How to insert a column into a DataFrame without generating the SettingWithCopyWarning

Question

I would like to insert a column into an existing DataFrame. Ideally without copying existing data. Whatever I try, later assignments to the resulting DataFrame generate a SettingWithCopyWarning, if the inserted data contains null values.

import pandas as pd

df = pd.DataFrame(data={'a': [1]})
df = df.assign(b=pd.Series(data=pd.NaT, index=df.index))
df['a'].iloc[0] = 5

Replacing assign with either of

df['b'] = pd.Series(data=pd.NaT, index=df.index)
df.insert(column='b', loc=0, value=pd.NaT)

results in the same warning.

Strange enough, if the inserted value is not null (replacing pd.NaT with, e.g., 0) doesn't generate a warning. Is that a bug?

Answer 1

Your issue seems to be with df['a'].iloc[0] = 5, where you're using chained assignment. Try this instead:

df.at[0, 'a'] = 5
# Or: df.loc[0, 'a'] = 5, but `.at` is preferred when assigning scalar