def json_to_url(json, symbol):
    """ Converts a JSON to a URL by the Poloniex API 

    Args:
        json: JSON data as a list of dict dates, where the keys are
            the raw market statistics.
        symbol: String of currency pair, like a ticker symbol.

    Returns:
        String URL to Poloniex API representing the given JSON.
    """
    start = json[0]['date']
    end = json[-1]['date']
    diff = end - start

    # Get period by a ratio from calculated period to valid periods
    # Ratio closest to 1 is the period
    # Valid values: 300, 900, 1800, 7200, 14400, 86400
    periods = [300, 900, 1800, 7200, 14400, 86400]

    diffs = {}
    for p in periods:
        diffs[p] = abs(1 - (p / (diff / len(json)))) # Get ratio

    period = min(diffs, key=diffs.get) # Find closest period
    
    url = ('https://poloniex.com/public?command'
           '=returnChartData&currencyPair={0}&start={1}'
           '&end={2}&period={3}').format(symbol, start, end, period) 
    return url
Converts a JSON to a URL by the Poloniex API 

    Args:
        json: JSON data as a list of dict dates, where the keys are
            the raw market statistics.
        symbol: String of currency pair, like a ticker symbol.

    Returns:
        String URL to Poloniex API representing the given JSON.