Edit pandas DataFrame using indexes

Question

Is there a general, efficient way to assign values to a subset of a DataFrame in pandas? I've got hundreds of rows and columns that I can access directly but I haven't managed to figure out how to edit their values without iterating through each row,col pair. For example:

In [1]: import pandas, numpy

In [2]: array = numpy.arange(30).reshape(3,10)

In [3]: df = pandas.DataFrame(array, index=list("ABC"))

In [4]: df
Out[4]: 
    0   1   2   3   4   5   6   7   8   9
A   0   1   2   3   4   5   6   7   8   9
B  10  11  12  13  14  15  16  17  18  19
C  20  21  22  23  24  25  26  27  28  29

In [5]: rows = ['A','C']

In [6]: columns = [1,4,7]

In [7]: df[columns].ix[rows]
Out[7]: 
    1   4   7
A   1   4   7
C  21  24  27

In [8]: df[columns].ix[rows] = 900

In [9]: df
Out[9]: 
    0   1   2   3   4   5   6   7   8   9
A   0   1   2   3   4   5   6   7   8   9
B  10  11  12  13  14  15  16  17  18  19
C  20  21  22  23  24  25  26  27  28  29

I believe what is happening here is that I'm getting a copy rather than a view, meaning I can't assign to the original DataFrame. Is that my problem? What's the most efficient way to edit those rows x columns (preferably in-pace, as the DataFrame may take up a lot of memory)?

Also, what if I want to replace those values with a correctly shaped DataFrame?

Answer 1

Use loc in an assignment expression (the = means it's not relevant whether it is a view or a copy!):

In [11]: df.loc[rows, columns] = 99

In [12]: df
Out[12]:
    0   1   2   3   4   5   6   7   8   9
A   0  99   2   3  99   5   6  99   8   9
B  10  11  12  13  14  15  16  17  18  19
C  20  99  22  23  99  25  26  99  28  29

If you're using a version prior to 0.11 you can use .ix.

As @Jeff comments:

This is an assignment expression (see 'advanced indexing with ix' section of the docs) and doesn't return anything (although there are assignment expressions which do return things, e.g. .at and .iat).

df.loc[rows,columns] can return a view, but usually it's a copy. Confusing, but done for efficiency.

Bottom line: use ix, loc, iloc to set (as above), and don't modify copies.

See 'view versus copy' section of the docs.