Is it possible to use a custom filter function in pandas?

Question

Can I use my helper function to determine if a shot was a three pointer as a filter function in Pandas? My actual function is much more complex, but i simplified it for this question.

def isThree(x, y):
    return (x + y == 3)

print data[isThree(data['x'], data['y'])].head()

Answer 1

Yes:

import numpy as np
import pandas as pd

data = pd.DataFrame({'x': np.random.randint(1,3,10),
                     'y': np.random.randint(1,3,10)})
print(data)

Output:

   x  y
0  1  2
1  2  1
2  2  1
3  1  2
4  2  1
5  2  1
6  2  1
7  2  1
8  2  1
9  2  2

def isThree(x, y):
    return (x + y == 3)

print(data[isThree(data['x'], data['y'])].head())

Output:

   x  y
0  1  2
1  2  1
2  2  1
3  1  2
4  2  1

Answer 2

Yes, so long as your function returns a Boolean Series with the same index you can slice your original DataFrame with the output. In this simple example, we can pass Series to your function:

import pandas as pd
import numpy as np

df = pd.DataFrame(np.random.randint(0, 4, (30, 2)))
def isThree(x, y):
    return x + y == 3

df[isThree(df[0], df[1])]
#    0  1
#2   2  1
#5   2  1
#9   0  3
#11  2  1
#12  0  3
#13  2  1
#27  3  0