Your df2 is a slice of another dataframe. You need to explicitly copy it with df2 = df2.copy() just prior to your attempt to drop
Consider the following dataframe:
import pandas as pd
import numpy as np
df1 = pd.DataFrame(np.arange(20).reshape(4, 5), list('abcd'), list('ABCDE'))
df1
Let me assign a slice of df1 to df2
df2 = df1[['A', 'C']]
df2 is now a slice of df1 and should trigger those pesky SettingWithCopyWarning‘s if we try to change things in df2. Let’s take a look.
df2.drop('c')
No problems. How about:
df2.drop('c', inplace=True)
There it is:
The problem is that pandas tries to be efficient and tracks that df2 is pointing to the same data as df1. It is preserving that relationship. The warning is telling you that you shouldn’t be trying to mess with the original dataframe via the slice.
Notice that when we look at df2, row ‘c’ has been dropped.
df2
And looking at df1 we see that row ‘c’ is still there.
df1
pandas made a copy of df2 then dropped row ‘c’. This is potentially inconsistent with what our intent may have been considering we made df2 a slice of and pointing to same data as df1. So pandas is warning us.
To not see the warning, make the copy yourself.
df2 = df2.copy()
# or
df2 = df1[['A', 'C']].copy()