Offer your insights or judgment on the input financial query or topic using your financial expertise. Reply as normal question answering

Algorithmic trading in linux using python 
A couple options that I know of: Interactive Brokers offers a "paper trading" mode to its account holders that allows you to start with a pretend stack of money and place simulated trades to test trading ideas. They also provide an API that allows you to interface with their platform programmatically for retrieving quotes, placing orders, and the such. As you noted, however, it's not free; you must hold a funded brokerage account in order to qualify for access to their platform. In order to maintain an account, there are minimums for required equity and monthly activity (measured in dollars that you spend on commissions), so you won't get access to their platform without having a decent amount of skin in the game. IB's native API is Java-based; IbPy is an unofficial wrapper that makes the interface available in Python. I've not used IB at all myself, but I've heard good things about their API and its accessibility via IbPy. Edit: IB now supports Python natively via their published API, so using IbPy is no longer needed, unless you wish to use Python 2.x. The officially supported API is based on Python 3. TD Ameritrade also offers an API that is usable by its brokerage clients. They do not offer any such "paper trading" mode, so you would need to "execute" transactions based on quotes at the corresponding trade times and then keep track of your simulated account history yourself. The API supports quote retrieval, price history, and trade execution, among other functions. TDA might be more attractive than IB if you're looking for a low-cost link into market data, as I believe their minimum-equity levels are lower. To get access, you'll need to sign up for an API developer account, which I believe requires an NDA. I don't believe there is an official Python implementation of the API, but if you're a capable Python writer, you shouldn't have trouble hooking up to the published interfaces. Some caveats: as when doing any strategy backtesting, you'll want to be sure to be pessimistic when doing so, so your optimism doesn't make your trades look more successful than they would be in the real world. At a minimum, you'll want to ensure that your simulations transact at the posted bid/ask prices, not necessarily the last trade's price, as well as any commissions and fees associated with the trade. A more robust scheme would also take into account the depth of the order book (also known as level 2 quotes), which can cause additional slippage in the prices at which you buy/sell your security. An even more robust scheme would take into account the potential latency of trade execution, looking at all prices over some time period that covers the maximum expected latency and simulating the trade at the worst-possible price.