如何將 function 應用於 pandas dataframe 的特定列？

Question

我想將 function 應用於 pandas 數據框的特定列。 這是一個例子：

# import modules
from pandas_datareader import data as pdr

# import parameters
start = "2020-01-01"
end = "2021-01-01"
symbols = ["AAPL"]

# get the data
data = pdr.get_data_yahoo(symbols, start, end)

def mult(row):
    return row['Close']*2, row['Open']/3


data[['Close', 'Open']].apply(mult, axis = 1)

print(data.head())

結果：

Attributes  Adj Close      Close       High        Low       Open       Volume
Symbols          AAPL       AAPL       AAPL       AAPL       AAPL         AAPL
Date                                                                          
2020-01-02  73.894333  75.087502  75.150002  73.797501  74.059998  135480400.0
2020-01-03  73.175926  74.357498  75.144997  74.125000  74.287498  146322800.0
2020-01-06  73.759003  74.949997  74.989998  73.187500  73.447502  118387200.0
2020-01-07  73.412109  74.597504  75.224998  74.370003  74.959999  108872000.0
2020-01-08  74.593048  75.797501  76.110001  74.290001  74.290001  132079200.0

關於為什么這不起作用的任何想法？

Answer 1

我認為問題在於您沒有將mult函數的返回分配給任何變量。

實現你想要的一種方法是：

# import modules
from pandas_datareader import data as pdr

# import parameters
start = "2020-01-01"
end = "2021-01-01"
symbols = ["AAPL"]

# get the data
data = pdr.get_data_yahoo(symbols, start, end)

def mult(df):
    df['Close'] = 2 * df['Close']
    df['Open'] = df['Open'] / 3
    return df

mult(data)

print(data.head())

Attributes  Adj Close       Close       High        Low       Open  \
Symbols          AAPL        AAPL       AAPL       AAPL       AAPL   
Date                                                                 
2020-01-02  73.894325  150.175003  75.150002  73.797501  24.686666   
2020-01-03  73.175926  148.714996  75.144997  74.125000  24.762499   
2020-01-06  73.759010  149.899994  74.989998  73.187500  24.482501   
2020-01-07  73.412117  149.195007  75.224998  74.370003  24.986666   
2020-01-08  74.593048  151.595001  76.110001  74.290001  24.763334

Answer 2

兩件事情：

(i) 你永遠不會將它分配回原來的 DataFrame，所以它永遠不會更新。

(ii) 如果您的 function 不再復雜，對於簡單的乘法，向量化運算更好，因此直接在列上執行乘法而不是 function：

data['Close'] *= 2
data['Open'] /= 3