Swapping decimal for cdecimal on Python 2

cdecimal and decimal

Python 3 includes a faster re-implementation of the decimal standard library, called cdecimal. How much faster?

“Typical performance gains are between 30x for I/O heavy benchmarks and 80x for numerical programs”.

Nice! Thankfully we don’t have to port all our code to Python 3 before we can use it; it runs on Python 2 as well. You can install it either by downloading via the external URL listed on its official pip page, or if that annoys you as much as it annoyed me, you can pip install it from the m3-cdecimal pip upload with pip install m3-cdecimal.

If your code is quite simple, you might be able to just swap from decimal import Decimal to from cdecimal import Decimal to get an instant speed boost. Unfortunately, if any of your dependencies imports decimal.Decimal, this might blow up in any of several ways.

A more all-encompassing solution is to swap the two in the module cache sys.modules, right at the top - before any other imports - with this snippet:

import sys
import cdecimal

# Ensure any import of decimal gets cdecimal instead.
sys.modules["decimal"] = cdecimal

As long as you get the first lines of every one of your entry points, this should work really well.

Unfortunately, at YPlan we run our application via a third party monitoring script, so we don’t have control over the very first lines of Python that get executed. It also turns out the script ends up importing some dependencies that import decimal, which would require a lot of brittle monkey-patching of these modules if we wanted to do the swap at our entry point(s).

At first I thought this meant we were out of options, but googling lead me to find an eleven year old blog post on patching Python’s standard library by Bob Ippolito that looked like what I wanted. It covers fixing a buggy Python interpreter in an old version of Mac OS X, but the main technique is still just as applicable, though something you’d rarely use. This post is therefore a bit of a recycle of that one for the current issue.

Python imports modules from a search path, and you might be familiar with extending this via the PYTHONPATH environment variable. However, you can also add to the search path by creating .pth files as noted in the Python site module docs. These are read during interpreter initialization and normally contain one directory per line to add to the search path. The real magic of .pth files is this little extra, which the manual explains with a single short sentence:

Lines starting with import (followed by space or tab) are executed.

Since importing a module allows arbitrary code execution, we can use this to swap decimal and cdecimal for all code run on our python interpreter, before any program code is executed. Using a virtualenv means our system Python will be unaffected so we don’t need to worry about breaking any OS tools.

First, we need to create a .pth file in the default search path at e.g. /path/to/virtualenv/lib/python2.7/site-packages/my_patches.pth, containing the one-liner:

import my_patches

This will then try to import the my_patches module, which we’ll create at /path/to/virtualenv/lib/python2.7/site-packages/my_patches.py with:

import sys
import cdecimal

# Ensure any import of decimal gets cdecimal instead.
sys.modules["decimal"] = cdecimal

Then it’s done! It’s quite easy to test, for example using ipython:

In [1]: import decimal, cdecimal

In [2]: decimal is cdecimal
Out[2]: True

You can also check other modules are importing cdecimal, e.g. simplejson.encoder which does from decimal import Decimal:

In [3]: import simplejson.encoder

In [4]: simplejson.encoder.Decimal is cdecimal.Decimal
Out[4]: True

Ta-daa! The only thing that remains is to benchmark and see if your code is actually faster. Enjoy!

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