Pandas - Adding Column to DataFrame, but Items Get Reordered

Question

I'm trying to simply add a column from another dataframe of the same length using df['columnname'] = df2['columnname'], but the column appears in df with the lines in a different order.

Is there any rhyme or reason to this? Am I doing pandas wrong? Would love any help for this low level data transformation.

Answer 1

Problem is different indices in both Dataframes, you can assign array or list for avoid it, only necessary same length of both DataFrames:

df = pd.DataFrame({'col': [0,1,2]})
df2 = pd.DataFrame({'columnname': [5,6,7]}, index=[1,0,2])

df['columnname'] = df2['columnname']
df['columnname1'] = df2['columnname'].to_numpy()
df['columnname2'] = df2['columnname'].to_list()

print (df)
   col  columnname  columnname1  columnname2
0    0           6            5            5
1    1           5            6            6
2    2           7            7            7