input: Write a function to convert a given SQL statement into a Python Pandas DataFrame. The function should take a SQL statement as input, execute it using a database connection, and return the result as a Pandas DataFrame.
  
  def sql_to_dataframe(sql_statement, conn):
      import pandas as pd
      df = pd.read_sql_query(sql_statement, conn)
      return df
  
  assert sql_to_dataframe("SELECT * FROM book;", conn).shape == (5, 3)
  assert sql_to_dataframe("SELECT title, author FROM book WHERE year > 2000;", conn).shape == (2, 2)
  assert sql_to_dataframe("SELECT * FROM book WHERE genre = 'Science Fiction';", conn).shape == (3, 3)
  assert sql_to_dataframe("SELECT * FROM book WHERE year = 2020;", conn).shape == (1, 3)
  assert sql_to_dataframe("SELECT * FROM book WHERE year < 2000;", conn).shape == (2, 3)
output: def sql_to_dataframe(sql_statement, conn):
      import pandas as pd
      df = pd.read_sql_query(sql_statement, conn)
      return df