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本教程解释了如何使用 insert() 方法在 Pandas DataFrame 中插入一列。

import pandas as pd

countries_df = pd.DataFrame(
    {
        "Country": ["Nepal", "Switzerland", "Germany", "Canada"],
        "Continent": ["Asia", "Europe", "Europe", "North America"],
        "Primary Language": ["Nepali", "French", "German", "English"],
    }
)
print("Countries DataFrame:")
print(countries_df, "\n")

输出：

Countries DataFrame:
       Country      Continent Primary Language
0        Nepal           Asia           Nepali
1  Switzerland         Europe           French
2      Germany         Europe           German
3       Canada  North America          English

我们将使用上例中所示的 countries_df DataFrame 来解释如何使用 insert() 方法在 Pandas DataFrame 中插入一列。

pandas.DataFrame.insert() 方法

语法

DataFrame.insert(loc, column, value, allow_duplicates=False)

它将名为 column 的列插入到 DataFrame 中，其值由 value 指定，位于 loc 位置。

使用 insert() 方法插入对所有行具有相同值的列

import pandas as pd

countries_df = pd.DataFrame(
    {
        "Country": ["Nepal", "Switzerland", "Germany", "Canada"],
        "Continent": ["Asia", "Europe", "Europe", "North America"],
        "Primary Language": ["Nepali", "French", "German", "English"],
    }
)
print("Countries DataFrame:")
print(countries_df, "\n")

countries_df.insert(3, "Capital", "Unknown")

print("Countries DataFrame after inserting Capital column:")
print(countries_df)

输出：

Countries DataFrame:
       Country      Continent Primary Language
0        Nepal           Asia           Nepali
1  Switzerland         Europe           French
2      Germany         Europe           German
3       Canada  North America          English

Countries DataFrame after inserting Capital column:
       Country      Continent Primary Language  Capital
0        Nepal           Asia           Nepali  Unknown
1  Switzerland         Europe           French  Unknown
2      Germany         Europe           German  Unknown
3       Canada  North America          English  Unknown

它在 countries_df DataFrame 的 3 索引位置插入 ·Capital·列，所有行的 ·Capital·列值均设置为 Unknown。

该位置从 0 开始，因此 3 位置指的是 DataFrame 中的 4 列。

在 DataFrame 中插入一列，指定每行的值

如果我们想使用 insert() 方法为要插入的列指定每一行的值，我们可以在 insert() 方法中传递一个值列表作为 value 参数。

import pandas as pd

countries_df = pd.DataFrame(
    {
        "Country": ["Nepal", "Switzerland", "Germany", "Canada"],
        "Continent": ["Asia", "Europe", "Europe", "North America"],
        "Primary Language": ["Nepali", "French", "German", "English"],
    }
)
print("Countries DataFrame:")
print(countries_df, "\n")

capitals = ["Kathmandu", "Zurich", "Berlin", "Ottawa"]

countries_df.insert(2, "Capital", capitals)

print("Countries DataFrame after inserting Capital column:")
print(countries_df)

输出：

Countries DataFrame:
       Country      Continent Primary Language
0        Nepal           Asia           Nepali
1  Switzerland         Europe           French
2      Germany         Europe           German
3       Canada  North America          English

Countries DataFrame after inserting Capital column:
       Country      Continent    Capital Primary Language
0        Nepal           Asia  Kathmandu           Nepali
1  Switzerland         Europe     Zurich           French
2      Germany         Europe     Berlin           German
3       Canada  North America     Ottawa          English

它在 DataFrame countries_df 中的索引 2 插入了列 Capital，并为 DataFrame 中的 Capital 列指定了每一行的值。

在 insert() 方法中设置 allow_duplicates = True 来添加已经存在的列

import pandas as pd

countries_df = pd.DataFrame(
    {
        "Country": ["Nepal", "Switzerland", "Germany", "Canada"],
        "Continent": ["Asia", "Europe", "Europe", "North America"],
        "Primary Language": ["Nepali", "French", "German", "English"],
        "Capital": ["Kathmandu", "Zurich", "Berlin", "Ottawa"],
    }
)
print("Countries DataFrame:")
print(countries_df, "\n")

capitals = ["Kathmandu", "Zurich", "Berlin", "Ottawa"]

countries_df.insert(4, "Capital", capitals, allow_duplicates=True)

print("Countries DataFrame after inserting Capital column:")
print(countries_df)

输出：

Countries DataFrame:
       Country      Continent Primary Language    Capital
0        Nepal           Asia           Nepali  Kathmandu
1  Switzerland         Europe           French     Zurich
2      Germany         Europe           German     Berlin
3       Canada  North America          English     Ottawa

Countries DataFrame after inserting Capital column:
       Country      Continent Primary Language    Capital    Capital
0        Nepal           Asia           Nepali  Kathmandu  Kathmandu
1  Switzerland         Europe           French     Zurich     Zurich
2      Germany         Europe           German     Berlin     Berlin
3       Canada  North America          English     Ottawa     Ottawa

它将列 Capital 添加到 countries_df DataFrame 中，尽管 countries_df DataFrame 中已经存在 Capital 列。

如果我们尝试插入已经存在于 DataFrame 中的列，而没有在 insert() 方法中设置 allow_duplicates = True，它就会向我们抛出一个错误信息：ValueError: cannot insert column, already exists.。