Convert a lambda function to a regular function

Question

I'm trying to understand how can I convert a lambda function to a normal one. I have this lambda function that it supposed to fill the null values of each column with the mode

def fill_nn(data):
    df= data.apply(lambda column: column.fillna(column.mode()[0]))
    return df

I tried this:

def fill_nn(df):
    for column in df:
        if df[column].isnull().any():
            return df[column].fillna(df[column].mode()[0])

Answer 1

Hi 👋 Hope you are doing well!

If I understood your question correctly then the best possible way will be similar to this:

import pandas as pd


def fill_missing_values(series: pd.Series) -> pd.Series:
    """Fill missing values in series/column."""

    value_to_use = series.mode()[0]
    return series.fillna(value=value_to_use)


df = pd.DataFrame(
    {
        "A": [1, 2, 3, 4, 5],
        "B": [None, 2, 3, 4, None],
        "C": [None, None, 3, 4, None],
    }
)

df = df.apply(fill_missing_values)  # type: ignore

print(df)
#    A    B    C
# 0  1  2.0  3.0
# 1  2  2.0  3.0
# 2  3  3.0  3.0
# 3  4  4.0  4.0
# 4  5  2.0  3.0

but personally, I would still use the lambda as it requires less code and is easier to handle (especially for such a small task).