input: Write a function to convert a given SQL statement into a Python Pandas DataFrame. The function should take a SQL string and a database connection as input, and return a Pandas DataFrame.
  
  def sql_to_dataframe(sql_query, conn):
      import pandas as pd
      df = pd.read_sql_query(sql_query, conn)
      return df
  
  assert isinstance(sql_to_dataframe("SELECT * FROM book;", conn), pd.DataFrame)
  assert sql_to_dataframe("SELECT * FROM book;", conn).shape != (0, 0)
output: def sql_to_dataframe(sql_query, conn):
      import pandas as pd
      df = pd.read_sql_query(sql_query, conn)
      return df