Flatten lists of lists. Including callback argument to filter iterable types.

list_flatten.py

import collections

# ignore functions
ignore_strings = lambda x: isinstance(x, basestring)
ignore_dicts =  lambda x: isinstance(x, dict)

def list_flatten(*ilist, **kwargs):
	"""
	Flattens nested indexable iterables, i.e lists of lists
	By default treats strings and dicts as scalar values
	LIST ORDER IS ALWAYS IN REVERSE. Use reversed() to reorder, if needed.
	
	Kwarg "ignore" is list of functions
	Each function takes a single argument that returns True if a value should be treated as a scalar
		or false if it should be iterated: must be indexable with integer e.g. iterable[0]
		Dicts are always treated as scalars.

	String can be treated as iterables if ignore=[]
	"""
	# ignore_checks is a list of functions 
	# returning True to indicate an iterable should be treated as single element
	ignore_checks = []
	if 'ignore' in kwargs and isinstance(kwargs['ignore'], list):
		ignore_checks = kwargs['ignore']
		#always ignore dicts
		if ignore_dicts not in ignore_checks:
			ignore_checks.append(ignore_dicts)
	else:
		#defaults to ignoring strings and dicts
		ignore_checks += [ignore_strings, ignore_dicts]

	# this would be to hard to read with indexed tuples, but they should be used instead
	itree = [{'item':ilist, 'index':len(ilist)-1}]
	newlist = []
	pdict = itree[0]
	while itree:
		if pdict['index'] >= 0 :
			citem = pdict['item'][pdict['index']]
			if isinstance(citem, collections.Iterable) \
			 and not True in [func(citem) for func in ignore_checks]:
				itree.append({'item': citem ,'index':len(citem)-1})
				pdict['index'] -= 1
				pdict = itree[-1]
				# continue
			else:
				newlist.append(citem)
				pdict['index'] -= 1
				# continue
		else:
			itree.pop()
			if len(itree) > 0:
				pdict = itree[-1]
				# continue
			else:
				#we are done
				break
	return newlist

if __name__ == "__main__":
	# simple tests
	tl1 = [1, 2]
	tl2 = ['a','b','c']
	tl3 = tl1 + [3, 4, [5, 6, 'seven']] + [8]
	tl4 = tl1 + (tl2)
	tl5 = ["WHAT","IS","THIS"]

	print list_flatten(1, [2, [3]], 4)
	print list_flatten(tl3)
	print list_flatten([tl1+tl2+[tl1+[tl2]]])
	print list_flatten([[7,6], 5, 4, [3]], tl5, 2, 1)
	print list_flatten([1,2], "String", ['a','b','c'],['1a','2a','3a'])
	print list_flatten([1,2], "String", dict([('a',1),('b',2)]), ['a','b','c'],['1a','2a','3a'])