In [19]:
#http://pandas.pydata.org/pandas-docs/stable/cookbook.html
import pandas as pd

In [20]:
df = pd.DataFrame({'AAA' : [4,5,6,7], 'BBB' : [10,20,30,40],'CCC' : [100,50,-30,-50]}); df


Out[20]:
AAA BBB CCC
0 4 10 100
1 5 20 50
2 6 30 -30
3 7 40 -50

In [21]:
#if-then...
#An if-then on one column
df.ix[df.AAA >= 5,'BBB'] = -1; df


Out[21]:
AAA BBB CCC
0 4 10 100
1 5 -1 50
2 6 -1 -30
3 7 -1 -50

In [22]:
#An if-then with assignment to 2 columns:
df.ix[df.AAA >= 5,['BBB','CCC']] = 555; df


Out[22]:
AAA BBB CCC
0 4 10 100
1 5 555 555
2 6 555 555
3 7 555 555

In [23]:
#Add another line with different logic, to do the -else
df.ix[df.AAA < 5,['BBB','CCC']] = 2000; df


Out[23]:
AAA BBB CCC
0 4 2000 2000
1 5 555 555
2 6 555 555
3 7 555 555

In [23]: