提问人：gregV 提问时间：8/22/2023 更新时间：8/23/2023 访问量：54
pandas：使用方法链接按切片分配列值
pandas: assign a column values by slice with method chaining
问：
在下面的玩具示例中，我尝试根据外部合并结果添加一个状态列。挑战在于保留 tom 博客中最好描述的链式方法。注释掉的行是我的尝试，但它不起作用
import pandas as pd
# Create sample data frames A and B
A = pd.DataFrame({
'key': ['A', 'B', 'C', 'D'],
'value': [1, 2, 3, 4]
})
B = pd.DataFrame({
'key': ['C', 'D', 'E', 'F'],
'value': [3, 4, 5, 6]
})
# Merge data frames A and B on the 'key' column and add an indicator column
merged = pd.merge(A, B, on='key', how='outer', indicator=True)
# add a status column
#{'both':'no change',
#'left_only': 'added',
#'right_only': 'removed'}
merged = (merged
.assign (status = 'no change')
#.assign(status = lambda x: x.loc[x._merge == 'left_only'], 'added')
.drop('_merge', axis=1)
)
答：
2赞
sammywemmy
8/22/2023
#1
像这样的东西应该就足够了 - 通常对于切片，因为你要分配，你需要使用条件（、、等）mapnp.wherenp.selectpd.where
(A
.merge(B, on='key', how='outer', indicator=True)
.assign(status = lambda f: f._merge.map({"left_only":"added",
"both":"no change",
"right_only":"removed"}))
)
1赞
taller
8/22/2023
#2
添加以获取状态。DataFrame.apply
merged = (merged
.assign(status = merged.apply(lambda x:
'added' if x._merge == "left_only" else "", axis=1))
.drop('_merge', axis=1)
)
key value_x value_y status
0 A 1.0 NaN added
1 B 2.0 NaN added
2 C 3.0 3.0
3 D 4.0 4.0
4 E NaN 5.0
5 F NaN 6.0
1赞
Scott Boston
8/22/2023
#3
这里有一种方法，可以在一行中使用“walrus”运算符，使用预定义的字典，并将列名更改为字符串：:=mapindicatormerge
import pandas as pd
# Create sample data frames A and B
A = pd.DataFrame({
'key': ['A', 'B', 'C', 'D'],
'value': [1, 2, 3, 4]
})
B = pd.DataFrame({
'key': ['C', 'D', 'E', 'F'],
'value': [3, 4, 5, 6]
})
d = {'both':'no_change',
'left_only':'added',
'right_only':'removed'}
merged = (merged_out:=pd.merge(A, B, on='key', how='outer', indicator='status'))\
.assign(status=merged_out['status'].map(d))
merged
输出：
key value_x value_y status
0 A 1.0 NaN added
1 B 2.0 NaN added
2 C 3.0 3.0 no_change
3 D 4.0 4.0 no_change
4 E NaN 5.0 removed
5 F NaN 6.0 removed
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