Python/ Pandas If 语句在 function 中解释

Question

I have the following example and I cannot understand why it doesn't work.

import pandas as pd

d = {'col1': [1, 2], 'col2': [3, 4]}
df = pd.DataFrame(data=d)

def balh(a, b):
    z = a + b
    if z.any() > 1:
        return z + 1
    else:
        return z

df['col3'] = balh(df.col1, df.col2)

Output:

My expected output would be see 5 and 7 not 4 and 6 in col3 , since 4 and 6 are grater than 1 and my intention is to add 1 if a + b are grater than 1

Answer 1

The any method will evaluate if any element of the pandas.Series or pandas.DataFrame is True . A non-null integer is evaluated as True . So essentially by if z.any() > 1 you are comparing the True returned by the method with the 1 integer.

You need to condition directly the pandas.Series which will return a boolean pandas.Series where you can safely apply the any method.

This will be the same for the all method.

def balh(a, b):
    z = a + b
    if (z > 1).any():
        return z + 1
    else:
        return z

Answer 2

As @arhr clearly explained the issue was the incorrect call to z.any() , which returns True when there is at least one non-zero element in z . It resulted in a True > 1 which is a False expression.

A one line alternative to avoid the if statement and the custom function call would be the following:

df['col3'] = df.iloc[:, :2].sum(1).transform(lambda x: x + int(x > 1))

This gets the first two columns in the dataframe then sums the elements along each row and transforms the new column according to the lambda function.

The iloc can also be omitted because the dataframe is instantiated with only two columns col1 and col2 , thus the line can be refactored to:

df['col3'] = df.sum(1).transform(lambda x: x + int(x > 1))

Example output:

   col1  col2  col3
0     1     3     5
1     2     4     7