python dataframe add

To add a new column to a Python dataframe, you can use the following syntax:

df['new_column_name'] = new_column_values

where `df` is the name of the dataframe, `new_column_name` is the name of the new column you want to add, and `new_column_values` is a list or array of the same length as the number of rows in the dataframe.

For example, let's say you have a dataframe `df` with columns `A` and `B`, and you want to add a new column `C` containing the sum of `A` and `B`:

import pandas as pd

df = pd.DataFrame({'A': [1, 2, 3], 'B': [4, 5, 6]})

df['C'] = df['A'] + df['B']

print(df)

This will output:

   A  B  C
0  1  4  5
1  2  5  7
2  3  6  9

You can also add a new column with a single value for all rows:

df['D'] = 'new_value'

This will add a new column `D` with the value `'new_value'` for all rows in the dataframe.