title: asyncio + asyncpg + pandas: obtain pandas.df with async selects from db - ERROR
body: <p>Edited my code - NOW it WORKS
  I'm trying to obtain some date from my Postgres db through asyncpg connection pool asynchronously.
  Basically my db contain about 100 different tables (per city) and i'm trying to gather all the data in one frame as fast as it possible.</p>
  
  <pre><code>    import pandas as pd
      import asyncpg
      import asyncio
      from time import time
  
  
      def make_t():
          lst = []
          # iterator for sql tuple
          for i in ['a',
                    'b',
                    'c']:
              i1 = i
              sql = """
      SELECT
      '%s' as city,
      MAX(starttime) AS max_ts
      FROM
      "table_%s"
      """
              lst.append(sql % (i, i1))
          return tuple(lst)
  
  
      async def get_data(pool, sql):
          start = time()
          async with pool.acquire() as conn:
             stmt = await conn.prepare(sql)
             columns = [a.name for a in stmt.get_attributes()]
             data = await stmt.fetch()
             print(f'Exec time: {time() - start}')
             return pd.DataFrame(data, columns=columns)
  
  
      async def main():
          dsn = 'postgres://user:pass@127.0.0.1:5432/my_base'
          cT = ['city', 'max_ts']
          sqls = make_t()
          pool = await asyncpg.create_pool(dsn=dsn, max_size=50)
          start = time()
          tasks = []
  
          for sql in sqls:
              tasks.append(loop.create_task(get_data(pool, sql)))
  
          tasks = await asyncio.gather(*tasks)
          df = pd.DataFrame(columns=cT)
          for task in tasks:
              # form df from corutine results
              df = df.append(task.result())
  
          print(f'total exec time: {time() - start} secs')
          print('exiting main')
          return df
  
  
      loop = asyncio.get_event_loop()
      df = loop.run_until_complete(main())
      loop.close()
  
      print('exiting program')
  </code></pre>
  
  <p>Python 3.6.5 :: Anaconda, Inc.</p>
  
  <p>Gets me this error:</p>
  
  <blockquote>
    <p>Traceback (most recent call last):
              File "", line 319, in 
              File "/Users/fixx/anaconda3/lib/python3.6/asyncio/base_events.py", line
    468, in run_until_complete
            return future.result()
              File "", line 308, in main
              File "/Users/fixx/anaconda3/lib/python3.6/asyncio/tasks.py", line 594, in gather
            for arg in set(coros_or_futures):
                TypeError: unhashable type: 'list'</p>
  </blockquote>
  
  <p>I cant figure out, why? My sqls in tuple!</p>