code: def dropnotnull(
      df: pd.DataFrame, column_name: Hashable
  ) -> pd.DataFrame:
      """Drop rows that do *not* have null values in the given column.
  
      This method does not mutate the original DataFrame.
  
      Examples:
          >>> import numpy as np
          >>> import pandas as pd
          >>> import janitor
          >>> df = pd.DataFrame({"a": [1., np.NaN, 3.], "b": [None, "y", "z"]})
          >>> df
               a     b
          0  1.0  None
          1  NaN     y
          2  3.0     z
          >>> df.dropnotnull("a")
              a  b
          1 NaN  y
          >>> df.dropnotnull("b")
               a     b
          0  1.0  None
  
      Args:
          df: A pandas DataFrame.
          column_name: The column name to drop rows from.
  
      Returns:
          A pandas DataFrame with dropped rows.
      """
      return df[pd.isna(df[column_name])]
  
imports: from typing import Hashable
  import pandas
  import pandas_flavor
  from janitor.utils import deprecated_alias
label_desc: Use image node_id 1 for calling a global function with example usage: dropnotnull(df, column_name) and returns: df