I recently wanted to use an OrderedCounter and an OrderedDefaultDict. After a bit of googling I discovered that OrderedCounter is quite easy to implement:
from collections import Counter, OrderedDict
class OrderedCounter(Counter, OrderedDict): 'Counter that remembers the order elements are first seen' def __repr__(self): return '%s(%r)' % (self.__class__.__name__, OrderedDict(self)) def __reduce__(self): return self.__class__, (OrderedDict(self),)
from https://rhettinger.wordpress.com/2011/05/26/super-considered-super/
Unfortunately an OrderedDefaultDict did not seem so easy. I did find http://stackoverflow.com/questions/6190331/can-i-do-an-ordered-default-dict-... which suggests:
|fromcollections importOrderedDict,CallableclassDefaultOrderedDict(OrderedDict):# Source: http://stackoverflow.com/a/6190500/562769def__init__(self,default_factory=No... isnotNoneandnotisinstance(default_factory,Callable)):raiseTypeError('first argument must be callable')OrderedDict.__init__(self,*a,**kw)self.default_factory =default_factory def__getitem__(self,key):try:returnOrderedDict.__getitem__(self,key)exceptKeyError:returnself.__missing__(key)def__missing__(self,key):ifself.default_factory isNone:raiseKeyError(key)self[key]=value =self.default_factory()returnvalue def__reduce__(self):ifself.default_factory isNone:args =tuple()else:args =self.default_factory,returntype(self),args,None,None,self.items()defcopy(self):returnself.__copy__()def__copy__(self):returntype(self)(self.default_factory,self)def__deepcopy__(self,memo):importcopy returntype(self)(self.default_factory,copy.deepcopy(self.items()))def__repr__(self):return'OrderedDefaultDict(%s, %s)'%(self.default_factory,OrderedDict.__repr__(self))|
This seems to me both easy to get wrong and hard to discover. It also risks getting out of sync with updates to collections. I'm wondering if this is sufficient justification to add OrderedCounter and OrderedDict to collections, either directly or as recipes.
Thanks, Ian F