Python Cookbook

Expanding and Compressing Tabs

Credit: Alex Martelli

Problem

You want to convert tabs in a string to the appropriate number of spaces, or vice versa.

Solution

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][0]==' ':
            numblanks = lensofar % tablen
            numtabs = (thislen-numblanks+tablen-1)/tablen
            pieces[i] = '\t'*numtabs + ' '*numblanks
    return ''.join(pieces)

Discussion

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])

See Also

Documentation for the expandtabs function in the string module in the Library Reference; Perl Cookbook Recipe 1.7.