How to assign new column to existing DataFrame in pandas

Question

I'm new to pandas. I'm trying to add new columns to my existing DataFrame but It's not getting assigned don't know why can anyone explain me what I'm missing this is what i tried

import pandas as pd
df = pd.DataFrame(data = {"test":["mkt1","mkt2","mkt3"],
                         "test2":["cty1","cty2","cty3"]})
print("Before",df.columns)
df.assign(test3="Hello")
print("After",df.columns)

Output

Before Index(['test', 'test2'], dtype='object')
After Index(['test', 'test2'], dtype='object')

@It_is_Chris df = df.assign(test3="Hello") check I've added in between two print functions where should place it can you provide a solution for it — Ankit Tiwari
– Ankit Tiwari, Commented Oct 27, 2021 at 15:33
sorry what is the question? You just need to replace df.assign(test3="Hello") to df = df.assign(test3="Hello") — sophocles
– sophocles, Commented Oct 27, 2021 at 15:36

BSP · Accepted Answer · 2021-10-27 15:39:21Z

1

Pandas assign method returns a new modified dataframe with a new column, it does not modify it in place.

import pandas as pd
df = pd.DataFrame(data = {"test":["mkt1","mkt2","mkt3"],
                         "test2":["cty1","cty2","cty3"]})
print("Before",df.columns)
df = df.assign(test3="Hello") # <--- Note the variable reassingment
print("After",df.columns)

answered Oct 27, 2021 at 15:39

BSP

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2 Comments

Ankit Tiwari Over a year ago

Hello @BSP I've got solution by sophocles but thanks for your answer ❤️️

BSP Over a year ago

@ I see he commented just before me. Anyway, the important idea behind your question is that pandas dataframes methods do not modify them in place but return a new modified copy. That is what I wanted to explain.

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