input: Write a python function to connect to a PostgreSQL database, fetch all records from a table named 'customers', and return the results.
  assert fetch_customers() != None
  assert isinstance(fetch_customers(), list)
  assert len(fetch_customers()) > 0
output: def fetch_customers():
      import psycopg2
      try:
          connection = psycopg2.connect(
              user="your_username",
              password="your_password",
              host="your_host",
              port="your_port",
              database="your_database"
          )
          cursor = connection.cursor()
          cursor.execute("SELECT * FROM customers;")
          result = cursor.fetchall()
          return result
      except (Exception, psycopg2.Error) as error:
          print("Error while connecting to PostgreSQL", error)
          return None
      finally:
          if connection:
              cursor.close()
              connection.close()