pandas.DataFrame.combine_first也可以。
(注意:由于“结果索引列将是各个索引和列的并集”,因此应检查索引和列是否匹配。)
import numpy as np
import pandas as pd
df = pd.DataFrame([["1","cat","mouse"],
["2","dog","elephant"],
["3","cat","giraf"],
["4",np.nan,"ant"]],columns=["Day","Cat1","Cat2"])
In: df["Cat1"].combine_first(df["Cat2"])
Out:
0 cat
1 dog
2 cat
3 ant
Name: Cat1, dtype: object
与其他答案比较:
%timeit df["Cat1"].combine_first(df["Cat2"])
181 µs ± 11.3 µs per loop (mean ± std. dev. of 7 runs, 10000 loops each)
%timeit df['Cat1'].fillna(df['Cat2'])
253 µs ± 10.3 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
%timeit np.where(df.Cat1.isnull(), df.Cat2, df.Cat1)
88.1 µs ± 793 ns per loop (mean ± std. dev. of 7 runs, 10000 loops each)
我没有在下面使用此方法:
def is_missing(Cat1,Cat2):
if np.isnan(Cat1):
return Cat2
else:
return Cat1
df['Cat1'] = df.apply(lambda x: is_missing(x['Cat1'],x['Cat2']),axis=1)
因为它将引发一个异常:
TypeError: ("ufunc 'isnan' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule ''safe''", 'occurred at index 0')
这意味着np.isnan可以应用于本机dtype的NumPy数组(例如np.float64),但是在应用于对象数组时会引发TypeError 。
所以我修改了方法:
def is_missing(Cat1,Cat2):
if pd.isnull(Cat1):
return Cat2
else:
return Cat1
%timeit df.apply(lambda x: is_missing(x['Cat1'],x['Cat2']),axis=1)
701 µs ± 7.38 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
fillna
需要一系列。