如何解决Pandas中设置不兼容数据类型的FutureWarning问题？

问题背景 #

现有代码当前可运行，但触发FutureWarning，未来版本会直接报错：

FutureWarning: Setting an item of incompatible dtype is deprecated and
will raise in a future error of pandas. Value 'GRAND TOTAL' has dtype incompatible with float64, please explicitly cast to a compatible dtype first.
total_df.at['New_Row', 'COMPOSITE'] = 'GRAND TOTAL'

原代码：

total.name = 'New_Row'
total_df = total.to_frame().T
total_df.at['New_Row', 'CURRENCY'] = ''
total_df.at['New_Row', 'MANDATE'] = Portfolio
total_df.at['New_Row', 'COMPOSITE'] = 'GRAND TOTAL'
total_df.set_index('COMPOSITE',inplace=True)

变量total的信息：

CURRENCY
MANDATE             Mandate_Test
USD AMOUNT          123
LOCAL AMOUNT        12
Beg. Mkt            123
End. Mkt            456
Name: New_Row, dtype: object

问题原因 #

total这个Series里没有COMPOSITE字段，转成DataFrame后，该列会被自动初始化为float64类型（缺失值默认用NaN，对应float类型）。直接给这个列赋值字符串类型的GRAND TOTAL，就会触发类型不兼容的告警。

修复方案 #

以下是几种直接可行的修复方式：

方案1：创建DataFrame时统一指定类型

转DataFrame时直接把所有列的类型设为object，避免后续赋值冲突：

total.name = 'New_Row'
# 转换时指定所有列为object类型
total_df = total.to_frame().T.astype(object)
total_df.at['New_Row', 'CURRENCY'] = ''
total_df.at['New_Row', 'MANDATE'] = Portfolio
total_df.at['New_Row', 'COMPOSITE'] = 'GRAND TOTAL'
total_df.set_index('COMPOSITE', inplace=True)

方案2：单独转换COMPOSITE列类型后赋值

先将COMPOSITE列转为object类型，再进行字符串赋值：

total.name = 'New_Row'
total_df = total.to_frame().T
total_df.at['New_Row', 'CURRENCY'] = ''
total_df.at['New_Row', 'MANDATE'] = Portfolio
# 先转换列类型
total_df['COMPOSITE'] = total_df['COMPOSITE'].astype(object)
total_df.at['New_Row', 'COMPOSITE'] = 'GRAND TOTAL'
total_df.set_index('COMPOSITE', inplace=True)

方案3：从源头构造包含COMPOSITE的Series

先给total添加COMPOSITE字段，避免转DataFrame时生成默认类型的列：

# 提前给total添加COMPOSITE字段，用空字符串占位
total['COMPOSITE'] = ''
total.name = 'New_Row'
total_df = total.to_frame().T
total_df.at['New_Row', 'CURRENCY'] = ''
total_df.at['New_Row', 'MANDATE'] = Portfolio
total_df.at['New_Row', 'COMPOSITE'] = 'GRAND TOTAL'
total_df.set_index('COMPOSITE', inplace=True)

内容的提问来源于stack exchange，提问作者TourEiffel