Reading \n unescaped from a file

Friedrich Rentsch anthra.norell at bluewin.ch
Thu Sep 3 22:21:32 CEST 2015



On 09/03/2015 06:12 PM, Rob Hills wrote:
> 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,
>
I'm sure you can do it with replace, except if your replacements add up 
into the dozens, it gets awkward, as you do the substitutions one by one 
on the whole text. But the real problem with this approach is that you 
are responsible for doing the replacements in the right sequence. It has 
been pointed out that order matters. What you want with overlapping 
targets is upstream takes precedence over downstream and longer over 
shorter. My suggestion automates this as it automates the construction 
of the regex.

Peter Otten found one rough sport and one mistake:

    substitutes = [self.table [item] for item in hits if item in 
valid_hits] + []

Adding an empty list is totally useless and can be omitted.

    output = input

Second last line should be; output = text


Regards

Frederic




More information about the Python-list mailing list