input: Write a function to convert a given SQL statement into a Python Pandas DataFrame. The function should take a SQL query string and a database connection object as input, execute the query using Pandas' read_sql_query method, and return the resulting DataFrame. 
  
  def sql_to_dataframe(query, conn):
      import pandas as pd
      df = pd.read_sql_query(query, conn)
      return df
  
  assert sql_to_dataframe("SELECT * FROM book;", conn) is not None
  assert isinstance(sql_to_dataframe("SELECT * FROM book;", conn), pd.DataFrame)
output: def sql_to_dataframe(query, conn):
      import pandas as pd
      df = pd.read_sql_query(query, conn)
      return df