Python: a new column name in a function with assign()

Question

In the following code, I created a function to return a DataFrame with a new column new_col with one-period shifted values of ori_col. But, a new column of the output has the name new_col rather than B. What am I missing here?

import pandas as pd

def add_col_diff(dframe, ori_col, new_col):
    shift_1 = dframe[ori_col].shift(1)
    return dframe.assign(new_col=shift_1)

data = {'A': [10, 11, 12]}
df = pd.DataFrame.from_dict(data)
df = add_col_diff(df, 'A', 'B')
df

This gives

     A  new_col
0   10  NaN
1   11  10.0
2   12  11.0

I'm actually completely confused by this. return dframe.assign('B'=shift_1) is not valid, so it presumably takes the argument name rather than its value? — roganjosh
– roganjosh, Commented Dec 29, 2018 at 1:20
Yes, return dframe.assign(B=shift_1) will work, but I don't see how you reasonably convey that — roganjosh
– roganjosh, Commented Dec 29, 2018 at 1:22

busybear · Accepted Answer · 2019-03-05 02:15:46Z

5

assign will set the keyword arguments as the column names. Simply having assign(new_col=shift_l) will make a column with the name new_col. Keyword arguments are not evaluated as variables.

If you want to use variables to define your column names, you can unpack from a dictionary. Try:

dframe.assign(**{new_col: shift_1})

An alternative is to use:

dframe[new_col] = shift_1

edited Mar 5, 2019 at 2:15

answered Dec 29, 2018 at 1:21

busybear

10.7k1 gold badge29 silver badges44 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

roganjosh Over a year ago

Do you know why it works like this? The OP's attempt seems pretty reasonable to me

John Over a year ago

Keyword arguments do not resolve variables. The answer is using the correct syntax of casting a dictionary to keywords ( IE: the dictionary will resolve the variables )

roganjosh Over a year ago

@John so this is a necessary evil of allowing multiple assignments in one call?

John Over a year ago

@roganjosh not necessarily, it's just the idiom for how keyword arguments are declared. As demonstrated above, you can assign a dictionary "key" + "Value" using a variable/single call.

busybear Over a year ago

If this weren't the case, function keywords would need quotes around them... too much energy for those extra key presses.

Collectives™ on Stack Overflow

Python: a new column name in a function with assign()

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related