How to set values for particular cells in pandas.Dataframe correctly?

Question

I've created a pandas DataFrame

df = DataFrame(np.arange(15).reshape(3,5), columns=['a','b','c', 'd', 'e'])

df  a   b   c   d   e
0   0   1   2   3   4
1   5   6   7   8   9
2  10  11  12  13  14

And I want to set values for particular cells:

flag = df['b'] > 3 

df[flag]['b']=10

But it doesn't work.

df  a   b   c   d   e
0   0   1   2   3   4
1   5   6   7   8   9
2  10  11  12  13  14

I use the following codes. It works, but i don't know why?

df['b'][flag] = 10

df  a   b   c   d   e
0   0   1   2   3   4
1   5  10   7   8   9
2  10  10  12  13  14

Answer 1

Do not use chained indexing for assigning values.

Instead, use pd.DataFrame.loc to specify rows and columns:

df.loc[df['b'] > 3, 'b'] = 10

The .loc indexer accepts lists, scalars, or Boolean arrays.

The pandas docs explain in detail why chained indexing should be avoided.