Pandas 重置索引深度总结

2年前 (2022) 程序员胖胖胖虎阿

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今天我们来讨论 Pandas 中的 reset_index() 方法，包括为什么我们需要在 Pandas 中重置 DataFrame 的索引，以及我们应该如何应用该方法

在本文我们将使用 Kaggle 上的数据集样本 Animal Shelter Analytics 来作为我们的测试数据

Pandas 中的 Reset_Index() 是什么？

如果我们使用 Pandas 的 read_csv() 方法读取 csv 文件而不指定任何索引，则生成的 DataFrame 将具有默认的基于整数的索引，第一行从 0 开始，随后每行增加 1：

import pandas as pd
import numpy as np

df = pd.read_csv('Austin_Animal_Center_Intakes.csv').head()
df

Output:

    Animal ID    Name    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
0    A786884    *Brock    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
1    A706918    Belle    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
2    A724273    Runster    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
3    A665644    NaN    10/21/2013 07:59:00 AM    10/21/2013 07:59:00 AM    Austin (TX)    Stray    Sick    Cat    Intact Female    4 weeks    Domestic Shorthair Mix    Calico
4    A682524    Rio    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

在某些情况下，我们可能希望拥有更有意义的行标签，因此我们将选择 DataFrame 的其中一列作为 DataFrame 索引。我们可以使用 read_csv() 方法的 index_col 参数直接执行此操作：

df = pd.read_csv('Austin_Animal_Center_Intakes.csv', index_col='Animal ID').head()
df

Output:

Animal ID    Name    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
A786884    *Brock    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
A706918    Belle    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
A724273    Runster    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
A665644    NaN    10/21/2013 07:59:00 AM    10/21/2013 07:59:00 AM    Austin (TX)    Stray    Sick    Cat    Intact Female    4 weeks    Domestic Shorthair Mix    Calico
A682524    Rio    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

或者我们还可以使用 set_index() 方法将 DataFrame 的任何列设置为 DataFrame 索引：

df = pd.read_csv('Austin_Animal_Center_Intakes.csv').head()
df.set_index('Animal ID', inplace=True)
df

Output:

Animal ID    Name    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
A786884    *Brock    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
A706918    Belle    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
A724273    Runster    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
A665644    NaN    10/21/2013 07:59:00 AM    10/21/2013 07:59:00 AM    Austin (TX)    Stray    Sick    Cat    Intact Female    4 weeks    Domestic Shorthair Mix    Calico
A682524    Rio    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

如果在某个时候我们需要恢复默认的数字索引呢，这时就可以使用 reset_index()函数了

df.reset_index()

Output:

    Animal ID    Name    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
0    A786884    *Brock    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
1    A706918    Belle    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
2    A724273    Runster    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
3    A665644    NaN    10/21/2013 07:59:00 AM    10/21/2013 07:59:00 AM    Austin (TX)    Stray    Sick    Cat    Intact Female    4 weeks    Domestic Shorthair Mix    Calico
4    A682524    Rio    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

此方法的默认行为包括用默认的基于整数的索引替换现有的 DataFrame 索引，并将旧索引转换为与旧索引同名的新列（或名称索引）。此外，默认情况下，reset_index() 方法会从 MultiIndex 中删除所有级别并且不会影响原始 DataFrame 数据，而是创建一个新的

何时使用 Reset_Index() 方法

reset_index() 方法将 DataFrame 索引重置为默认数字索引，在以下情况下特别有用：

执行数据整理时——尤其是过滤数据或删除缺失值等预处理操作，会导致较小的 DataFrame 具有不再连续的数字索引
当索引应该被视为一个常见的 DataFrame 列时
当索引标签没有提供有关数据的任何有价值的信息时

如何调整 Reset_Index() 方法

前面的讨论中，我们看到了当我们不向它传递任何参数时，reset_index() 方法是如何工作的，当然如果有需要，我们可以通过调整方法的各种参数来更改此默认行为。让我们看看最有用的三种参数：level、drop 和 inplace

level

此参数采用整数、字符串、元组或列表作为可能的数据类型，并且仅适用于具有 MultiIndex 的 DataFrame，如下所示：

df_multiindex = pd.read_csv('Austin_Animal_Center_Intakes.csv', index_col=['Animal ID', 'Name']).head()
df_multiindex

Output:

Animal ID    Name    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
A786884    *Brock    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
A706918    Belle    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
A724273    Runster    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
A665644    NaN    10/21/2013 07:59:00 AM    10/21/2013 07:59:00 AM    Austin (TX)    Stray    Sick    Cat    Intact Female    4 weeks    Domestic Shorthair Mix    Calico
A682524    Rio    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

事实上，如果现在我们检查上面 DataFrame 的索引，我们会发现它不是一个常见的 DataFrame 索引，而是一个 MultiIndex 对象：

df_multiindex.index

Output:

MultiIndex([('A786884',  '*Brock'),
            ('A706918',   'Belle'),
            ('A724273', 'Runster'),
            ('A665644',       nan),
            ('A682524',     'Rio')],
           names=['Animal ID', 'Name'])

默认情况下，reset_index() 方法参数 level (level=None) 会移除 MultiIndex 的所有级别：

df_multiindex.reset_index()

Output:

    Animal ID    Name    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
0    A786884    *Brock    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
1    A706918    Belle    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
2    A724273    Runster    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
3    A665644    NaN    10/21/2013 07:59:00 AM    10/21/2013 07:59:00 AM    Austin (TX)    Stray    Sick    Cat    Intact Female    4 weeks    Domestic Shorthair Mix    Calico
4    A682524    Rio    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

我们看到 DataFrame 的两个索引都被转换为通用 DataFrame 列，而索引被重置为默认的基于整数的索引

相反，如果我们显式传递 level 的值，则此参数会从 DataFrame 索引中删除选定的级别，并将它们作为常见的 DataFrame 列返回（除非我们选择使用 drop 参数从 DataFrame 中完全删除此信息）。比较以下操作：

df_multiindex.reset_index(level='Animal ID')

Output:

Name    Animal ID    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
*Brock    A786884    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
Belle    A706918    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
Runster    A724273    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
NaN    A665644    10/21/2013 07:59:00 AM    10/21/2013 07:59:00 AM    Austin (TX)    Stray    Sick    Cat    Intact Female    4 weeks    Domestic Shorthair Mix    Calico
Rio    A682524    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

最开始 Animal ID 是 DataFrame 的索引之一，当我们设置 level 参数后，将其从索引中删除并作为称为 Animal ID 的公共列插入到 DataFrame 中

df_multiindex.reset_index(level='Name')

Output:

Animal ID    Name    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
A786884    *Brock    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
A706918    Belle    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
A724273    Runster    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
A665644    NaN    10/21/2013 07:59:00 AM    10/21/2013 07:59:00 AM    Austin (TX)    Stray    Sick    Cat    Intact Female    4 weeks    Domestic Shorthair Mix    Calico
A682524    Rio    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

在这里，Name 最初是 DataFrame 的索引之一，设置完level参数后，就变成了一个常用的列，叫做Name

drop

此参数决定在索引重置后是否将旧索引保留为通用 DataFrame 列，或者将其从 DataFrame 中完全删除。默认情况下 (drop=False) 是进行保留的，正如我们在前面的所有示例中看到的那样。否则，如果我们不想将旧索引保留为列，我们可以在索引重置后将其从 DataFrame 中完全删除（drop=True）：

df

Output:

Animal ID    Name    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
A786884    *Brock    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
A706918    Belle    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
A724273    Runster    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
A665644    NaN    10/21/2013 07:59:00 AM    10/21/2013 07:59:00 AM    Austin (TX)    Stray    Sick    Cat    Intact Female    4 weeks    Domestic Shorthair Mix    Calico
A682524    Rio    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

增加 drop 参数

df.reset_index(drop=True)

Output:

    Name    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
0    *Brock    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
1    Belle    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
2    Runster    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
3    NaN    10/21/2013 07:59:00 AM    10/21/2013 07:59:00 AM    Austin (TX)    Stray    Sick    Cat    Intact Female    4 weeks    Domestic Shorthair Mix    Calico
4    Rio    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

在上面的 DataFrame 中，旧索引中包含的信息已完全从 DataFrame 中删除了

drop 参数也适用于具有 MultiIndex 的 DataFrame，就像我们之前创建的那样：

df_multiindex

Output:

Animal ID    Name    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
A786884    *Brock    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
A706918    Belle    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
A724273    Runster    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
A665644    NaN    10/21/2013 07:59:00 AM    10/21/2013 07:59:00 AM    Austin (TX)    Stray    Sick    Cat    Intact Female    4 weeks    Domestic Shorthair Mix    Calico
A682524    Rio    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

增加 drop 参数

df_multiindex.reset_index(drop=True)

Output:

    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
0    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
1    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
2    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
3    10/21/2013 07:59:00 AM    10/21/2013 07:59:00 AM    Austin (TX)    Stray    Sick    Cat    Intact Female    4 weeks    Domestic Shorthair Mix    Calico
4    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

两个旧索引都已从 Dataframe 中完全删除，并且索引已重置为默认值

当然，我们可以结合 drop 和 level 参数，指定要从 DataFrame 中完全删除哪些旧索引：

df_multiindex.reset_index(level='Animal ID', drop=True)

Output:

    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
Name                                        
*Brock    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
Belle    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
Runster    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
NaN    10/21/2013 07:59:00 AM    10/21/2013 07:59:00 AM    Austin (TX)    Stray    Sick    Cat    Intact Female    4 weeks    Domestic Shorthair Mix    Calico
Rio    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

旧索引 Animal ID 已从索引和 DataFrame 本身中删除，另一个索引 Name 被保留为 DataFrame 的当前索引

inplace

该参数决定是直接修改原来的 DataFrame 还是新建一个 DataFrame 对象。默认情况下，它会使用新索引 (inplace=False) 创建一个新的 DataFrame，并保持原始 DataFrame 不变。让我们使用默认参数再次运行 reset_index() 方法，然后将结果与原始 DataFrame 进行比较：

df.reset_index()

Output:

    Animal ID    Name    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
0    A786884    *Brock    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
1    A706918    Belle    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
2    A724273    Runster    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
3    A665644    NaN    10/21/2013 07:59:00 AM    10/21/2013 07:59:00 AM    Austin (TX)    Stray    Sick    Cat    Intact Female    4 weeks    Domestic Shorthair Mix    Calico
4    A682524    Rio    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

df

Output:

Animal ID    Name    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
A786884    *Brock    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
A706918    Belle    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
A724273    Runster    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
A665644    NaN    10/21/2013 07:59:00 AM    10/21/2013 07:59:00 AM    Austin (TX)    Stray    Sick    Cat    Intact Female    4 weeks    Domestic Shorthair Mix    Calico
A682524    Rio    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

即使我们将索引重置为运行第一段代码的默认数字，原始 DataFrame 仍然保持不变。如果我们需要将原始 DataFrame 重新分配给对其应用 reset_index() 方法的结果，我们可以直接重新分配它（df = df.reset_index()）或将参数 inplace=True 传递给该方法：

df.reset_index(inplace=True)
df

Output:

    Animal ID    Name    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
0    A786884    *Brock    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
1    A706918    Belle    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
2    A724273    Runster    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
3    A665644    NaN    10/21/2013 07:59:00 AM    10/21/2013 07:59:00 AM    Austin (TX)    Stray    Sick    Cat    Intact Female    4 weeks    Domestic Shorthair Mix    Calico
4    A682524    Rio    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

我们看到现在更改已直接应用于原始 DataFrame 之上了

应用实例：删除缺失值后重置索引

让我们将到目前为止讨论的所有内容付诸实践，看看当我们从 DataFrame 中删除缺失值时，重置 DataFrame 索引是如何有用的

首先，让我们恢复我们最开始时创建的第一个 DataFrame，它具有默认数字索引：

df = pd.read_csv('Austin_Animal_Center_Intakes.csv').head()
df

Output:

    Animal ID    Name    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
0    A786884    *Brock    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
1    A706918    Belle    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
2    A724273    Runster    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
3    A665644    NaN    10/21/2013 07:59:00 AM    10/21/2013 07:59:00 AM    Austin (TX)    Stray    Sick    Cat    Intact Female    4 weeks    Domestic Shorthair Mix    Calico
4    A682524    Rio    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

我们看到 DataFrame 中有一个缺失值，让我们使用 dropna() 方法删除具有缺失值的整行

df.dropna(inplace=True)
df

Output:

    Animal ID    Name    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
0    A786884    *Brock    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
1    A706918    Belle    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
2    A724273    Runster    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
4    A682524    Rio    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

该行已从 DataFrame 中删除，但是索引不再是连续的：0、1、2、4。让我们重新设置它：

df.reset_index()

Output:

    index    Animal ID    Name    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
0    0    A786884    *Brock    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
1    1    A706918    Belle    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
2    2    A724273    Runster    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
3    4    A682524    Rio    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

现在索引是连续的了，但是由于我们没有显式传递 drop 参数，旧索引被转换为列，具有默认名称 index，下面让我们从 DataFrame 中完全删除旧索引：

df.reset_index(drop=True)

Output:

    Animal ID    Name    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
0    A786884    *Brock    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
1    A706918    Belle    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
2    A724273    Runster    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
3    A682524    Rio    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray

现在我们彻底摆脱了无意义的旧索引，当前索引是连续的。最后一步是使用 inplace 参数将这些修改保存到我们的原始 DataFrame 中：

df.reset_index(drop=True, inplace=True)
df

Output:

    Animal ID    Name    DateTime    MonthYear    Found Location    Intake Type    Intake Condition    Animal Type    Sex upon Intake    Age upon Intake    Breed    Color
0    A786884    *Brock    01/03/2019 04:19:00 PM    01/03/2019 04:19:00 PM    2501 Magin Meadow Dr in Austin (TX)    Stray    Normal    Dog    Neutered Male    2 years    Beagle Mix    Tricolor
1    A706918    Belle    07/05/2015 12:59:00 PM    07/05/2015 12:59:00 PM    9409 Bluegrass Dr in Austin (TX)    Stray    Normal    Dog    Spayed Female    8 years    English Springer Spaniel    White/Liver
2    A724273    Runster    04/14/2016 06:43:00 PM    04/14/2016 06:43:00 PM    2818 Palomino Trail in Austin (TX)    Stray    Normal    Dog    Intact Male    11 months    Basenji Mix    Sable/White
3    A682524    Rio    06/29/2014 10:38:00 AM    06/29/2014 10:38:00 AM    800 Grove Blvd in Austin (TX)    Stray    Normal    Dog    Neutered Male    4 years    Doberman Pinsch/Australian Cattle Dog    Tan/Gray