Question

我需要为pandas数据帧的每一行返回多个计算列。

在以下代码段中执行ValueError: Shape of passed values is (4, 2), indices imply (4, 3)函数时出现此错误：apply：

import pandas as pd

my_df = pd.DataFrame({
  'datetime_stuff': ['2012-01-20', '2012-02-16', '2012-06-19', '2012-12-15'],
  'url': ['http://www.something', 'http://www.somethingelse', 'http://www.foo', 'http://www.bar' ],
  'categories': [['foo', 'bar'], ['x', 'y', 'z'], ['xxx'], ['a123', 'a456']],   
})

my_df['datetime_stuff'] = pd.to_datetime(my_df['datetime_stuff'])
my_df.sort_values(['datetime_stuff'], inplace=True)

print(my_df.head())

def calculate_stuff(row):
  if row['url'].startswith('http'):
    categories = row['categories'] if type(row['categories']) == list else []
    calculated_column_x = row['url'] + '_other_stuff_'
  else:
    calculated_column_x = None
  another_column = 'deduction_from_fields'
  return calculated_column_x, another_column

print(my_df.shape)

my_df['calculated_column_x'], my_df['another_column'] = zip(*my_df.apply(calculate_stuff, axis=1))

我正在处理的数据帧的每一行都比上面的示例更复杂，我正在应用的函数calculate_stuff为每一行使用了许多不同的列，然后返回多个新列。

但是，上一个示例仍然提出与ValueError数据框相关的shape，我无法理解如何修复。

如何创建可从现有列开始计算的多个新列（每行）？

Answer 1

当您从正在应用的函数返回列表或元组时，pandas会尝试将其重新插入您运行的数据帧中。相反，返回一个系列。

重新配置代码

my_df = pd.DataFrame({
  'datetime_stuff': ['2012-01-20', '2012-02-16', '2012-06-19', '2012-12-15'],
  'url': ['http://www.something', 'http://www.somethingelse', 'http://www.foo', 'http://www.bar' ],
  'categories': [['foo', 'bar'], ['x', 'y', 'z'], ['xxx'], ['a123', 'a456']],   
})

my_df['datetime_stuff'] = pd.to_datetime(my_df['datetime_stuff'])
my_df.sort_values(['datetime_stuff'], inplace=True)

def calculate_stuff(row):
  if row['url'].startswith('http'):
    categories = row['categories'] if type(row['categories']) == list else []
    calculated_column_x = row['url'] + '_other_stuff_'
  else:
    calculated_column_x = None
  another_column = 'deduction_from_fields'

  # I changed this VVVV
  return pd.Series((calculated_column_x, another_column), ['calculated_column_x', 'another_column'])

my_df.join(my_df.apply(calculate_stuff, axis=1))

     categories datetime_stuff                       url                    calculated_column_x         another_column
0    [foo, bar]     2012-01-20      http://www.something      http://www.something_other_stuff_  deduction_from_fields
1     [x, y, z]     2012-02-16  http://www.somethingelse  http://www.somethingelse_other_stuff_  deduction_from_fields
2         [xxx]     2012-06-19            http://www.foo            http://www.foo_other_stuff_  deduction_from_fields
3  [a123, a456]     2012-12-15            http://www.bar            http://www.bar_other_stuff_  deduction_from_fields

应用返回多个新列的函数时，pandas形状问题

1 个答案: