通过标签选择的 Pandas 有时返回 Series，有时返回 DataFrame [英] Pandas selecting by label sometimes return Series, sometimes returns DataFrame

问题描述

在 Pandas 中，当我选择一个索引中只有一个条目的标签时，我会返回一个系列，但是当我选择一个包含多个条目的条目时，我会返回一个数据框.

In Pandas, when I select a label that only has one entry in the index I get back a Series, but when I select an entry that has more then one entry I get back a data frame.

这是为什么?有没有办法确保我总是能取回数据框?

Why is that? Is there a way to ensure I always get back a data frame?

In [1]: import pandas as pd

In [2]: df = pd.DataFrame(data=range(5), index=[1, 2, 3, 3, 3])

In [3]: type(df.loc[3])
Out[3]: pandas.core.frame.DataFrame

In [4]: type(df.loc[1])
Out[4]: pandas.core.series.Series

推荐答案

虽然行为不一致，但我认为很容易想象这样方便的情况.无论如何，要每次都获得一个 DataFrame，只需将一个列表传递给 loc.还有其他方法，但在我看来这是最干净的.

Granted that the behavior is inconsistent, but I think it's easy to imagine cases where this is convenient. Anyway, to get a DataFrame every time, just pass a list to loc. There are other ways, but in my opinion this is the cleanest.

In [2]: type(df.loc[[3]])
Out[2]: pandas.core.frame.DataFrame

In [3]: type(df.loc[[1]])
Out[3]: pandas.core.frame.DataFrame

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