Copy all values in a column to a new column in a pandas dataframe

I think the correct access method is using the index:

df_2.loc[:,'D'] = df_2['B']

You can simply assign the B to the new column , Like -

df['D'] = df['B']

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Example/Demo -

In [1]: import pandas as pd

In [2]: df = pd.DataFrame([['a.1','b.1','c.1'],['a.2','b.2','c.2'],['a.3','b.3','c.3']],columns=['A','B','C'])

In [3]: df
Out[3]:
     A    B    C
0  a.1  b.1  c.1
1  a.2  b.2  c.2
2  a.3  b.3  c.3

In [4]: df['D'] = df['B']                  #<---What you want.

In [5]: df
Out[5]:
     A    B    C    D
0  a.1  b.1  c.1  b.1
1  a.2  b.2  c.2  b.2
2  a.3  b.3  c.3  b.3

In [6]: df.loc[0,'D'] = 'd.1'

In [7]: df
Out[7]:
     A    B    C    D
0  a.1  b.1  c.1  d.1
1  a.2  b.2  c.2  b.2
2  a.3  b.3  c.3  b.3

The problem is in the line before the one that throws the warning. When you create df_2 that's where you're creating a copy of a slice of a dataframe. Instead, when you create df_2, use .copy() and you won't get that warning later on.

df_2 = df[df['B'] == 'b.2'].copy()