part of the string which you cannot enter like this—for example the quotation marks themselves—escape sequences allow you to do it. They usually are \x where x is something you want to enter. Now because all escape sequences start with a backslash, the backslash itself also turns into a special character which you cannot specify directly within a string literal. So you need to escape it too.

That means that the string literal '\\' actually represents a string with a single character: The backslash. Raw strings, that are string literals with an r in front of the opening quotation character, ignore (most) escape sequences. So r'\\x' is actually the string where two backslashes are followed by an x. So it’s identical to the string described by the non-raw string literal '\\\\x'.

All this only applies to string literals though. The string itself holds no information about whether it was created with a raw string literal or not, or whether there was some escape sequence need or not. It just contains all the characters that make out the string.

That also means that as soon as you get a string from somewhere, for example by reading it from a file, then you don’t need to worry about escaping something in there to make sure that it’s a correct string. It just is.

So in your code, when you open the file at z:\python\mot1 input.txt, you need to specify that filename as a string first. So you have to use a string literal, either with escaping the backslashes, or by using a raw string.

Then, when you read the new filename from that file, you already have a real string, and don’t need to bother with anything more. Assuming that it was correctly written to the file, you can just use it like that.

Upvotes: 1

jonrsharpe
jonrsharpe

Reputation: 122154

The backslash \ in Python strings (and in code blocks on StackOverflow!) means, effectively, "treat the next character differently". As it is reserved for this purpose, when you actually have a backslash in your strings, it must be "escaped" by a preceding backslash:

>>> myString = "\\" # the first one "escapes" the second

>>> myString = "\" # no escape, so...
SyntaxError: EOL while scanning string literal

>>> print("\\") # when we actually print out the string
\

The short story is, you can basically ignore this in your strings. If you pass rbfile to open, Python will interpret it correctly.

Upvotes: 0

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