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

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So first of all, pandas updates using the index. When an update command does not update anything, check both left-hand side and right-hand side. If you don’t update the indices to follow your identification logic, you can do something along the lines of

>>> df.loc[df.filename == 'test2.dat', 'n'] = df2[df2.filename == 'test2.dat'].loc[0]['n']
>>> df
Out[331]: 
    filename   m     n
0  test0.dat  12  None
1  test2.dat  13    16

If you want to do this for the whole table, I suggest a method I believe is superior to the previously mentioned ones: since your identifier is filename, set filename as your index, and then use update() as you wanted to. Both merge and the apply() approach contain unnecessary overhead:

>>> df.set_index('filename', inplace=True)
>>> df2.set_index('filename', inplace=True)
>>> df.update(df2)
>>> df
Out[292]: 
            m     n
filename           
test0.dat  12  None
test2.dat  13    16

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