title: How to get trade history from Poloniex about BTC to USD in Python?
body: <p>I would like to get whole trade history from Poloniex about BTC to USD in Python.
  I have entered my key and secret.
  I have got the code like below:</p>
  
  <pre><code>import urllib
  import urllib2
  import json
  import time
  import hmac,hashlib
  
  def createTimeStamp(datestr, format="%Y-%m-%d %H:%M:%S"):
      return time.mktime(time.strptime(datestr, format))
  
  class poloniex:
      def __init__(self, APIKey, Secret):
          self.APIKey = APIKey
          self.Secret = Secret
  
      def post_process(self, before):
          after = before
  
          # Add timestamps if there isnt one but is a datetime
          if('return' in after):
              if(isinstance(after['return'], list)):
                  for x in xrange(0, len(after['return'])):
                      if(isinstance(after['return'][x], dict)):
                          if('datetime' in after['return'][x] and 'timestamp' not in after['return'][x]):
                              after['return'][x]['timestamp'] = float(createTimeStamp(after['return'][x]['datetime']))
  
          return after
  
      def api_query(self, command, req={}):
  
          if(command == "returnTicker" or command == "return24Volume"):
              ret = urllib2.urlopen(urllib2.Request('https://poloniex.com/public?command=' + command))
              return json.loads(ret.read())
          elif(command == "returnOrderBook"):
              ret = urllib2.urlopen(urllib2.Request('https://poloniex.com/public?command=' + command + '&amp;currencyPair=' + str(req['currencyPair'])))
              return json.loads(ret.read())
          elif(command == "returnMarketTradeHistory"):
              ret = urllib2.urlopen(urllib2.Request('https://poloniex.com/public?command=' + "returnTradeHistory" + '&amp;currencyPair=' + str(req['currencyPair'])))
              return json.loads(ret.read())
          else:
              req['command'] = command
              req['nonce'] = int(time.time()*1000)
              post_data = urllib.urlencode(req)
  
              sign = hmac.new(self.Secret, post_data, hashlib.sha512).hexdigest()
              headers = {
                  'Sign': sign,
                  'Key': self.APIKey
              }
  
              ret = urllib2.urlopen(urllib2.Request('https://poloniex.com/tradingApi', post_data, headers))
              jsonRet = json.loads(ret.read())
              return self.post_process(jsonRet)
  
  
      def returnTicker(self):
          return self.api_query("returnTicker")
  
      def return24Volume(self):
          return self.api_query("return24Volume")
  
      def returnOrderBook (self, currencyPair):
          return self.api_query("returnOrderBook", {'currencyPair': currencyPair})
  
      def returnMarketTradeHistory (self, currencyPair):
          return self.api_query("returnMarketTradeHistory", {'currencyPair': currencyPair})
  
  
      # Returns all of your balances.
      # Outputs: 
      # {"BTC":"0.59098578","LTC":"3.31117268", ... }
      def returnBalances(self):
          return self.api_query('returnBalances')
  
      # Returns your open orders for a given market, specified by the "currencyPair" POST parameter, e.g. "BTC_XCP"
      # Inputs:
      # currencyPair  The currency pair e.g. "BTC_XCP"
      # Outputs: 
      # orderNumber   The order number
      # type          sell or buy
      # rate          Price the order is selling or buying at
      # Amount        Quantity of order
      # total         Total value of order (price * quantity)
      def returnOpenOrders(self,currencyPair):
          return self.api_query('returnOpenOrders',{"currencyPair":currencyPair})
  
  
      # Returns your trade history for a given market, specified by the "currencyPair" POST parameter
      # Inputs:
      # currencyPair  The currency pair e.g. "BTC_XCP"
      # Outputs: 
      # date          Date in the form: "2014-02-19 03:44:59"
      # rate          Price the order is selling or buying at
      # amount        Quantity of order
      # total         Total value of order (price * quantity)
      # type          sell or buy
      def returnTradeHistory(self,currencyPair):
          return self.api_query('returnTradeHistory',{"currencyPair":currencyPair})
  
      # Places a buy order in a given market. Required POST parameters are "currencyPair", "rate", and "amount". If successful, the method will return the order number.
      # Inputs:
      # currencyPair  The curreny pair
      # rate          price the order is buying at
      # amount        Amount of coins to buy
      # Outputs: 
      # orderNumber   The order number
      def buy(self,currencyPair,rate,amount):
          return self.api_query('buy',{"currencyPair":currencyPair,"rate":rate,"amount":amount})
  
      # Places a sell order in a given market. Required POST parameters are "currencyPair", "rate", and "amount". If successful, the method will return the order number.
      # Inputs:
      # currencyPair  The curreny pair
      # rate          price the order is selling at
      # amount        Amount of coins to sell
      # Outputs: 
      # orderNumber   The order number
      def sell(self,currencyPair,rate,amount):
          return self.api_query('sell',{"currencyPair":currencyPair,"rate":rate,"amount":amount})
  
      # Cancels an order you have placed in a given market. Required POST parameters are "currencyPair" and "orderNumber".
      # Inputs:
      # currencyPair  The curreny pair
      # orderNumber   The order number to cancel
      # Outputs: 
      # succes        1 or 0
      def cancel(self,currencyPair,orderNumber):
          return self.api_query('cancelOrder',{"currencyPair":currencyPair,"orderNumber":orderNumber})
  
      # Immediately places a withdrawal for a given currency, with no email confirmation. In order to use this method, the withdrawal privilege must be enabled for your API key. Required POST parameters are "currency", "amount", and "address". Sample output: {"response":"Withdrew 2398 NXT."} 
      # Inputs:
      # currency      The currency to withdraw
      # amount        The amount of this coin to withdraw
      # address       The withdrawal address
      # Outputs: 
      # response      Text containing message about the withdrawal
      def withdraw(self, currency, amount, address):
          return self.api_query('withdraw',{"currency":currency, "amount":amount, "address":address})
  
  polo = poloniex('my_key', 'my_security')
  print(polo.returnTradeHistory('BTC_ETH'))
  #print(polo.returnTradeHistory('BTC_USDT))
  </code></pre>
  
  <p>I have used method like:</p>
  
  <pre><code>print(polo.returnTradeHistory('BTC_ETH'))
  </code></pre>
  
  <p>but I get empty array [], I don't know why?
  Could you also tell me how can I get this trade history and get info about BTC to USD, because something like BTC_USD hasn't worked.</p>