Financial Python

Studies in Finance and Python

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Use python and sqlite3 to build a database

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The previous post showed how matplotlib can pull market data from Yahoo. Using some python-fu, you can easily create CSV files for given stocks. CSV files are great as they are easy to parse and don’t require a lot of overhead (in terms of setting things up, you can just open the file directly). Of course, Yahoo, Google, and others offer decent tools to view stock prices, so creating CSV files might be a bit redundant. Furthermore, if you really want to go whole hog and pull prices for thousands of assets, I imagine one would have to think carefully about an appropriate filesystem.

On the other hand, python includes a built in wrapper for sqlite, the ubiquitous file-based database that’s embedded in so many mobile and desktop applications. Sqlite can be run on any platform and is a completely open source, flyweight piece of software (which explains it’s popularity). This post will outline how to create a simple two table sqlite database to store stock prices.

First if all, sqlite has it’s own interpreter you can access from the command line, so if you’re an sql cowboy, you can instantly execute sql statements from the prompt to explore a given database. Here are a few examples:

The .help command lists some of the sqlite specific commands available at the command line.
The .schema command prints the tables in the database (just as you might think).
You can also input sql statements directly.
Anyway, enough about sqlite itself, let’s get to the python that built the database above. The attached script used four functions to package data for a defined set of tickers, using quotes_historical_yahoo(), and inserts the data into an sqlite database. The key is using a dictionary of tuples to pass around data.

stocks = {'data':[('GOOG', 'stock'), ('AAPL','stock')],
 'headers':('ticker', 'tag')}

In the example above, the ‘data’ key references a list of tuples that provide information on the ticker and type of asset in the tuple. The ‘header’ key references the table field names (as defined in the database schema) associated with the ‘data’ key. The same structure is used to write market data to the database, where the ‘data’ key refers to numeric data (e.g., foreign keys, date string, open, close, etc.) and the ‘header’ key refers to the formal fields to which each piece of data is associated.

raw_quotes = quotes_historical_yahoo(ticker, start, end) #list of tuples
data = []
for quote in raw_quotes:
 date_raw = datetime.datetime.fromordinal(int(quote[0]))
 year, month, day = date_raw.year, date_raw.month,
 date_string = str(year)+'-'+str(month)+'-'+str(day)
 record = (f_key, quote[0], date_string, year, month, day,
 quote[1], quote[2], quote[3], quote[4], quote[5])

headers = ('asset_id',
return {'data':data, 'headers':headers}

I use a fair amount of string substitution in the code, which isn’t strictly recommended, but I’m not particularly worried about security. Anyway, the write2sql() function needs to know how many fields are associated with the data. Rather than create two separate functions for adding stocks to the asset table and market data to the prices table, I used string operations to expand or contract the substitution marks to match the number of headers. I’m sure there are more elegant ways to do it, but, hey, it works.

Finally, I’ll plug the Sqlite Manager add-on for Firefox. It’s a basic and easy way to inspect sqlite files.


Here’s the complete code: Read the rest of this entry »

Written by DK

August 5, 2009 at 3:36 am

Posted in Finance, Python

Tagged with , , , ,