python - 未应用 kind 参数时 Panda 的 sort_values 使用的排序算法

Question

在 Pandas 的 sort_values 方法中，kind 参数仅在对单个列或标签进行排序时应用。为什么会这样？在这些未应用 kind 参数的情况下使用什么排序算法？是稳定排序吗？

(有关文档，请参阅 https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.sort_values.html。)

Answer 1

这是一个docstring from the source file ，声明 get_group_index_sorter(group_index, ngroups):

algos.groupsort_indexer implements `counting sort` and it is at least
O(ngroups), where
    ngroups = prod(shape)
    shape = map(len, keys)
that is, linear in the number of combinations (cartesian product) of unique
values of groupby keys. This can be huge when doing multi-key groupby.
np.argsort(kind='mergesort') is O(count x log(count)) where count is the
length of the data-frame;

---

Both algorithms are `stable` sort and that is necessary for correctness of

---

groupby operations. e.g. consider:
    df.groupby(key)[col].transform('first')

PS这里是一个“调用链”:

pandas.core.frame.DataFrame.sort_values() -> \
  pandas.core.sorting.lexsort_indexer() ->  \
    pandas.core.sorting.indexer_from_factorized() -> \
      pandas.core.sorting.get_group_index_sorter()