Pandas assign() has no effect when used in user-defined function

Question

When I use the DataFrame.assign() method in my own function foobar, it has no effect to the global DataFrame.

#!/usr/bin/env python3
import pandas as pd

def foobar(df):
    # has no affect to the "global" df
    df.assign(Z = lambda x: x.A + x.B)

    return df    
    

data = {'A': range(3),
        'B': range(3)}

df = pd.DataFrame(data)
df = foobar(df)
# There is no 'Z' column in this df
print(df)

The result output

I assume this has something to do with the difference of views and copy's in Pandas. But I am not sure how to handle this the right and elegant Pandas-way.

You need to assign it back i.e., df = df.assign(Z=lambda x: x.A + x.B) or it's effectively doing nothing. — Mustafa Aydın
– Mustafa Aydın, Commented Jun 21, 2021 at 10:52

bruno-uy · Accepted Answer · 2021-06-21 10:53:52Z

3

Pandas assign returns a DataFrame so you need to assign the result to the same df. Try this:

def foobar(df):
    df = df.assign(Z = lambda x: x.A + x.B)

    return df

answered Jun 21, 2021 at 10:53

bruno-uy

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1 Comment

buhtz Over a year ago

Thank you very much. It really was a misunderstanding based on assumptions grounded in the object orientation, not a typo.

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