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Every now and then I "discover" a cute one-liner data transformation idiom. Here is one to revert the order of columns in a 2-D iterable. It comes in handy to invert a dictionary (i.e. to swap keys and values).

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# the idiom:

def theIdiom(a2Diterable): 
  return zip(*reversed(zip(*a2Diterable))) 

# sample use case:

def invertedDict(aDict):
  return dict(zip(*reversed(zip(*aDict.items()))))

I used this for unescaping XML/HTML, in order to derive the reverse entity mapping from a given "forward" mapping (most notably: htmlentitydefs.entitydefs). See the python documentation on xml.sax.saxutils' escape and unescape functions for more information.

Cheers and happy column reverting.

3 comments

Matteo Dell'Amico 14 years, 4 months ago  # | flag

I find it more readable, and more or less as concise, to use generator expressions:

def invertedDict(d):
    return dict((k, v) for k, v in d.iteritems())

maybe using reversed instead of explicit unpacking in some cases.

Matteo Dell'Amico 14 years, 4 months ago  # | flag

Whoops, stupid error. It should obviously be

def invertedDict(d):
    return dict((v, k) for k, v in d.iteritems())
Zoran Isailovski (author) 14 years, 4 months ago  # | flag

Right,... I admit,

[ tuple(reversed(row)) for row in iterable ]

does the job equally well. Thx.

(Now you made me wonder if I've got a addiction to pre-generator map/filter/reduce idioms... ;-)

Created by Zoran Isailovski on Tue, 17 Jul 2007 (PSF)
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