在 Pandas 中将多列转换为类别。申请？
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Converting multiple columns to categories in Pandas. apply?
提问by Amelio Vazquez-Reina
Consider a Dataframe. I want to convert a set of columns to_convertto categories.
考虑一个数据框。我想将一组列转换to_convert为类别。
I can certainly do the following:
我当然可以做到以下几点：
for col in to_convert:
df[col] = df[col].astype('category')
but I was surprised that the following does not return a dataframe:
但我很惊讶以下不返回数据帧：
df[to_convert].apply(lambda x: x.astype('category'), axis=0)
which of course makes the following not work:
这当然会使以下内容不起作用：
df[to_convert] = df[to_convert].apply(lambda x: x.astype('category'), axis=0)
Why does apply(axis=0) return a Series even though it is supposed to act on the columns one by one?
为什么apply( axis=0) 返回一个系列，即使它应该一一作用于列？
采纳答案by Jeff
This was just fixed in master, and so will be in 0.17.0, see the issue here
这只是在 master 中修复，因此将在 0.17.0 中，请在此处查看问题
In [7]: df = DataFrame({'A' : list('aabbcd'), 'B' : list('ffghhe')})
In [8]: df
Out[8]:
A B
0 a f
1 a f
2 b g
3 b h
4 c h
5 d e
In [9]: df.dtypes
Out[9]:
A object
B object
dtype: object
In [10]: df.apply(lambda x: x.astype('category'))
Out[10]:
A B
0 a f
1 a f
2 b g
3 b h
4 c h
5 d e
In [11]: df.apply(lambda x: x.astype('category')).dtypes
Out[11]:
A category
B category
dtype: object
回答by joelostblom
Note that since pandas 0.23.0you no longer applyto convert multiple columns to categorical data types. Now you can simply do df[to_convert].astype('category')instead (where to_convertis a set of columns as defined in the question).
请注意，从 pandas 0.23.0 开始，您不再apply需要将多列转换为分类数据类型。现在你可以简单地做df[to_convert].astype('category')（哪里to_convert是问题中定义的一组列）。
