Is there a way to merge a DataFrame on the first non null variable similar to a coalesce?

Question

I am trying to left merge two DataFrames but I want to left_on the first non null variable. Similar to a coalesce in SQL. Is there a way to do that?

In my example below I am left merging on a variable called 'clean_email' but I need to do something like coalesce(clean_email,email)

df = df.merge(df2, how='left',left_on='clean_email',right_on='clean_email')

Answer 1

use pandas.Series.combine_first¶

Combine Series values, choosing the calling Series’s values first.

import pandas as pd
import numpy as np  

s1 = pd.Series([1, np.nan])
s2 = pd.Series([3, 4])
s1.combine_first(s2)