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3.14 Expanding and Compressing Tabs
Credit: Alex Martelli
Changing tabs to the appropriate number of spaces is a reasonably frequent task, easily accomplished with Python strings' built-in expandtabs method. Because strings are immutable, the method returns a new string object (a modified copy of the original one). However, it's easy to rebind a string variable name from the original to the modified-copy value:
mystring = mystring.expandtabs( )
This doesn't change the string object to which mystring originally referred, but it does rebind the name mystring to a newly created string object in which tabs are expanded into runs of spaces.
Changing spaces into tabs is a rare and peculiar need. Compression, if that's what you're after, is far better performed in other ways, so Python doesn't offer a built-in way to unexpand spaces into tabs. We can, of course, write one. String processing tends to be fastest in a split/process/rejoin approach, rather than with repeated overall string transformations:
def unexpand(astring, tablen=8): import re pieces = re.split(r'( +)', astring.expandtabs(tablen)) lensofar = 0 for i in range(len(pieces)): thislen = len(pieces[i]) lensofar += thislen if pieces[i]==' ': numblanks = lensofar % tablen numtabs = (thislen-numblanks+tablen-1)/tablen pieces[i] = '\t'*numtabs + ' '*numblanks return ''.join(pieces)
If expandtabs didn't exist, we could write it up as a function. Here is a regular expression-based approach, similar to the one used in the recipe's unexpand function:
def expand_with_re(astring, tablen=8): import re pieces = re.split(r'(\t)', astring) lensofar = 0 for i in range(len(pieces)): if pieces[i]=='\t': pieces[i] = ' '*(tablen-lensofar%tablen) lensofar += len(pieces[i]) return ''.join(pieces)
When the regular expression contains a (parenthesized) group, re.split gives us the splitters too. This is useful here for massaging the pieces list into the form we want for the final ''.join. However, a string split by '\t', followed by interleaving the spaces joiners of suitable lengths, looks a bit better in this case:
def expand(astring, tablen=8): result =  for piece in astring.split('\t'): result.append(piece) result.append(' '*(tablen-len(piece)%tablen)) return ''.join(result[:-1])
3.14.4 See Also
Documentation for the expandtabs function in the string module in the Library Reference; Perl Cookbook Recipe 1.7.
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