在python3.6中，如下例子：
keys = list(newKeywords.keys())
resultkeys = list(itertools.combinations(keys,3))
当上面中的Keys太长时会发生MemoryError错误，解决办法：
keys = list(newKeywords.keys())
resultkeys = itertools.combinations(keys,3)
for rkey in resultkeys:
print(rkey)
据下面链接中所说，只有当前用到的组合才存放在内存中。
原文如下：
by doing this, python will keep in memory only the currently used permutation, not all the permutations (in term of memory usage, it is really better ;) )
reference:https://stackoverflow.com/questions/6503388/prevent-memory-error-in-itertools-permutation