How to add a new column to an existing DataFrame?

You can add a new column to an existing pandas DataFrame by using the assign() method or the [] notation.

Using the assign() method:

df = df.assign(new_column_name=new_column_values)

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Using the [] notation:

df['new_column_name'] = new_column_values

Note that new_column_values should be a list or array of the same length as the number of rows in the DataFrame.