python - 试图理解 Pandas 中的 .apply()

Question

我试图避免循环遍历数据帧，所以最近开始使用 .apply()。

但是我并不真正理解这种行为。我在下面有一个 super 简单的玩具示例。询问用户列中的每个水果是否都是苹果(它们都是苹果，所以每个答案都是 Y)。

import pandas as pd
df= pd.DataFrame({'fruit':['apple','apple', 'apple','apple', 'apple'],'result':['']*5})
df

   fruit result
0  apple       
1  apple       
2  apple       
3  apple       
4  apple   

设置一个 .apply() 函数来询问用户水果是否是苹果:

def check_fruit(row):

    # get the current fruit in the row
    current_fruit = row['fruit']

    # print output for user
    print('\n===============================================')
    print('Is this an apple?')
    print('===============================================\n')
    print(f'Current Fruit: {current_fruit}\n')

    # user input - they are asked if the displayed fruit
    # is an apple or not and must enter y/n
    choice = input('Please enter Y/N: ')

     # if they choose yes
    if (choice == 'Y' or choice == 'y'):

        # add the word 'correct' to row column
        row['result']=='Correct'

        return row
    # if they choose no
    elif (choice == 'N' or choice == 'n'):

        # add the word 'Incorrect' to row column
        row['result']=='Incorrect'

        return row

现在应用它 - 注意输出。为什么 apple 在数据框中只有 5 行时打印了 6 次？

df= df.apply(check_fruit,axis=1)

===============================================
Is this an apple?
===============================================

Current Fruit: apple

Please enter Y/N: y


===============================================
Is this an apple?
===============================================

Current Fruit: apple

Please enter Y/N: y


===============================================
Is this an apple?
===============================================

Current Fruit: apple

Please enter Y/N: y


===============================================
Is this an apple?
===============================================

Current Fruit: apple

Please enter Y/N: y


===============================================
Is this an apple?
===============================================

Current Fruit: apple

Please enter Y/N: y


===============================================
Is this an apple?
===============================================

Current Fruit: apple

Please enter Y/N: y

其次，为什么应用函数没有返回该行？应用该函数后，“结果”列仍然为空。

   fruit result
0  apple       
1  apple       
2  apple       
3  apple       
4  apple

这可能是我所知道的非常明显的事情......

知道我哪里出错了吗？

(ps.我知道输入没有错误检查，现在只关注 .apply())

Answer 1

请参阅 pd.DataFrame.apply 的文档:

Notes

---

In the current implementation apply calls func twice on the first column/row to decide whether it can take a fast or slow code path. This can lead to unexpected behavior if func has side-effects, as they will take effect twice for the first column/row.

您的函数 check_fruit 确实有副作用，即要求用户进行一些输入，这种情况发生的次数超出您的预期。

通常，apply 和其他数据框函数旨在与以某种方式转换数据的函数一起使用，而不是与应用程序逻辑一起使用。在这种情况下，不显式写出循环不会给您带来任何特别的好处，因此您能做的最好的事情可能就是手动遍历每一行:

import pandas as pd

def check_fruit(row):
    # ...

df = pd.DataFrame({'fruit': ['apple', 'apple', 'apple', 'apple', 'apple'],
                   'result': [''] * 5})
for row in df.iterrows():
    check_fruit(row)