How to add a value to all elements of a certain column of a dataframe

One would think that the answer to this question would be something as simple as this:

df[column_name]=df[column_name] + adjustment

Sadly this does not work and after trying this we get this error message:

<ipython-input-29-9df0f3935efe>:25: SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead
See the caveats in the documentation: https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy
  df[column_name]=df[column_name] + adjustment

Sadly after trying using .loc and reading for 30 mins I am still unable to solve my trivial problem and thus I ask here since I am sure that what I am trying to do is very easy to solve with 1 line of code

Kind regards

One solution that works but still gives the same warning is the following:

df.loc[:,column_name]=df.loc[:,column_name].add(adjustment)

Would be great to see a solution that does not produce such warning!

Based on the comment, the warning is printed because the df is a chunk from a bigger dataframe.

Consider this dataframe main_df:

  col1  col2
0    a     1
1    b     3
2    c     2
3    d     4
4    e     1

When you get a chunk from this dataframe, for example df = main_df.loc[1:3]:

  col1  col2
1    b     3
2    c     2
3    d     4

Then you get a warning when you try to modify a column. For example df['col2'] = df['col2'] + 10.

You can circumvent this making a copy df = df.copy() before modifying the column.