pandas 何时使用类别而不是对象？
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When to use Category rather than Object?
提问by user4640449
I have a CSV dataset with 40 features that I am handling with Pandas. 7 features are continuous (int32) and the rest of them are categorical.
我有一个 CSV 数据集，其中包含 40 个我正在使用 Pandas 处理的功能。7 个特征是连续的 ( int32)，其余特征是分类的。
My question is :
我的问题是：
Should I use the dtype('category')of Pandas for the categorical features, or can I let the default dtype('object')?
我应该将dtype('category')Pandas 用于分类特征，还是可以使用默认值dtype('object')？
回答by chrisaycock
Use a category when there is lots of repetition that you expect to exploit.
当您希望利用大量重复时，请使用类别。
For example, suppose I want the aggregate size per exchange for a large table of trades. Using the default objectis totally reasonable:
例如，假设我想要一个大型交易表的每个交易所的总规模。使用默认值object是完全合理的：
In [6]: %timeit trades.groupby('exch')['size'].sum()
1000 loops, best of 3: 1.25 ms per loop
But since the list of possible exchanges is pretty small, and because there is lots of repetition, I could make this faster by using a category:
但是由于可能的交换列表非常小，并且因为有很多重复，我可以通过使用一个来加快速度category：
In [7]: trades['exch'] = trades['exch'].astype('category')
In [8]: %timeit trades.groupby('exch')['size'].sum()
1000 loops, best of 3: 702 μs per loop
Note that categories are really a form of dynamic enumeration. They are most useful if the range of possible values is fixed and finite.
请注意，类别实际上是一种动态枚举形式。如果可能值的范围是固定且有限的，则它们最有用。
回答by willk
The Pandas documentation has a concise sectionon when to use the categoricaldata type:
Pandas 文档有一个关于何时使用数据类型的简洁部分categorical：
The categorical data type is useful in the following cases:
- A string variable consisting of only a few different values. Converting such a string variable to a categorical variable will save some memory, see here.
- The lexical order of a variable is not the same as the logical order (“one”, “two”, “three”). By converting to a categorical and specifying an order on the categories, sorting and min/max will use the logical order instead of the lexical order, see here.
- As a signal to other Python libraries that this column should be treated as a categorical variable (e.g. to use suitable statistical methods or plot types).
分类数据类型在以下情况下很有用：
