Function py_get_exchange_rate 

pub fn py_get_exchange_rate(
    from_currency: &str,
    to_currency: &str,
    price_type: PriceType,
    quotes_bid: HashMap<String, f64>,
    quotes_ask: HashMap<String, f64>,
) -> PyResult<Option<f64>>

Calculates the exchange rate between two currencies using provided bid and ask quotes.

Errors

Returns an error if:

• price_type is equal to Last or Mark (cannot calculate from quotes).
• quotes_bid or quotes_ask is empty.
• quotes_bid and quotes_ask lengths are not equal.
• The bid or ask side of a pair is missing.