Row-wise average for a subset of columns with missing values

You can simply:

df['avg'] = df.mean(axis=1)

       Monday  Tuesday  Wednesday        avg
Mike       42      NaN         12  27.000000
Jenna     NaN      NaN         15  15.000000
Jon        21        4          1   8.666667

because .mean() ignores missing values by default: see docs.

To select a subset, you can:

df['avg'] = df[['Monday', 'Tuesday']].mean(axis=1)

       Monday  Tuesday  Wednesday   avg
Mike       42      NaN         12  42.0
Jenna     NaN      NaN         15   NaN
Jon        21        4          1  12.5

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