I have a series of large CSV files "basename.csv" like:

B1,3,5,6

B2,2,1,5

B3,1,9,0

C1,4,7,9

C2,1,9,3

C3,8,5,2

I would like to split them into different files like:

basename\_B.csv

B1,3,5,6

B2,2,1,5

B3,1,9,0

basename\_C.csv

C1,4,7,9

C2,1,9,3

C3,8,5,2

I have already done similar things in the past with for loops and ifs, but I was wondering if there is a more efficient way of doing this with Pandas or whatever. 

SOLUTION

Adapting the solution from @chthonicdaemon and @jezrael, I came up with this:

```
def split_csv():
    for dfile in glob.glob('*.csv'):
        df = pd.read_csv(dfile, header=None)
        for letter, group in df.groupby(df[0].str[0]):
            group.to_csv((os.path.splitext(dfile)[0]) + '_{}.csv'.format(letter), index=False, header=False)

split_csv()

```