ActiveState Code

Guido inspired SQL-like GROUPBY class that also encapsulates the logic in a Unix-like "sort | uniq".

Comments

1. 1. At 12:40 p.m. on 14 jan 2004, Wesley Dyk said:

   Zope Friendly Version. I use a Zope installation that uses Python 2.1 and it doesn't support iterators. Also, it doesn't allow variable names that start with '_'. So I made a modification to use in Zope for use in creating web reports. Just create a new script or a function within a script with the parameters seq and key (just like __init__ in the recipe). Use this code inside the function or script:

   groupdict = {}
   for item in seq:
     k = key(item)
     groupdict.setdefault(k, []).append(item)
   return groupdict.items()
   

   Since the Zope version that I have uses Python 2.1, it can't use iteritems, so it has to return an actual list. This means that a copy of the list of key, value pairs is created. This could drop performance if you have a large sequence.

2. 2. At 7:30 a.m. on 13 apr 2004, Maxim Krikun said:

   redundant list creation. Note that in this line

   self.setdefault(k, []).append(value)
   

   an empty list is instantiated on each iteration.

   I prefer to use for similar tasks a dict subclass with KeyError catched inside, like follows:

   class idict(dict):
       '''self-initializing dictionnary'''
       def __init__(self,init):
           dict.__init__(self)
           self.init=init
       def __getitem__(self,key):
           try:
               return dict.__getitem__(self,key)
           except KeyError:
               if callable(self.init):
                   self[key]=self.init()
               else:
                   self[key]=self.init
               return dict.__getitem__(self,key)
   

   Then the groupby could be defined as a function:

   def groupby(seq, key=lambda x:x):
       d=idict(list)
       for value in seq:
           d[key(value)].append(value)
       return d.items()
   

   This class also provides a simple way to count list entries:

   def count_list_items(ll):
       d=idict(0)
       for l in ll:
           d[l]+=1
       return d.items()
   

3. 3. At 3:13 p.m. on 30 oct 2004, Jonathan Wright said:

   Can someone put the idict in its own recipe? Recently I have used the idict class a number of times. It seems useful enough to warrant its own recipe.

   Thanks, Jonathan.

4. 4. At 11:56 a.m. on 2 may 2005, Raymond Hettinger (the author) said:

   idict() is pennywise and pound foolish. The cost of setdefault() instantiating an empty list is miniscule in comparison with the overhead of a __setitem__ call to idict().

5. 5. At 3:36 p.m. on 3 feb 2006, runsun pan said:

   An application of this nice recipe:

   groupbyhead: Group a list of items according to the starting character(s) of items.

   https://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/259173

6. 6. At 2:52 a.m. on 17 aug 2007, Louis Riviere said:

   Getting rid of setdefault ; using defaultdict instead. def group(data, key=None):

   ____d=defaultdict(list)

   ____for v in data:

   ________k=key(v) if key else v

   ________d[k].append(v)

   ____return d.items()

7. 7. At 6:37 p.m. on 3 oct 2007, Tim J said:

   defaultdict is New in version 2.5. Alas. I itch for it weekly.

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Tags

• algorithms