Best practice for default arguments

One of your co-workers (who obviously didn't take this course) has written this function for adding a column to a panda's DataFrame. Unfortunately, they used a mutable variable as a default argument value! Please show them a better way to do this so that they don't get unexpected behavior.

def add_column(values, df=pandas.DataFrame()):
  """Add a column of `values` to a DataFrame `df`.
  The column will be named "col_<n>" where "n" is
  the numerical index of the column.

  Args:
    values (iterable): The values of the new column
    df (DataFrame, optional): The DataFrame to update.
      If no DataFrame is passed, one is created by default.

  Returns:
    DataFrame
  """
  df['col_{}'.format(len(df.columns))] = values
  return df

Change the default value of df to an immutable variable to follow best practices.
Update the code of the function so that a new DataFrame is created if the caller didn't pass one.

Exercise

Best practice for default arguments

Instructions