pandas combine_first 和 fillna 有什么区别？
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What is the difference between combine_first and fillna?
提问by kjmerf
These two functions seem equivalent to me. You can see that they accomplish the same goal in the code below, as columns c and d are equal. So when should I use one over the other?
这两个功能在我看来是等价的。您可以在下面的代码中看到它们实现了相同的目标，因为 c 列和 d 列相等。那么我什么时候应该使用一个？
Here is an example:
下面是一个例子：
import pandas as pd
import numpy as np
df = pd.DataFrame(np.random.randint(0, 10, size=(10, 2)), columns=list('ab'))
df.loc[::2, 'a'] = np.nan
Returns:
返回：
a b
0 NaN 4
1 2.0 6
2 NaN 8
3 0.0 4
4 NaN 4
5 0.0 8
6 NaN 7
7 2.0 2
8 NaN 9
9 7.0 2
This is my starting point. Now I will add two columns, one using combine_first and one using fillna, and they will produce the same result:
这是我的出发点。现在我将添加两列，一列使用 combine_first，另一列使用 fillna，它们将产生相同的结果：
df['c'] = df.a.combine_first(df.b)
df['d'] = df['a'].fillna(df['b'])
Returns:
返回：
a b c d
0 NaN 4 4.0 4.0
1 8.0 7 8.0 8.0
2 NaN 2 2.0 2.0
3 3.0 0 3.0 3.0
4 NaN 0 0.0 0.0
5 2.0 4 2.0 2.0
6 NaN 0 0.0 0.0
7 2.0 6 2.0 2.0
8 NaN 4 4.0 4.0
9 4.0 6 4.0 4.0
Credit to this question for the data set: Combine Pandas data frame column values into new column
归功于数据集的这个问题：Combining Pandas data frame column values into new column
回答by piRSquared
combine_firstis intended to be used when there is exists non-overlapping indices. It will effectively fill in nulls as well as supply values for indices and columns that didn't exist in the first.
combine_first旨在在存在非重叠索引时使用。它将有效地填充空值以及为第一个中不存在的索引和列提供值。
dfa = pd.DataFrame([[1, 2, 3], [4, np.nan, 5]], ['a', 'b'], ['w', 'x', 'y'])
w x y
a 1.0 2.0 3.0
b 4.0 NaN 5.0
dfb = pd.DataFrame([[1, 2, 3], [3, 4, 5]], ['b', 'c'], ['x', 'y', 'z'])
x y z
b 1.0 2.0 3.0
c 3.0 4.0 5.0
dfa.combine_first(dfb)
w x y z
a 1.0 2.0 3.0 NaN
b 4.0 1.0 5.0 3.0 # 1.0 filled from `dfb`; 5.0 was in `dfa`; 3.0 new column
c NaN 3.0 4.0 5.0 # whole new index
Notice that all indices and columns are included in the results
请注意，所有索引和列都包含在结果中
Now if we fillna
现在如果我们 fillna
dfa.fillna(dfb)
w x y
a 1 2.0 3
b 4 1.0 5 # 1.0 filled in from `dfb`
Notice no new columns or indices from dfbare included. We only filled in the null value where dfashared index and column information.
请注意，不包含新的列或索引dfb。我们只在dfa共享索引和列信息的地方填空值。
In your case, you use fillnaand combine_firston one column with the same index. These translate to effectively the same thing.
在您的情况下，您在具有相同索引的一列上使用fillna和combine_first。这些转化为有效的同一件事。
