使用 .loc 通过索引从另一个数据帧中分配值返回： ValueError: Must have equal len keys and value when setting with an iterable答案

【问题标题】：using .loc to assign values from another dataframe by index returns : ValueError: Must have equal len keys and value when setting with an iterable使用 .loc 通过索引从另一个数据帧中分配值返回： ValueError: Must have equal len keys and value when setting with an iterable
【发布时间】：2021-10-01 06:44:21
【问题描述】：

df_state_group：

Index	State	assigned_airport
4	Florida	NaN
5	Florida	NaN
6	Florida	NaN
7	Florida	NaN

df1:

Index	State	assigned_airport
0	Washington	NaN
1	New York	NaN
2	Illinois	NaN
3	Texas	NaN
4	Florida	NaN
5	Florida	NaN
6	Florida	NaN
7	Florida	NaN
8	Ohio	NaN
9	Colorado	NaN
10	Michigan	NaN
11	Indiana	NaN

df_airports：

Index	airports
0	a
1	b
2	c
3	d
4	e
5	f

index_nearest_airport：

[3]

期望的结果：

df1:

Index	State	assigned_airport
0	Washington	NaN
1	New York	NaN
2	Illinois	NaN
3	Texas	NaN
4	Florida	d
5	Florida	d
6	Florida	d
7	Florida	d
8	Ohio	NaN
9	Colorado	NaN
10	Michigan	NaN
11	Indiana	NaN

df_state_group 是佛罗里达州 df1 的子集

我试过了：

df1.loc[df_state_group.index, 'assigned_airport'] = df_airports.loc[index_nearest_airport,'airports'].values

但我不断收到错误：ValueError: Must have equal len keys and value when setting with an iterable

【问题讨论】：

我想你忘了在你的问题中添加 index_nearest_airport
@Anurag Dabas 就在最后一个df之前，值为[3]
实际上您的值只有一个元素，但您分配的位置需要 4 个值。因此长度不匹配。
对于 Python 3.9，我无法发表评论。您在 index_nearest_airport 中有单个元素。但是如果有更多的值，我不知道赋值的逻辑：/。

标签： python pandas indexing .loc

【解决方案1】：

这样就可以了：

(df1.assign(assigned_airport= 
     pd.DataFrame(np.repeat(df_airports.loc[index_nearest_airport,'airports'].values
     ,df_state_group.shape[0]), columns = ['assigned_airport'],index=df_state_group.Index)))

或

(df1.loc[df_state_group.Index, 'assigned_airport'] = 
     np.repeat(df_airports.loc[index_nearest_airport,'airports'].values,
     df_state_group.shape[0]))

Index	State	assigned_airport
0	Washington	NaN
1	New York	NaN
2	Illinois	NaN
3	Texas	NaN
4	Florida	d
5	Florida	d
6	Florida	d
7	Florida	d
8	Ohio	NaN
9	Colorado	NaN
10	Michigan	NaN
11	Indiana	NaN

【讨论】：

非常感谢完美运行，顺便说一句，这与 python 3.9.6 更新有什么关系吗？因为以前的代码在我更新我的 python 版本之前可以正常工作，而且我注意到如果要分配的数组单个值不是 dtype = object