Updating a Value of A Panda Dataframe with a Function

Question

I have a function which updates a dataframe that I have passed in:

def update_df(df, x, i):
    for i in range(x):
        list = ['name' + str(i), i + 2, i - 1]
        df.loc[i] = list

    return df, i   

df = pd.DataFrame(columns=['lib', 'qty1', 'qty2'])
i = 0
df, i = update_df(df, 6, i)

What I would like to do, is be able to modify the dataframe after running it through the update_df(df, x, i) function I created but I am not having much luck with it. The following is an example of what I am trying to do I am just trying to is concatenate the values of the last two rows in the lib column together:

temp = df.loc[i][0]+df.loc[i-1][0]
print(temp)
df.loc[i][0] = temp
print(df)

The following is the output I get:

name5name4
     lib  qty1  qty2
0  name0     2    -1
1  name1     3     0
2  name2     4     1
3  name3     5     2
4  name4     6     3
5  name5     7     4

But what I hope to get is:

name5name4
     lib  qty1  qty2
0  name0     2    -1
1  name1     3     0
2  name2     4     1
3  name3     5     2
4  name4     6     3
5  name5name4     7     4

This is part of a larger project where I will be constantly writing to a dataframe and then eventually write the dataframe to a file. Periodically I will want to update the last row of the dataframe which is where I am looking to figure out how to update it appropriately. I would like to avoid making a copy of the dataframe and just stick with one that I update throughout my code.

Answer 1

If all you want is to concatenate the last two values in the lib column, and reassign the last row's lib column to that value:

df.loc[df.index[-1], "lib"] = df[-2:]["lib"].sum()