get ' A value试图在DataFrame的切片副本上设置.设置列时

Question

我知道不应该在pandas数据框架的视图上设置值，我没有这样做，但我得到这个错误。我有一个这样的函数:

def do_something(df):
# id(df) is xxx240
idx = get_skip_idx(df)  # another function that returns a boolean series
if any(idx):
df = df[~idx]
# id(df) is xxx744, df is now a local variable which is a copy of the input argument
assert not df._is_view  # This doesn't fail, I'm not having a view
df['date_fixed'] = pd.to_datetime(df['old_date'].str[:10], format='%Y-%m-%d')
# I'm getting the warning here which doesn't make any sense to me

我使用的是pandas 1.4.1。对我来说，这听起来像是一个bug，我想在提交罚单之前确认我没有错过任何东西。

Answer 1

我的理解是_is_view可以返回假阴性，并且您实际上正在处理原始数据框架的视图。

一种解决方法是将df[~idx]替换为df[~idx].copy():

import pandas as pd
df = pd.DataFrame(
{
"value": [1, 2, 3],
"old_date": ["2022-04-20 abcd", "2022-04-21 efgh", "2022-04-22 ijkl"],
}
)

def do_something(df, idx):
if any(idx):
df = df[~idx].copy()
df["date_fixed"] = pd.to_datetime(df["old_date"].str[:10], format="%Y-%m-%d")
return df

print(do_something(df, pd.Series({0: True, 1: False, 2: False})))
# No warning
value         old_date date_fixed
1      2  2022-04-21 efgh 2022-04-21
2      3  2022-04-22 ijkl 2022-04-22