Updating a Value of A Panda Dataframe with a Function-CodePudding

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.

CodePudding user response：

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()