Hidden Chaining responsible for SettingWithCopyWarning

[paulelvers]

Hidden chaining is another frequent issue responsible for SettingWithCopyWarning. It may occur the following way:

import pandas as pd
import numpy as np
d = {'col1': [1, 2, 3], 'col2': [3, 4, np.nan]}
df = pd.DataFrame(data=d)

# This will cause the warning
df_no_na = df.dropna()
df_no_na.loc[:, 'new_column'] = df_no_na['col1'] + df_no_na['col2']

# This won't cause the warning
df_no_na = df.dropna().copy()
df_no_na.loc[: ,'new_column'] = df_no_na['col1'] + df_no_na['col2']

The point is that it also occurs when using df.loc[:, 'new_column'] so using .loc[] does not always prevent this warning from happening. The reason for the warning is obviously the same as in the example in your notebook, however i think this issue of chaining a get and an assignment is less obvious, hence 'hidden' :-).