Reading \n unescaped from a file

[Rob Hills]

Hi Friedrich,

On 03/09/15 16:40, Friedrich Rentsch wrote:
>
> On 09/02/2015 04:03 AM, Rob Hills wrote:
>> Hi,
>>
>> I am developing code (Python 3.4) that transforms text data from one
>> format to another.
>>
>> As part of the process, I had a set of hard-coded str.replace(...)
>> functions that I used to clean up the incoming text into the desired
>> output format, something like this:
>>
>>      dataIn = dataIn.replace('\r', '\\n') # Tidy up linefeeds
>>      dataIn = dataIn.replace('<','<') # Tidy up < character
>>      dataIn = dataIn.replace('>','>') # Tidy up < character
>>      dataIn = dataIn.replace('o','o') # No idea why but lots of
>> these: convert to 'o' character
>>      dataIn = dataIn.replace('f','f') # .. and these: convert to
>> 'f' character
>>      dataIn = dataIn.replace('e','e') # ..  'e'
>>      dataIn = dataIn.replace('O','O') # ..  'O'
>>
>> These statements transform my data correctly, but the list of statements
>> grows as I test the data so I thought it made sense to store the
>> replacement mappings in a file, read them into a dict and loop through
>> that to do the cleaning up, like this:
>>
>>          with open(fileName, 'r+t', encoding='utf-8') as mapFile:
>>              for line in mapFile:
>>                  line = line.strip()
>>                  try:
>>                      if (line) and not line.startswith('#'):
>>                          line = line.split('#')[:1][0].strip() # trim
>> any trailing comments
>>                          name, value = line.split('=')
>>                          name = name.strip()
>>                          self.filterMap[name]=value.strip()
>>                  except:
>>                      self.logger.error('exception occurred parsing
>> line [{0}] in file [{1}]'.format(line, fileName))
>>                      raise
>>
>> Elsewhere, I use the following code to do the actual cleaning up:
>>
>>      def filter(self, dataIn):
>>          if dataIn:
>>              for token, replacement in self.filterMap.items():
>>                  dataIn = dataIn.replace(token, replacement)
>>          return dataIn
>>
>>
>> My mapping file contents look like this:
>>
>> \r = \\n
>> “ = "
>> < = <
>> > = >
>> ' = '
>> F = F
>> o = o
>> f = f
>> e = e
>> O = O
>>
>> This all works "as advertised" */except/* for the '\r' => '\\n'
>> replacement. Debugging the code, I see that my '\r' character is
>> "escaped" to '\\r' and the '\\n' to '\\\\n' when they are read in from
>> the file.
>>
>> I've been googling hard and reading the Python docs, trying to get my
>> head around character encoding, but I just can't figure out how to get
>> these bits of code to do what I want.
>>
>> It seems to me that I need to either:
>>
>>    * change the way I represent '\r' and '\\n' in my mapping file; or
>>    * transform them somehow when I read them in
>>
>> However, I haven't figured out how to do either of these.
>>
>> TIA,
>>
>>
>
> I have had this problem too and can propose a solution ready to run
> out of my toolbox:
>
>
> class editor:
>
>     def compile (self, replacements):
>         targets, substitutes = zip (*replacements)
>         re_targets = [re.escape (item) for item in targets]
>         re_targets.sort (reverse = True)
>         self.targets_set = set (targets)
>         self.table = dict (replacements)
>         regex_string = '|'.join (re_targets)
>         self.regex = re.compile (regex_string, re.DOTALL)
>
>     def edit (self, text, eat = False):
>         hits = self.regex.findall (text)
>         nohits = self.regex.split (text)
>         valid_hits = set (hits) & self.targets_set  # Ignore targets
> with illegal re modifiers.
>         if valid_hits:
>             substitutes = [self.table [item] for item in hits if item
> in valid_hits] + []  # Make lengths equal for zip to work right
>             if eat:
>                 output = ''.join (substitutes)
>             else:
>                 zipped = zip (nohits, substitutes)
>                 output = ''.join (list (reduce (lambda a, b: a + b,
> [zipped][0]))) + nohits [-1]
>         else:
>             if eat:
>                 output = ''
>             else:
>                 output = input
>         return output
>
> >>> substitutions = (
>     ('\r', '\n'),
>     ('<', '<'),
>     ('>', '>'),
>     ('o', 'o'),
>     ('f', 'f'),
>     ('e', 'e'),
>     ('O', 'O'),
>     )
>
> Order doesn't matter. Add new ones at the end.
>
> >>> e = editor ()
> >>> e.compile (substitutions)
>
> A simple way of testing is running the substitutions through the editor
>
> >>> print e.edit (repr (substitutions))
> (('\r', '\n'), ('<', '<'), ('>', '>'), ('o', 'o'), ('f', 'f'), ('e',
> 'e'), ('O', 'O'))
>
> The escapes need to be tested separately
>
> >>> print e.edit ('abc\rdef')
> abc
> def
>
> Note: This editor's compiler compiles the substitution list to a
> regular expression which the editor uses to find all matches in the
> text passed to edit. There has got to be a limit to the size of a text
> which a regular expression can handle. I don't know what this limit
> is. To be on the safe side, edit a large text line by line or at least
> in sensible chunks.
>
> Frederic
>

Thanks for the suggestion.  I had originally done a simple set of
hard-coded str.replace() functions which worked fine and are fast enough
for me not to have to delve into the complexity and obscurity of regex.

I had also contemplated simply declaring my replacement dict in its own
.py file and then importing it.

I ended up stubbornly pursuing the idea of loading everything from a
text file just because I didn't understand why it wasn't working.

Cheers,

-- 
Rob Hills
Waikiki, Western Australia