来自Alpha Vantage的5种不同FX的数据框

Question

因此，我尝试获取一个包含5种不同货币FX的数据框，但是当我尝试将我需要的列附加到结果数据框中时，将其添加到将具有所有5种货币FX的数据框中。错误[：] [1]

    let client = new XMLHttpRequest();
client.open('GET', 'http://localhost:5000/api/auth/check');
client.onload = function (x) {
    console.log(client.responseText);
}

client.setRequestHeader('Content-Type', 'application/json');
client.send('{}');

symbols = ["MXN", "EUR", "BRL", "ARS", "CHF"]

for symbol in symbols:
        data = {     
            "function": "FX_DAILY",
            "to_symbol": symbol,
            "from_symbol":"USD",
            "apikey":avkey,
            "interval":"1min",
            "outputsize":"full"} 
        response = requests.get(base_url, data) 
        data = response.json()
        print(symbol)
        a = pd.DataFrame(data["Time Series FX (Daily)"],dtype=float).T
        b = pd.DataFrame(columns = ["MXN", "EUR", "BRL", "ARS", "CHF"])
        b[symbol].append(a["4. close"])