Pandas高级教程之:GroupBy用法

缺乏、安全感 2022-10-04 15:46 279阅读 0赞

### 文章目录 ###

*  简介
 *  分割数据
 *   *  多index
     *  get\_group
     *  dropna
     *  groups属性
     *  index的层级
 *  group的遍历
 *  聚合操作
 *   *  通用聚合方法
     *  同时使用多个聚合方法
     *  NamedAgg
     *  不同的列指定不同的聚合方法
 *  转换操作
 *  过滤操作
 *  Apply操作

# 简介 #

pandas中的DF数据类型可以像数据库表格一样进行groupby操作。通常来说groupby操作可以分为三部分：分割数据，应用变换和和合并数据。

本文将会详细讲解Pandas中的groupby操作。

# 分割数据 #

分割数据的目的是将DF分割成为一个个的group。为了进行groupby操作，在创建DF的时候需要指定相应的label：

df = pd.DataFrame(
       ...:     {
       ...:         "A": ["foo", "bar", "foo", "bar", "foo", "bar", "foo", "foo"],
       ...:         "B": ["one", "one", "two", "three", "two", "two", "one", "three"],
       ...:         "C": np.random.randn(8),
       ...:         "D": np.random.randn(8),
       ...:     }
       ...: )
       ...:
    
    df
    Out[61]: 
         A      B         C         D
    0  foo    one -0.490565 -0.233106
    1  bar    one  0.430089  1.040789
    2  foo    two  0.653449 -1.155530
    3  bar  three -0.610380 -0.447735
    4  foo    two -0.934961  0.256358
    5  bar    two -0.256263 -0.661954
    6  foo    one -1.132186 -0.304330
    7  foo  three  2.129757  0.445744

默认情况下，groupby的轴是x轴。可以一列group，也可以多列group：

In [8]: grouped = df.groupby("A")
    
    In [9]: grouped = df.groupby(["A", "B"])

## 多index ##

在*0.24*版本中，如果我们有多index，可以从中选择特定的index进行group：

In [10]: df2 = df.set_index(["A", "B"])
    
    In [11]: grouped = df2.groupby(level=df2.index.names.difference(["B"]))
    
    In [12]: grouped.sum()
    Out[12]: 
                C         D
    A                      
    bar -1.591710 -1.739537
    foo -0.752861 -1.402938

## get\_group ##

get\_group 可以获取分组之后的数据：

In [24]: df3 = pd.DataFrame({"X": ["A", "B", "A", "B"], "Y": [1, 4, 3, 2]})
    
    In [25]: df3.groupby(["X"]).get_group("A")
    Out[25]: 
       X  Y
    0  A  1
    2  A  3
    
    In [26]: df3.groupby(["X"]).get_group("B")
    Out[26]: 
       X  Y
    1  B  4
    3  B  2

## dropna ##

默认情况下，NaN数据会被排除在groupby之外，通过设置 dropna=False 可以允许NaN数据：

In [27]: df_list = [[1, 2, 3], [1, None, 4], [2, 1, 3], [1, 2, 2]]
    
    In [28]: df_dropna = pd.DataFrame(df_list, columns=["a", "b", "c"])
    
    In [29]: df_dropna
    Out[29]: 
       a    b  c
    0  1  2.0  3
    1  1  NaN  4
    2  2  1.0  3
    3  1  2.0  2

# Default ``dropna`` is set to True, which will exclude NaNs in keys
    In [30]: df_dropna.groupby(by=["b"], dropna=True).sum()
    Out[30]: 
         a  c
    b        
    1.0  2  3
    2.0  2  5
    
    # In order to allow NaN in keys, set ``dropna`` to False
    In [31]: df_dropna.groupby(by=["b"], dropna=False).sum()
    Out[31]: 
         a  c
    b        
    1.0  2  3
    2.0  2  5
    NaN  1  4

## groups属性 ##

groupby对象有个groups属性，它是一个key-value字典，key是用来分类的数据，value是分类对应的值。

In [34]: grouped = df.groupby(["A", "B"])
    
    In [35]: grouped.groups
    Out[35]: {('bar', 'one'): [1], ('bar', 'three'): [3], ('bar', 'two'): [5], ('foo', 'one'): [0, 6], ('foo', 'three'): [7], ('foo', 'two'): [2, 4]}
    
    In [36]: len(grouped)
    Out[36]: 6

## index的层级 ##

对于多级index对象，groupby可以指定group的index层级：

In [40]: arrays = [
       ....:     ["bar", "bar", "baz", "baz", "foo", "foo", "qux", "qux"],
       ....:     ["one", "two", "one", "two", "one", "two", "one", "two"],
       ....: ]
       ....: 
    
    In [41]: index = pd.MultiIndex.from_arrays(arrays, names=["first", "second"])
    
    In [42]: s = pd.Series(np.random.randn(8), index=index)
    
    In [43]: s
    Out[43]: 
    first  second
    bar    one      -0.919854
           two      -0.042379
    baz    one       1.247642
           two      -0.009920
    foo    one       0.290213
           two       0.495767
    qux    one       0.362949
           two       1.548106
    dtype: float64

group第一级：

In [44]: grouped = s.groupby(level=0)
    
    In [45]: grouped.sum()
    Out[45]: 
    first
    bar   -0.962232
    baz    1.237723
    foo    0.785980
    qux    1.911055
    dtype: float64

group第二级：

In [46]: s.groupby(level="second").sum()
    Out[46]: 
    second
    one    0.980950
    two    1.991575
    dtype: float64

# group的遍历 #

得到group对象之后，我们可以通过for语句来遍历group：

In [62]: grouped = df.groupby('A')
    
    In [63]: for name, group in grouped:
       ....:     print(name)
       ....:     print(group)
       ....: 
    bar
         A      B         C         D
    1  bar    one  0.254161  1.511763
    3  bar  three  0.215897 -0.990582
    5  bar    two -0.077118  1.211526
    foo
         A      B         C         D
    0  foo    one -0.575247  1.346061
    2  foo    two -1.143704  1.627081
    4  foo    two  1.193555 -0.441652
    6  foo    one -0.408530  0.268520
    7  foo  three -0.862495  0.024580

如果是多字段group，group的名字是一个元组：

In [64]: for name, group in df.groupby(['A', 'B']):
       ....:     print(name)
       ....:     print(group)
       ....: 
    ('bar', 'one')
         A    B         C         D
    1  bar  one  0.254161  1.511763
    ('bar', 'three')
         A      B         C         D
    3  bar  three  0.215897 -0.990582
    ('bar', 'two')
         A    B         C         D
    5  bar  two -0.077118  1.211526
    ('foo', 'one')
         A    B         C         D
    0  foo  one -0.575247  1.346061
    6  foo  one -0.408530  0.268520
    ('foo', 'three')
         A      B         C        D
    7  foo  three -0.862495  0.02458
    ('foo', 'two')
         A    B         C         D
    2  foo  two -1.143704  1.627081
    4  foo  two  1.193555 -0.441652

# 聚合操作 #

分组之后，就可以进行聚合操作：

In [67]: grouped = df.groupby("A")
    
    In [68]: grouped.aggregate(np.sum)
    Out[68]: 
                C         D
    A                      
    bar  0.392940  1.732707
    foo -1.796421  2.824590
    
    In [69]: grouped = df.groupby(["A", "B"])
    
    In [70]: grouped.aggregate(np.sum)
    Out[70]: 
                      C         D
    A   B                        
    bar one    0.254161  1.511763
        three  0.215897 -0.990582
        two   -0.077118  1.211526
    foo one   -0.983776  1.614581
        three -0.862495  0.024580
        two    0.049851  1.185429

对于多index数据来说，默认返回值也是多index的。如果想使用新的index，可以添加 as\_index = False：

In [71]: grouped = df.groupby(["A", "B"], as_index=False)
    
    In [72]: grouped.aggregate(np.sum)
    Out[72]: 
         A      B         C         D
    0  bar    one  0.254161  1.511763
    1  bar  three  0.215897 -0.990582
    2  bar    two -0.077118  1.211526
    3  foo    one -0.983776  1.614581
    4  foo  three -0.862495  0.024580
    5  foo    two  0.049851  1.185429
    
    In [73]: df.groupby("A", as_index=False).sum()
    Out[73]: 
         A         C         D
    0  bar  0.392940  1.732707
    1  foo -1.796421  2.824590

上面的效果等同于reset\_index

In [74]: df.groupby(["A", "B"]).sum().reset_index()

grouped.size() 计算group的大小：

In [75]: grouped.size()
    Out[75]: 
         A      B  size
    0  bar    one     1
    1  bar  three     1
    2  bar    two     1
    3  foo    one     2
    4  foo  three     1
    5  foo    two     2

grouped.describe() 描述group的信息：

In [76]: grouped.describe()
    Out[76]: 
          C                                                    ...         D                                                  
      count      mean       std       min       25%       50%  ...       std       min       25%       50%       75%       max
    0   1.0  0.254161       NaN  0.254161  0.254161  0.254161  ...       NaN  1.511763  1.511763  1.511763  1.511763  1.511763
    1   1.0  0.215897       NaN  0.215897  0.215897  0.215897  ...       NaN -0.990582 -0.990582 -0.990582 -0.990582 -0.990582
    2   1.0 -0.077118       NaN -0.077118 -0.077118 -0.077118  ...       NaN  1.211526  1.211526  1.211526  1.211526  1.211526
    3   2.0 -0.491888  0.117887 -0.575247 -0.533567 -0.491888  ...  0.761937  0.268520  0.537905  0.807291  1.076676  1.346061
    4   1.0 -0.862495       NaN -0.862495 -0.862495 -0.862495  ...       NaN  0.024580  0.024580  0.024580  0.024580  0.024580
    5   2.0  0.024925  1.652692 -1.143704 -0.559389  0.024925  ...  1.462816 -0.441652  0.075531  0.592714  1.109898  1.627081
    
    [6 rows x 16 columns]

## 通用聚合方法 ##

下面是通用的聚合方法：

<table> 
 <thead> 
  <tr> 
   <th align="left">函数</th> 
   <th align="left">描述</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left"><code>mean()</code></td> 
   <td align="left">平均值</td> 
  </tr> 
  <tr> 
   <td align="left"><code>sum()</code></td> 
   <td align="left">求和</td> 
  </tr> 
  <tr> 
   <td align="left"><code>size()</code></td> 
   <td align="left">计算size</td> 
  </tr> 
  <tr> 
   <td align="left"><code>count()</code></td> 
   <td align="left">group的统计</td> 
  </tr> 
  <tr> 
   <td align="left"><code>std()</code></td> 
   <td align="left">标准差</td> 
  </tr> 
  <tr> 
   <td align="left"><code>var()</code></td> 
   <td align="left">方差</td> 
  </tr> 
  <tr> 
   <td align="left"><code>sem()</code></td> 
   <td align="left">均值的标准误</td> 
  </tr> 
  <tr> 
   <td align="left"><code>describe()</code></td> 
   <td align="left">统计信息描述</td> 
  </tr> 
  <tr> 
   <td align="left"><code>first()</code></td> 
   <td align="left">第一个group值</td> 
  </tr> 
  <tr> 
   <td align="left"><code>last()</code></td> 
   <td align="left">最后一个group值</td> 
  </tr> 
  <tr> 
   <td align="left"><code>nth()</code></td> 
   <td align="left">第n个group值</td> 
  </tr> 
  <tr> 
   <td align="left"><code>min()</code></td> 
   <td align="left">最小值</td> 
  </tr> 
  <tr> 
   <td align="left"><code>max()</code></td> 
   <td align="left">最大值</td> 
  </tr> 
 </tbody> 
</table>

## 同时使用多个聚合方法 ##

可以同时指定多个聚合方法：

In [81]: grouped = df.groupby("A")
    
    In [82]: grouped["C"].agg([np.sum, np.mean, np.std])
    Out[82]: 
              sum      mean       std
    A                                
    bar  0.392940  0.130980  0.181231
    foo -1.796421 -0.359284  0.912265

可以重命名：

In [84]: (
       ....:     grouped["C"]
       ....:     .agg([np.sum, np.mean, np.std])
       ....:     .rename(columns={"sum": "foo", "mean": "bar", "std": "baz"})
       ....: )
       ....: 
    Out[84]: 
              foo       bar       baz
    A                                
    bar  0.392940  0.130980  0.181231
    foo -1.796421 -0.359284  0.912265

## NamedAgg ##

NamedAgg 可以对聚合进行更精准的定义，它包含 column 和aggfunc 两个定制化的字段。

In [88]: animals = pd.DataFrame(
       ....:     {
       ....:         "kind": ["cat", "dog", "cat", "dog"],
       ....:         "height": [9.1, 6.0, 9.5, 34.0],
       ....:         "weight": [7.9, 7.5, 9.9, 198.0],
       ....:     }
       ....: )
       ....: 
    
    In [89]: animals
    Out[89]: 
      kind  height  weight
    0  cat     9.1     7.9
    1  dog     6.0     7.5
    2  cat     9.5     9.9
    3  dog    34.0   198.0
    
    In [90]: animals.groupby("kind").agg(
       ....:     min_height=pd.NamedAgg(column="height", aggfunc="min"),
       ....:     max_height=pd.NamedAgg(column="height", aggfunc="max"),
       ....:     average_weight=pd.NamedAgg(column="weight", aggfunc=np.mean),
       ....: )
       ....: 
    Out[90]: 
          min_height  max_height  average_weight
    kind                                        
    cat          9.1         9.5            8.90
    dog          6.0        34.0          102.75

或者直接使用一个元组：

In [91]: animals.groupby("kind").agg(
       ....:     min_height=("height", "min"),
       ....:     max_height=("height", "max"),
       ....:     average_weight=("weight", np.mean),
       ....: )
       ....: 
    Out[91]: 
          min_height  max_height  average_weight
    kind                                        
    cat          9.1         9.5            8.90
    dog          6.0        34.0          102.75

## 不同的列指定不同的聚合方法 ##

通过给agg方法传入一个字典，可以指定不同的列使用不同的聚合：

In [95]: grouped.agg({"C": "sum", "D": "std"})
    Out[95]: 
                C         D
    A                      
    bar  0.392940  1.366330
    foo -1.796421  0.884785

# 转换操作 #

转换是将对象转换为同样大小对象的操作。在数据分析的过程中，经常需要进行数据的转换操作。

可以接lambda操作：

In [112]: ts.groupby(lambda x: x.year).transform(lambda x: x.max() - x.min())

填充na值：

In [121]: transformed = grouped.transform(lambda x: x.fillna(x.mean()))

# 过滤操作 #

filter方法可以通过lambda表达式来过滤我们不需要的数据：

In [136]: sf = pd.Series([1, 1, 2, 3, 3, 3])
    
    In [137]: sf.groupby(sf).filter(lambda x: x.sum() > 2)
    Out[137]: 
    3    3
    4    3
    5    3
    dtype: int64

# Apply操作 #

有些数据可能不适合进行聚合或者转换操作，Pandas提供了一个 `apply` 方法，用来进行更加灵活的转换操作。

In [156]: df
    Out[156]: 
         A      B         C         D
    0  foo    one -0.575247  1.346061
    1  bar    one  0.254161  1.511763
    2  foo    two -1.143704  1.627081
    3  bar  three  0.215897 -0.990582
    4  foo    two  1.193555 -0.441652
    5  bar    two -0.077118  1.211526
    6  foo    one -0.408530  0.268520
    7  foo  three -0.862495  0.024580
    
    In [157]: grouped = df.groupby("A")
    
    # could also just call .describe()
    In [158]: grouped["C"].apply(lambda x: x.describe())
    Out[158]: 
    A         
    bar  count    3.000000
         mean     0.130980
         std      0.181231
         min     -0.077118
         25%      0.069390
                    ...   
    foo  min     -1.143704
         25%     -0.862495
         50%     -0.575247
         75%     -0.408530
         max      1.193555
    Name: C, Length: 16, dtype: float64

可以外接函数：

In [159]: grouped = df.groupby('A')['C']
    
    In [160]: def f(group):
       .....:     return pd.DataFrame({'original': group,
       .....:                          'demeaned': group - group.mean()})
       .....: 
    
    In [161]: grouped.apply(f)
    Out[161]: 
       original  demeaned
    0 -0.575247 -0.215962
    1  0.254161  0.123181
    2 -1.143704 -0.784420
    3  0.215897  0.084917
    4  1.193555  1.552839
    5 -0.077118 -0.208098
    6 -0.408530 -0.049245
    7 -0.862495 -0.503211

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