How can I add and drop columns and rows in a pandas dataframe followed by saving it down into a csv (adding and dropping works but doesn't save)?

So I have some code that drops a few columns in a pandas dataframe and add one column to the pandas dataframe:

import pandas as pd
df = pd.read_csv('TradedInstrument_20230331_real.txt',  sep='t', encoding='unicode_escape')

cols = ['stopTrdgAllag', 'stopTrdgsAllowTrdg', 'stopTrdleInTrdg', 'presholdChf', 'billentCode', 'preTradeBsholdChf', 'billinntDesc']

x = ['e'] * len(df.index)
df['new_col'] = x

df.drop(columns=cols).to_csv('output.csv', index=False)

So what I am trying to do is add a new column new_col with e as entries in each row. And also to drop all of the columns that are in cols. While the code works fine to achieve this, when I run the above, in the output, only the columns are dropped and the row is NOT added!

I can kind of see why this is, it is because the to_csv only incorporates the dropping of the columns. So how can I add the adding of the new column too?