How to update values in a specific row in a Python Pandas DataFrame?

Question

With the nice indexing methods in Pandas I have no problems extracting data in various ways. On the other hand I am still confused about how to change data in an existing Da

Answer 1

There are probably a few ways to do this, but one approach would be to merge the two dataframes together on the filename/m column, then populate the column 'n' from the right dataframe if a match was found. The n_x, n_y in the code refer to the left/right dataframes in the merge.

In[100] : df = pd.merge(df1, df2, how='left', on=['filename','m'])

In[101] : df
Out[101]: 
    filename   m   n_x  n_y
0  test0.dat  12  None  NaN
1  test2.dat  13  None   16

In[102] : df['n'] = df['n_y'].fillna(df['n_x'])

In[103] : df = df.drop(['n_x','n_y'], axis=1)

In[104] : df
Out[104]: 
    filename   m     n
0  test0.dat  12  None
1  test2.dat  13    16