Callback Function when Requesting Market Data using ib_insync and Interactive Brokers

Question

I am trying to get market data for multiple stocks using ib_insync and Interactive Brokers. To do this I am using the pendingTickersEvent callback to wait for the data to arrive. However all that happens is that data for the last symbol in the watch_dict (AAPL) is returned multiple times.

If I use ib.sleep(2) instead of ib.pendingTickersEvent += wait_for_market_data it works ok but is too slow.

This is my first time using an asynchronous library so I've probably doe something silly but I can't work out what it is.

from ib_insync import *

watch_dict = {"TSLA": "NYSE",
              "MSFT": "NYSE",
              "AAPL": "NYSE"}

def wait_for_market_data(tickers):

    print(market_data)
    print()

# Create connection
ib = IB()
ib.connect('127.0.0.1', 7497, clientId=1)

while True:

    # Define stocks
    for ticker in list(watch_dict.keys()):

        print(ticker)
        stock = Stock(ticker, watch_dict[ticker], 'USD')
        print(stock)

        # Request current prices
        market_data = ib.reqMktData(stock, '', False, False)

        #ib.sleep(2)
        #print(market_data)
        ib.pendingTickersEvent += wait_for_market_data

ib.run()