Unexpected whitespace in python generated strings

I am using Python to generate an ASCII file composed of very long lines. This is one example line (let's say line 100 in the file, '[...]' are added by me to shorten the line):

{6 1,14 1,[...],264 1,270 2,274 2,[...],478 1,479 8,485 1,[...]}
 

If I open the ASCII file that I generated with ipython:

f = open('myfile','r')
 print repr(f.readlines()[99])
 

I do obtain the expected line printed correctly ('[...]' are added by me to shorten the line):

'{6 1,14 1,[...],264 1,270 2,274 2,[...],478 1,479 8,485 1,[...]}\n'
 

On the contrary, if I open this file with the program that is suppose to read it, it will generate an exception, complaining about an unexpected pair after 478 1.
So I tried to open the file with vim. Still vim shows no problem, but if I copy the line as printed by vim and paste it in another text editor (in my case TextMate), this is the line that I obtain ('[...]' are added by me to shorten the line):

{6 1,14 1,[...],264 1,270      2,274 2,[...],478 1,4     79 8,485 1,[...]}
 

This line indeed has a problem after the pair 478 1.
I tried to generate my lines in different ways (concatenating, with cStringIO, ...), but I always obtain this result. When using the cStringIO, for example, the lines are generated as in the following (even though I tried to change this, as well, with no luck):

def _construct_arff(self,attributes,header,data_rows):
   """Create the string representation of a Weka ARFF file.
      *attributes* is a dictionary with attribute_name:attribute_type
        (e.g., 'num_of_days':'NUMERIC')
      *header* is a list of the attributes sorted
        (e.g., ['age','name','num_of_days'])
      *data_rows* is a list of lists with the values, sorted as in the header
        (e.g., [ [88,'John',465],[77,'Bob',223]]"""
 
   arff_str = cStringIO.StringIO()
   arff_str.write('@relation %s\n' % self.relation_name)
 
   for idx,att_name in enumerate(header):
     try:
       name = att_name.replace("\\","\\\\").replace("'","\\'")
       arff_str.write("@attribute '%s' %s\n" % (name,attributes[att_name]))
     except UnicodeEncodeError:
       arff_str.write('@attribute unicode_err_%s %s\n' 
                      % (idx,attributes[att_name]))
 
   arff_str.write('@data\n')
   for data_row in data_rows:
     row = []
     for att_idx,att_name in enumerate(header):
       att_type = attributes[att_name]
       value = data_row[att_idx]
       # numeric attributes can be sparse: None and zeros are not written
       if ((not att_type == constants.ARRF_NUMERIC)
           or not ((value == None) or value == 0)):
         row.append('%s %s' % (att_idx,value))
     arff_str.write('{' + (','.join(row)) + '}\n')
   return arff_str.getvalue()
 

UPDATE: As you can see from the code above, the function transforms a given set of data to a special arff file format. I noticed that one of the attributes I was creating contained numbers as strings (e.g., '1', instead of 1). By forcing these numbers into integers:

features[name] = int(value)
 

I recreated the arff file successfully. However I don't see how this, which is a value, can have an impact on the formatting of *att_idx*, which is always an integer, as also pointed out by @JohnMachin and @gnibbler (thanks for your answers, btw). So, even if my code runs now, I still don't see why this happens. How can the value, if not properly transformed into int, influence the formatting of something else?

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