Function to print the date of the first non-null value for each column in a DataFrame, sorting the output by date.

func.py

def print_first_non_null_dates(df):
    non_null_dates = {}
    
    for column in df.columns:
        non_null_values = df[column].dropna()
        if not non_null_values.empty:
            first_non_null_date = non_null_values.index[0]
            non_null_dates[column] = first_non_null_date

    sorted_dates = sorted(non_null_dates.items(), key=lambda x: x[1])
    
    for column, date in sorted_dates:
        print(f"'{column}': {date.strftime('%Y-%m-%d')}")